arXiv:0807.0532v2 [astro-ph] 4 Jul 2008 -...
Transcript of arXiv:0807.0532v2 [astro-ph] 4 Jul 2008 -...
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TheL1615/L1616 PM S population:April20,2013
T he star form ation in the L1615/L1616 C om etary C loud
DavideGandol�1;2;3,Juan M .Alcal�a2,Silvio Leccia2,Antonio Frasca1,Loredana Spezzi1;2,
Elvira Covino2,Leonardo Testi4,EttoreM arilli1,JouniKainulainen5
A B ST R A C T
The present work aim s at perform ing a com prehensive census and charac-
terisation ofthe pre-m ain sequence (PM S) population in the com etary cloud
L1615/L1616,in orderto assessthe signi�cance ofthe triggered starform ation
scenario and investigatetheim pactofm assivestarson itsstarform ation history
and m assspectrum .Ourstudy isbased on UB V R C IC and JH K s photom etry,
aswellasopticalm ulti-objectspectroscopy.W eperform ed aphysicalparam etri-
sation ofthe young stellar population in L1615/L1616. W e identi�ed 25 new
T Tauristarsm ainly projected on thedensehead ofthecom etary cloud,alm ost
doubling the currentnum berofknown m em bers. W e studied the spatialdistri-
bution ofthecloud m em bersasa function oftheageand H� em ission.Thestar
form ation e�ciency in thecloud is� 7{8% ,asexpected form olecularcloudsin
thevicinity ofOB associations.Theslopeoftheinitialm assfunction (IM F),in
the m assrange 0:1�M � 5:5 M � ,isconsistentwith thatofotherT and OB as-
sociations,providing furthersupportofan universalIM F down to thehydrogen
burninglim it,regardlessofenvironm entalconditions.Thecom etaryappearance,
aswellasthehigh starform atione�ciency,can beexplained in term softriggered
starform ation induced by thestrong UV radiation from OB starsorsupernovae
shockwaves. The age spread as wellas both the spatialand age distribution
ofthe PM S objects provide strong evidence ofsequential,m ultiple events and
possibly stillongoing starform ation activity in thecloud.
1INAF,O sservatorio Astro�sico diCatania,Via S.So�a 78,95123 Catania,Italy;dgandol�@ oact.inaf.it
2INAF, O sservatorio Astronom ico di Capodim onte, Salita M oiariello 16, 80131 Napoli, Italy;
jm ae@ na.astro.it
3Th�uringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, G erm any; davide@ tls-
tautenburg.de
4INAF,O sservatorio Astronom ico diArcetri,L.go E.Ferm i5,50125 Firenze,Italy;lt@ arcetri.astro.it
5O bservatory,PO Box 14,FIN-00014 University ofHelsinki,Finland;jouni.kainulainen@ helsinki.�
{ 2 {
Subject headings: com etary cloud: individual(L1615/L1616) | dust,extinc-
tion | stars: circum stellar m atter | stars: form ation | stars: fundam ental
param eters| stars:pre-m ain sequence
1. Introduction
TheLyndscloudsL1615andL1616(Lynds1962)arebothlocatedatanangulardistance
ofabout6� W estofthe Orion OB1 associations. These clouds actually form a com etary-
shaped singlecloud with a \head-tail" distribution subtending about400(5.2 pcassum ing a
distanceof450pc;Alcal�a etal.2004)roughly in theEast-W estdirection.Thedensehead of
thecloud com plex (i.e.L1616 only),pointing toward Eastand facing thebrightOrion-belt
stars,harbourstheIRAS Sm allScale StructureX0504-034 (Helou & W alker1988;Ram esh
1995)and abrightre ection nebula,NGC 1788(= DG 51,Ced40,vdB 33,RNO 35,LBN 916;
Lynds1965),which isillum inated by a sm allclusterofstars(Stankeetal.2002).The two
interm ediate-m assstarsHD 293815 and KisoA-0974 15 arethebrightestvisiblem em bersof
thiscluster.
L1615/L1616,likem any othercom etary cloudso�them ain Orion starform ingregions,
clearly showsevidenceofongoingstarform ation activity which m ighthavebeen triggered by
thestrongim pactoftheUV radiation from them assive,lum inousstarsin theOrion com plex
(M addalena etal.1986;Stankeetal.2002;Alcal�a etal.2004;Kun etal.2004;Leeetal.
2005,2007). The illum ination ofdense clum ps in m olecular clouds by OB stars could be
responsible for their collapse and subsequent star form ation. The UV radiation from the
OB starsm ay sweep them olecularm aterialofthecloud into a com etary shapewith adense
corelocated atthehead ofthecom etary cloud.
The radiation and wind ofOB starsm ay also have an im portantim pacton the m ass
accretion during the star form ation process. W hile in a T association a protostar m ay
accum ulate a signi�cant fraction ofm ass,the m ass accretion ofa low-m ass protostarin a
region exposed to the wind ofOB stars can be term inated earlier because ofthe photo-
evaporation ofthe circum stellar m atter (Kroupa 2001,2002). Therefore,m any low-m ass
protostarsm ay notcom plete theiraccretion and hence can resultasbrown dwarfs(BDs).
Thism echanism m ighta�ectthe low-m assend ofthe initialm assfunction (IM F).Recent
studies have provided som e inform ation aboutthe shape ofthe IM F in the very low-m ass
and sub-stellarregim es. W hile the IM F in the Orion Nebula Clusterappearsto rise below
0.1 M � (Hillenbrand & Carpenter 2000),in T association like Taurus-Auriga and ChaII
thereissom eindication ofade�citofsub-stellarobjects(Luhm an etal.2000;Brice~no etal.
2002;Spezzietal.2008). Other studies ofthe young cluster IC 348,which is devoid of
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very m assive stars,have also revealed a de�citofBDsrelative to theOrion Nebula Cluster
(Preibisch etal.2003;M uench etal.2003;Lada etal.2006).
Because ofitsvicinity to the Orion OB associationsthe L1615/L1616 com etary cloud
constitutesan ideallaboratorytoinvestigatethetriggered starform ation scenario.Them ost
recentcensusofthepre-m ain sequence (PM S)starsin L1616 wasprovided by Alcal�a etal.
(2004),who presented a m ulti-wavelength study ofthe region,from X-ray to near-infrared
wavelengths. They found 22 new low-m ass PM S stars distributed m ainly to the East of
L1616,in about1 square degree �eld. By adding the 22 new PM S starsto the previously
con�rm ed m em bers of the cloud (Cohen & Kuhi1979;Sterzik etal.1995;Nakano etal.
1995;Stankeetal.2002) and counting the m illim eter radio-source,i.e. the Class-0 pro-
tostarM M S1A found by Stankeetal.(2002),Alcal�a etal.(2004)ended up with a sam ple
of33 young stellarobjectsassociated with L1616.However,thelatterwork could notinves-
tigateim portantaspectsofthestarform ation in thiscloud,liketheIM F and theim pactof
environm entalconditionson them assspectrum ,becausetheirsam plewasratherincom plete.
Therefore, the aim of the present study is to perform a com prehensive census and
characterisation ofthe PM S population in L1615/L1616,in order to investigate the star
form ation history,the relevance ofthe triggered scenario,and to study the IM F.To this
aim ,we reportboth opticaland near-infrared observations,aswellasm ulti-objectoptical
spectroscopy in L1615/L1616. An im portant goalofour study is to com pare the m ass
function in L1615/L1616 with thatofotherT and OB associations.
Theoutlineofthepaperisasfollows:wedescribeourobservationsand data reduction
in Section 2 and 3;the results are reported in Section 4,while the data analysis and the
physicalproprieties ofthe new PM S starsare presented in Section 5. Ourdiscussion and
conclusions are developed in Section 6. Som e detailson the spectral-type classi�cation as
wellasreddening,radius,and lum inosity determ ination aregiven in theAppendix A and B,
respectively.
2. Photom etry
2.1. O ptical
M ost ofthe opticalphotom etry com es from previous B ,V ,R C and IC broad-band
im aging observationsperform ed by Alcal�a etal.(2004),who obtained CCD m osaic im ages
with the W ide Field Im ager (W FI) cam era at the ESO/M PIA 2.2m telescope (La Silla
Observatory,Chile,program No. 64.I-0355),covering a sky-area ofabout360� 340 around
NGC 1788.Thepre-reduction oftheraw im ages,aswellastheastrom etricand photom etric
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calibration have been already discussed in Alcal�a etal.(2004). In order to im prove the
photom etry oftheblended sources,wecarried outpointspread function �tting photom etry
on theim agesusing theDAOPHOT package(Stetson 1987)undertheIRAF1 environm ent.
Further photom etric observations ofsix previously known m em bers ofL1615/L1616
(Alcal�a etal.2004),stilllacking opticalphotom etry and falling outsidethesky-area covered
by W FI,were carried out in Novem ber 17th and 18th,2006 with the 91cm telescope at
Catania AstrophysicalObservatory (OAC).The observationswere perform ed in the John-
son UB V standard system underphotom etric sky conditions,by using a photon-counting
single-head photom eter equipped with an EM I9893QA/350 photom ultiplier. In order to
determ inethetransform ation coe�cientsto theJohnson standard system ,severalstandard
stars selected from the GeneralCatalogue ofPhotom etric Data (GCPD,M erm illiod etal.
1997)and from Landolt(1992)werealso observed in between thetargetsobservations.The
observing set-up and the data reduction were the standard ones already adopted during
previousobservationalcam paignsoflow-m assstars(see,e.g.,M arillietal.2007).
2.2. N ear-infrared
Near-infrared J,H and K s photom etry ofa � 300� 300 region centered on NGC 1788
wasobtained using the SOFIcam era atthe ESO NTT telescope (program No.70.C-0629)
on Decem ber 17th,2002 and January 16th,2003. 36 pointingsofthe 50� 50 �eld ofview
were required to coverthe�eld in each band.Severaldithered exposureswere obtained for
a totalintegration tim eperpointing of80 secondsin theJ band and 180 secondsin theH
and K sbands.Thedatawerecorrected forelectroniccrosstalk,di�erential at�elding,and
illum ination following standard SOFIproceduresunderIRAF.
Photom etriccalibration wasobtained observing a setofstandard starsfrom thelistof
Persson etal.(1998)(LasCam panasObservatory standards). Source extraction and aper-
ture photom etry were perform ed using the SExtractorsoftware package (Bertin & Arnouts
1996); aperture corrections were estim ated from sources away from crowded regions and
applied to allextracted sources.Thelim iting m agnitudesat3� levelachieved in oursurvey
are19,18 and 17.5 m ag atJ,H ,and K s,respectively.
1IRAF is distributed by the NationalO pticalAstronom y O bservatory,which is operated by the Asso-
ciation ofthe UniversitiesforResearch in Astronom y,inc. (AURA)undercooperative agreem entwith the
NationalScience Foundation.
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3. Spectroscopy
Based on IC versus (R C � IC ) colour-m agnitude diagram and theoreticalisochrones
from Bara�eetal.(1998),Alcal�a etal.(2004)selected a sam pleofabout200 PM S starand
BD candidates in L1615/L1616,by using the spectroscopically con�rm ed young low-m ass
stars to de�ne the PM S locus,as shown in their Figure12. In addition to this sam ple,
8 X-ray em itting stars detected by the ROSAT All-Sky Survey (RASS),as wellas m id-
infrared sources(Stankeetal.2002)and H� em ission-lineobjects(Nakano etal.1995),were
also selected as further PM S star candidates by Alcal�a etal.(2004). W e perform ed the
spectroscopicfollow-up ofabout70% ofthecandidatesin thissam ple,down toIC � 19m ag,
in orderto de�nitively assess theirnature and single outthe young objects. Additionally,
about30% of�eld starswith IC . 20:0 m ag wasalso observed in orderto detectpossible
\veiled" PM S starswhich m ighthaveescaped theselection criterion (seelaterFigure8).
Interm ediate- and low-resolution spectra were acquired during di�erent runs at the
ESO Very LargeTelescope(VLT;ParanalObservatory,Chile),by using theFOcalReducer
and low-dispersion Spectrograph 2 (FORS2;Appenzelleretal.1998)and theVIsibleM ulti-
Object Spectrograph (VIM OS;LeFevreetal.2003). M ulti-object interm ediate-resolution
spectroscopy was also perform ed with the Fibre Large Area M ulti-Elem ent Spectrograph
(FLAM ES;Pasquinietal.2002),but these observations willbe described in m ore detail
in a forthcom ing paper. The journalofthe FORS2 and VIM OS observations is given in
Table 1. The distribution on the sky ofthe area covered by the FORS2,VIM OS,and
FLAM ES spectroscopicfollow-upsisshown in Figure1.In thefollowing sub-sectionsa brief
description ofthespectroscopic observationsand data reduction ispresented.
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Fig.1.| Sky areassurveyed by theFORS2 (continuouslines),VIM OS (dashed lines),and
FLAM ES (dotted lines)spectroscopicfollow-upsoverlaid on theIRAS 100�m dustem ission
m ap covering a sky-area ofabout1�� 1� around L1615/L1616.Notethatthegapsofabout
20between thefourVIM OS quadrantsarenotreproduced in the�gure.
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3.1. FO R S2 observations
Interm ediate-resolution (�/��=2140) FORS2 spectra ofthe PM S candidates m ainly
projected on theNGC 1788 re ection nebula were collected in service m ode (ESO program
No.70.C-0536)on thenights7,22,25 February and 8 M arch 2003,underm ostly clearand
stable weatherconditions,with seeing typically in the range 0:008� 1:000. These observations
were perform ed setting the spectrograph in the m ulti-object spectroscopy m ode. A total
of5 m ask con�gurations,each including 18{19 objects,were observed. Three consecutive
spectra of960 secwereobtained foreach m ask con�guration in orderto rem ove cosm ic-ray
hitsand im prove the signalto noise (S/N)ratio. Bias, at-spectrum lam p,and Ar/He/Ne
lam p exposures for each m ask were taken in daytim e according to the FORS2 standard
calibration plan.ThetypicalS/N ratio fora IC � 16 m ag starwason theorderof90.
The standard data reduction process was carried out with a sem i-autom atic pipeline
that we have developed on the basis ofboth M IDAS2 (W arm els 1991) and IRAF pack-
age.Thereduction includesbiassubtraction, at-�eld division,wavelength calibration,sky
subtraction and one-dim ensionalspectra extraction. Relative ux calibration wasachieved
observing �vespectrophotom etricstandard starsfrom Ham uy etal.(1992,1994).
3.2. V IM O S observations
The VIM OS observationswere perform ed in service m ode (ESO program No. 074.C-
0111)during the period from 9 Decem ber2004 to 12 M arch 2005.The weatherconditions
were photom etric with seeing varying between 0:006 and 1:002. The larger sky-area covered
with VIM OS allowed us to extend the spectroscopic survey in a wider region around the
NGC 1788re ection nebula3.Furtherm ore,wecarried outboth interm ediate-(�/��=2500)
and low-resolution (�/��=580)spectroscopy,in orderto detecttheLii�6708 �A absorption
line and m easure its equivalent width,as wellas to obtain a wider wavelength range for
spectraltypeclassi�cation purpose(seeSection 5.2).
A totalof3 interm ediate-and 6 low-resolution m ask con�gurations,each including an
average num berofabout100 targets,were observed. To e�ciently pursue ourprogram m e
and m axim ise the num berofobserved �elds,only one exposure of900 sec and one of2000
secwereobtained foreach low-and interm ediate-resolution m ask con�guration,respectively.
2M IDAS isdeveloped and m aintained by ESO .
3Note,however,that the gaps ofabout 20 between the four VIM O S quadrantsprevented us from per-
form ing a uniform coverage.
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ThetypicalS/N ratio atIC � 16 m ag wasabout80 and 180 fortheinterm ediate-and low-
resolution spectra,respectively.
Fivespectrophotom etricstandard starsfrom Oke(1990)and Ham uy etal.(1992,1994)
wereobserved to perform theabsolute ux calibrations.Biasand at�eld fram esaswellas
wavelength calibration exposureswere acquired during daytim e,following theservice m ode
VIM OS calibration plan.
TheVIM OSspectrawereautom aticallyreduced byusingtheESO pipeline,fulldetailsof
which aregiven by Izzo etal.(2004).In orderto check thereliability ofthedata reduction,
the spectra were also independently processed by using the VIM OS Interactive Pipeline
GraphicalInterface(VIPGI;Scodeggio etal.2005),obtaining consistentresults.
3.3. FLA M ES-G IR A FFE observations
The FLAM ES-GIRAFFE observationswere perform ed in visitorm ode (ESO program
No. 076.C-0385) during the nights 28 February 2006 and 1,2 M arch 2006,under good
seeing conditions(0:007� 1:000),setting thespectrograph in theM EDUSA con�guration.The
low-resolution grating LR06 was used in conjunction with an order separating �lter and
a slit width of100. The adopted con�guration yielded a spectralcoverage ofabout 750 �A
(6438{7184 �A)with a m ean resolving power�/��=8600.
Sincetheseobservationsarepartofaseparatestudyon radialvelocitiesin L1615/L1616,
these data willbe described in m ore detailin a future paper. Here we have used the
FLAM ES spectra ofonly three objects,nam ely TTS050644.4�032913,RX J0507.3�0326,
and TTS050741.0�032253 (see Table 2),forwhich both FORS2 and VIM OS observations
arem issing.
4. PM S objects in L1615/L1616
Sincelithium israpidly destroyed in theconvectivelayersoflow-m assstarsin theearly
phases oftheir stellar evolution (Bodenheim er 1965),the presence ofstrong Lii�6708 �A
absorption line has been considered as the prim ary criterion for de�nitely assessing the
PM S nature ofthe observed stars. The H� em ission hasbeen used asfurtherindicatorof
youth,though itspresencealonedoesnotguaranteethePM S natureoftheobserved object.
Therefore,alltheK and M starswith H� in em ission butlackingtheLiiabsorption linehave
been rejected and classi�ed asolderdKeand dM e�eld starsprojected onto L1615/L1616.
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In addition to the above criteria,the presence offorbidden em ission lines,as wellas
non-photosphericUV and infrared continuum excesses(asevidenceofbipolarjets,accretion
colum ns,and circum stellardisk,respectively) have yielded furtherargum entson the PM S
natureoftheyoung starsidenti�ed in L1615/L1616.
Based on these criteria,we have identi�ed 25 new PM S starswith spectraltype later
thanaboutK1.ByaddingthesesourcestothepreviouslyreportedPM Sobjects(Alcal�a etal.
2004),the num berofoptically detected young starsknown so farin L1615/L1616 risesto
56 objects4. The fullsam ple is listed in Table 2;coordinatesare those from the 2M icron
AllSky Survey (2M ASS;Cutrietal.2003)pointsourcecatalogue,which haveaprecision of
about0:001. Allthe PM S starsrevealed by ourspectroscopic follow-up,butnotdetected in
previousX-rayand H� surveys,havebeen designated with \TTS"followed by theirposition,
according to theIAU convention.By adding them illim eterClass-0 protostardiscovered by
Stankeetal.(2002),which isnotreported in Table2,theyoung population ofL1615/L1616
increasesto57.Thus,ourspectroscopicsurvey alm osthasdoubled thenum berofcon�rm ed
PM S objectsassociated to L1615/L1616.Som eexam plesofFORS2and VIM OS spectra are
shown in Figure2.
In total,4 out ofthe 8 X-ray em itting sources listed by Alcal�a etal.(2004),nam ely
RX J0506.8�0305,RX J0507.4�0317,RX J0507.6�0318,and RX J0507.3�0326,have been
spectroscopically con�rm ed asPM S stars;theotherX-ray sources(RX J0506.5�0320,
RX J0506.4�0323,RX J0506.4�0328W ,and RX J0506.4�0328E)turned outto be �eld ac-
tive stars. Note that lithium has been detected for the �rst tim e in the spectrum ofthe
m id-infrared sourceL1616M IR4 (Stankeetal.2002).
TheS/N ratio ofthespectra ofthefaintsourcesTTS050654.5�032046 and
TTS050730.9�031846 turned outto be insu�cientfora reliable m easurem entoftheirLii
equivalent width. Nevertheless,the presence ofa broad and strong H� em ission in both
spectra (Table 4) can not be explained by chrom ospheric activity only and is typicalof
classicalT Tauristars. Indeed,according to the de�nition by W hite& Basri(2003),the
upper lim it on the H� equivalent width for chrom ospheric activity,in the spectralrange
M 3{M 5.5,is40 �A.TTS050654.5�032046 and TTS050730.9�031846 havespectraltypeM 4
and M 5.5andequivalentwidth 60�A and 290�A,respectively;thus,theyareclearlyaccretors.
ThestarKisoA-097417was�rstclassi�ed asaH� em ission-lineobjectby Nakano etal.
(1995). Although Alcal�a etal.(2004)did notobtain any spectrum ofthissource,they in-
cluded itin theirlistofPM S stars. W e observed thisobjectduring the FLAM ES spectro-
4Note thatthe starK isoA-0974 17 (Nakano etal.1995;Alcal�a etal.2004)hasbeen excluded from our
listofPM S objects(seebelow).
{ 10 {
scopicrun.NeitherH� em ission norLiiabsorption linewereobserved in itsspectrum .This
could be due to a m ism atch between the H� em ission source and the opticalcounterpart
detected by Nakano etal.(1995).Thusweexcluded thisobjectfrom ourlistofPM S stars.
The optical(UB V R C IC ) and near-infrared (JH K s) m agnitudes ofthe 56 optically
con�rm ed PM S stars in L1615/L1616 is presented in Table 3. Som e ofthe objects lack
W FI,OAC,and/orSOFIphotom etry becausethey falloutsidethesky-area covered by our
survey,are saturated in our im ages or are too faint to be observed with the 91cm tele-
scope at the OAC observatory. Nevertheless,for these stars we retrieved IC m agnitudes
from theTASS M ark IV photom etricsurvey (Droegeetal.2006),IC J m agnitudesfrom the
DEep Near-Infrared Survey (DENIS;Epchtein etal.1997),and JH K s photom etry from
the 2M ASS pointsource catalogue. Both the SOFIand DENIS NIR m agnitudeswere ho-
m ogenised to the 2M ASS photom etric system using the transform ation equationsprovided
by Carpenter(2001).Fora few sources,weused theopticalphotom etry from theliterature
(Lee 1968;M undt& Bastian 1980;Tayloretal.1989;Alcal�a etal.1996;Cieslinskietal.
1997;Frasca etal.2003;Vieira etal.2003)asexplained in thefootnotesofTable3.
{ 11 {
Fig. 2.| VIM OS and FORS2 spectra of three PM S objects in L1615/L1616, nam ely
TTS050650.7-032008,TTS050733.6-032517,and RX J0507.6-0318. The spectra have been
arbitrarily norm alised to the ux at6760 �A.Foreach objectthe whole observed spectrum
isshown in the leftpanel.The spectralrangeencom passing the H� and Lii�6708 �A lines
isplotted in m oredetailin therightpanel.
{ 12 {
5. D ata analysis
5.1. H � and Liiequivalent w idths
H� and Lii �6708 �A line equivalent widths, W (H�) and W (Li), respectively, were
determ ined by directintegration ofthelinepro�les,asdescribed in Alcal�a etal.(2004)and
Spezzietal.(2008),adopting the convention that a negative equivalent width m eans an
em ission line.Them ain sourceoferroron thesem easurem entscom esfrom theuncertainty
in the placem entofthe photospheric continuum ,especially forstarslaterthan M 1,whose
spectra are strongly a�ected by m olecular absorption bands. W (H�) and W (Li) for the
L1615/L1616m em bersarereported in thethird and fourth colum nsofTable4,respectively.
According totheintensity oftheH� em ission lineand thede�nitionsby W hite& Basri
(2003),weassigned theclassical(CTTS)orweak (W TTS)T Tauristarclassi�cation to the
PM S objectsin L1615/L1616 (Table4).
Theadditionalem ission linesobserved in thespectra,including thelinesofHei�5876,
�6678.7 and �7065.2�A,and Nai�5889.9 and �5895.9�A,aswellasthe forbidden linesof
[N ii]�6583.5 �A,[O i]�6300.3 and �6363.8 �A,and [Sii]�6716.4 and �6730.8 �A,are also
reported in Table 4. Note that alm ost only those PM S stars classi�ed as CTTSs show
forbidden em ission linesin theirspectra.
5.2. Spectraltypes and e� ective tem peratures
Spectraltypeswere determ ined using a m odi�ed version ofROTFIT,a code forspec-
traltype and vsinideterm ination developed by Frasca etal.(2003,2006)underthe IDL5
environm ent. The generalidea ofthe m ethod,already adopted in Alcal�a etal.(2006)and
Spezzietal.(2008),isto recoverthespectraltypeofa starby com paring itsspectrum with
a suitable grid oftem plates,taking into accountthe am ountofextinction along the line of
sight.Since ourspectra are ux calibrated,the m ethod allowed usto geta rough estim ate
oftheextinction (A V )along thelineofsightto thestar.
Tothisaim ,alibraryofrelative uxcalibrated and extinction-corrected tem plates,with
awavelength coverageencom passing thatoftheobserved spectra,isneeded.A suitablegrid
ofinterm ediate-resolution (R � 1000� 5000)tem plateshasbeen obtained by collectingspec-
traofdwarfand giantstarsfrom thelibrariesprovided by M art��n etal.(1999),Hawley etal.
5IDL isdistributed by ITT VisualInform ation Solutions,Boulder,Colorado.
{ 13 {
(2002),LeBorgneetal.(2003),Valdesetal.(2004),and Bochanskietal.(2007).
In perform ingthespectraltypeclassi�cation,we�rsttook intoaccountthePM S nature
ofthestarsweweredealingwith.DuetothepresenceofTiO andVO absorptionbandswhich
arestronglysensitivetothesurfacegravity (Torres-Dodgen & W eaver1993),m ostofM -type
PM S objectsshow opticalspectroscopic featuresthatcan be closely reproduced averaging
dwarfand giant spectra with the sam e spectraltype (Luhm an 1999;Luhm an etal.2003;
Guieu etal.2006).Though thespectra ofG-typeand K-typePM S starshavebeen usually
com pared with tem plateofnorm aldwarfstarsin severalworks(e.g.Basri& Batalha 1990),
we found that also the late K-type objects in our sam ple are best represented by using
an average ofdwarfand giant tem plates ofthe sam e spectraltype. Indeed TiO and VO
absorption m olecular bands are stillpresent in late K-type stars,although they are very
weak.
Thus,both late-type dwarfand gianttem plateswere included in ourgrid ofreference
spectra;m oreover,forspectraltypeslaterorequalthan K5,wealsobuiltup an adhocgrid of
tem platesby averagingdwarfand giantspectraofthesam espectralsub-class.Furtherm ore,
starting from spectraltypeslaterthan K7,weconstructed tem platesforthehalfsub-classes
(e.g.K8.5,M 0.5,M 1.5,etc.) by averaging thetwo contiguousspectra.Indeed,theintensity
ofthe absorption m olecular bands characterising late-type stars displays a rather strong
variation from one spectralsub-classto theother,allowing to appreciate di�erencesofhalf
a sub-class.Som edetailson thespectra �tting procedureisprovided in Appendix A.
ThespectraltypesofthePM S objectsin oursam plearereported in thesecond colum n
ofTable 4. From an inspection ofthe residualsofthe �tting procedure we have estim ated
an accuracy ofabout� 1 sub-classforstarsearlierthan K7 and about� 0:5 sub-classfor
cooler objects. A typicaluncertainty on A V of� 0:2 m ag was found. Forallthe objects
observed atleasttwice,with di�erentspectrographsand/orresolving powers,we obtained
consistent results,regardless ofspectralresolution and wavelength coverage. Forthe sake
ofhom ogeneity, we re-determ ined the spectraltypes also for the previously known low-
m assm em bers,by using thespectra from Alcal�a etal.(2004).A generalagreem entwithin
one/two spectralsub-classes wasfound between ourclassi�cation and the one reported in
theliterature(Cohen & Kuhi1979;Alcal�a etal.2004).
Forthetwoearly-typestarsHD 293815andKisoA-097415,wehaveadoptedthespectral
typesfrom Sharpless(1952)and Vieira etal.(2003),respectively.Both starswere spectro-
scopically observed byus,butm any photosphericfeaturessuitableforan accurateearly-type
classi�cation were notcovered in ourspectra. Nevertheless,we attem pted a spectraltype
estim ateforboth starsby applyingtheHern�andezetal.(2004)’scriteria.Them ain features
weused aretheHei�5876 and �7065.2�A absorption lines.Theresultsareconsistentwith
{ 14 {
thosereported by Sharpless(1952)and Vieira etal.(2003).
In linewith ourspectraltypeclassi�cation,thee�ectivetem perature(Te�)ofeach PM S
starwas assigned on the basis ofthe following criteria: the dwarftem perature scale from
thecom pilation ofKenyon & Hartm ann (1995)wasadopted foralltheobjectswith spectral
type earlierorequalto K4;forlaterspectraltypesan interm ediate tem perature scale was
derived byaveragingthegiantcom pilation provided byvan Belleetal.(1999)with thedwarf
onefrom Kenyon & Hartm ann (1995)forK0-M 0 and from Leggettetal.(1996)forM 1-M 7
spectraltypes,respectively. The Te� values corresponding to the halfspectralsub-classes
were com puted by linear interpolation. The e�ective tem peratures assigned to each PM S
object are reported in the second colum n ofTable 5. In assessing the errors on Te� the
uncertainty on thespectraltypeclassi�cation wastaken into account.
5.3. Extinction,stellar radii,and lum inosities
The m ethod we used for com puting extinction,stellar radii,and lum inosities follows
the generalguidelines described in Rom aniello etal.(2002,2006),although we m odi�ed
their schem e to m atch our purposes. The eight broad-band m agnitudes available for the
L1615/L1616PM S populationallowed ustoconstructthespectralenergydistribution (SED)
ofeach PM S starcovering a wide spectralrange,from opticalto near-infrared wavelengths
(seeTable6).Byusingsim ultaneously allthephotosphericcoloursencom passed bytheSED,
wecom puted foreach objectboth theinterstellarextinction (A V )and theratio oftotal-to-
selective extinction (R V = A V =E B�V ),as wellas the stellar radius (R ?) and lum inosity
(L?).
Apart for eventualultraviolet and infrared excesses caused by the presence ofan in-
teracting accretion disk around the PM S star and by the related boundary layer,or hot
accretion spots,the shape ofthe observed SED ism ostly determ ined by the e�ective tem -
peratureoftheobject,theam ountofdustalong thelineofsight,and thedust-grain m ean
size in the interstellar/cicum stellarenvironm ent. Therefore,once the e�ective tem perature
is known,both the value ofA V and R V m ay be recovered sim ultaneously by �tting the
observed SED with a grid oftheoreticalones. Indeed,both the star radius and distance
causejusta rigid shiftofthe ux on a logarithm icscale,withouta�ecting theshapeofthe
SED.TheoreticalSEDsm ay beobtained by using stellaratm ospherem odelswith thesam e
e�ective tem peratureasthestarand reddened by variousam ountofA V and R V .
A detailed description oftheSED �tting procedureisreported in Appendix B.Herewe
wantto stressthat,ourtwo-param eter�tting procedurecan beapplied only to theobjects
{ 15 {
which aresigni�cantly a�ected by extinction,i.etothestarswhich arestilldeeply em bedded
in theirparentcloud.Indeed,forslightly reddened starsthevalueofA V isrelatively insensi-
tiveto changesin theslopeofthereddening law (R V ),asshown in Appendix B.Therefore,
foralltheobjectsforwhich A V turned outto belessthan about0.5 m ag wereiterated the
�tting procedure�xing R V to thestandard valueof3.1.Thesam eassum ption wasadopted
foralltheobjectslacking opticalphotom etry and fortheveiled starTTS050649.8�032104.
ThevaluesofA V ,R V ,R ?,and L? arelisted in Table5.Foralltheobjectsspectroscop-
ically observed in this work,we found thatthe value ofA V derived from the SED �tting
procedureisin agreem ent,within � 0:2 m ag,with theoneobtained from thespectraltype
classi�cation.
R V wasderived for15 outofthe 56 PM S starsin L1615/L1616 (Table 5). W e found
a weighted m ean value of3:32� 0:04,closely m atching the standard one of3.1,typicalof
the di�use interstellarm edium . Forthe Herbig starKisoA-0974 15 we obtained a value of
R V = 5:5 (see Table 5). This m ay be related to the presence ofdust grains around the
interm ediate-m ass starwhich are largerin size than those characteristic ofthe interstellar
m edium ,asalready found by di�erentauthorsin large sam plesofHAeBe stars(Th�eetal.
1981;Herbstetal.1982;W aters& W aelkenset 1998;W hittetetal.2001;Hern�andezetal.
2004).
5.4. M asses and A ges
The m ass and age of each m em ber of L1615/L1616 have been derived by com par-
ing the location ofthe object on the H-R diagram with the theoreticalPM S evolutionary
tracksby Bara�eetal.(1998)& Chabrieretal.(2000),D’Antona & M azzitelli(1997),and
Palla & Stahler(1999)(Figure3).
Since stellar evolutionary m odels are stillrather uncertain, particularly in the low-
m ass star and sub-stellar regim es (M < 0:5M � ,Bara�eetal. 2002),the use ofdi�erent
evolutionary tracks allowed us to estim ate the m odel-dependent uncertainties associated
with thederived stellarparam eters.
Table 7 lists m asses and ages ofthe L1615/L1616 m em bers as inferred by the three
setsofm odels.Notethattheevolutionary tracksby Bara�eetal.(1998)& Chabrieretal.
(2000),D’Antona & M azzitelli(1997),and Palla & Stahler(1999)areavailablein them ass
range0:003� M � 1:40 M � ,0:017� M � 3 M � ,and 0:1� M � 6 M � ,respectively.Note
also thatisochronesby Bara�eetal.(1998)& Chabrieretal.(2000)are available only for
agesgreaterorequalthan 1 M yr.
{ 16 {
Fig. 3.| H-R diagram for the young population in L1615/L1616;the PM S evolutionary
tracksbyBara�eetal.(1998)& Chabrieretal.(2000)(upperpanel),D’Antona & M azzitelli
(1997)(centralpanel),and Palla & Stahler(1999)(lowerpanel)areover-plotted.The H-R
diagram fortheinterm ediate-m assm em bersofL1615/L1616,nam elyHD 293815and KisoA-
0974 15,isshown in theinsetofthelowerpanel.
{ 17 {
6. D iscussion
Once the physicalparam eters ofthe PM S objects are known,we can study the star
form ationin L1615/L1616.In thenextsub-sectionswediscussthehistory,rateand e�ciency
ofstarform ation,and som eissuesrelated to them ass-spectrum .
6.1. T he star form ation history
{ 18 {
Fig.4.| IRAS 100�m dustem ission m ap coveringasky-areaof5�� 5� around L1615/L1616
(left panel). The Classicaland W eak T Tauristars are represented with �lled dots and
squares,respectively. The upperarrow showsthe direction to the Orion OB1 associations,
located atabout7.5� (� 60 pcata distance of450 pc)to the North-EastofL1615/L1616.
The lowerarrow pointstoward the Orion A GiantM olecularCloud,also located atabout
7.5� to the South-EastofL1615/L1616. The dashed square de�nesthe area surveyed with
W FI.Thisregion iszoom ed in the rightpanel,where the two interm ediate-m assm em bers
HD 293815 and KisoA-0974 15 arem arked with �ve-pointed starsym bols.
{ 19 {
Based on them ostcom pletecensusoftheL1615/L1616’spopulation perform ed in this
work,we further investigated the triggered star form ation scenario suggested by Ram esh
(1995),Stankeetal.(2002),and Alcal�a etal.(2004).
In left panelofFigure 4 the IRAS 100 �m dust em ission m ap ofa 5� � 5� sky-area
around L1615/L1616 isshown.About64% oftheCTTSsareclustered,in thedensestpart
ofL1615/L1616,i.e.within the boundaries ofthe NGC 1788 re ection nebula and to the
W estofthebrightrim ofthecloud (Figure4,rightpanel).Such rim islocated totheEastof
NGC 1788,at� 6:50from thehead ofthecom etary cloud (about0.85pcatadistanceof450
pc)and itisdirectly exposed to the UV radiation from the Orion OB stars. On the other
hand,the W TTSsare m ore scattered (only � 22% isprojected on NGC 1788)and m ainly
occupy thesideofthecloud facing theOB1 associationsand to theEastofthebrightrim .
By applying the Kolm ogorov-Sm irnov testto the W TTS and CTTS age distributions
wefound thatthetwopopulationsaresim ilaratacon�dencelevelof� 80% .Thus,itseem s
there are no age di�erences between CTTSs and W TTSsin ourPM S sam ple. Thisresult
isconsistentwith previousPM S populationsstudies(see Feigelson & M ontm erle 1999,and
referencetherein);CTTSsarepredicted tohaveagesbetween about0.5and 3M yr,although
som e starsretain CTTS characteristicseven atagesasold as20 M yr. On the otherhand,
m any W TTSsoccupy the sam e region on the H-R diagram asCTTSsdo,whereassom e of
them areapproaching thezero-agem ain sequence.
The age distribution ofthe L1615/L1616 population peaksbetween 1 and 3 M yr,de-
pending on the adopted evolutionary tracks(Figure 5,rightpanels). Thisisin agreem ent
with the �ndings by Alcal�a etal.(2004),but the age spread found by us is signi�cantly
higherthan whatthese authorsclaim ,exceeding thevalueexpected on thebasisoftheun-
certaintieson lum inosity and tem perature. The L1615/L1616 m em bersspan a wide range
in age,from lessthan 0.1 M yrup to about30 M yr. Thism ightsuggestm ultiple eventsof
star form ation in the cloud,which would further support the hypothesis oftriggered star
form ation.
In ordertoinvestigateapossibleagedi�erencebetween \on-cloud"and \o�-cloud"PM S
objects,we divided the sam ple in two groups,�xing asdividing line the brightrim ofthe
cloud.Twenty of56 objectsin oursam ple arelocated within theboundariesofNGC 1788,
i.e.to theW estofthebrightrim ,while36 areo�-cloud m em bers.Theagedistributionsof
thetwogroupsareshown in Figure6.Theon-cloud PM S starsarestatistically youngerthan
those located to the Eastofthe brightrim ,regardlessofthe adopted evolutionary tracks.
Applying a Kolm ogorov-Sm irnov test we found that the probability thatthe two sets are
sub-sam pleofthesam estatisticalpopulation isvery low;in particular,wefound con�dence
levels of1.29,0.18,and 0.19% when using the PM S evolutionary tracks by Bara�eetal.
{ 20 {
(1998)& Chabrieretal.(2000),D’Antona & M azzitelli(1997),and Palla & Stahler(1999),
respectively.W ethusconcluded thatthereisa clearagedi�erencebetween thetwo groups
ofstars.
Theabove�ndingsfurthersupportthescenariooftriggeredstarform ationinL1615/L1616,
as proposed by Stankeetal.(2002). In this context,the spatialdispersion and older age
ofthe \o�-cloud" m em berscan be explained asa consequence ofthe \rocketacceleration"
e�ect. As recently pointed out by Kun etal.(2004),this acceleration continues after the
onset ofstar form ation and the parentalcloud is further accelerated with respect to the
newly form ed objects. As a consequence, the cloud is soon swept o� the newly form ed
stars. This hypothesis,together with the rapid dispersion typicalofsm allclouds,causes
the spatialdisplacem ent ofthe oldest cloud m em bers. The conspicuous num ber ofPM S
starsfound apparently isolated from classicalstarform ing regions(e.g.Alcal�a etal.1995;
Covino etal.1997;Guilloutetal.1998a,b;Frasca etal.2003;Zickgrafetal.2005)m ightbe
a consequence ofthism echanism aswell.
{ 21 {
Fig. 5.| M ass (left panel) and age (right panel) distributions ofthe L1615/L1616 PM S
population derived byusingtheevolutionarytracksbyBara�eetal.(1998)& Chabrieretal.
(2000) (upper panels),D’Antona & M azzitelli(1997) (centralpanels) and Palla & Stahler
(1999)(lowerpanels).
Fig. 6.| Age distributions ofthe on-cloud and o�-cloud PM S stars in L1615/L1616 de-
rived by using theevolutionary tracksby Bara�eetal.(1998)& Chabrieretal.(2000)(up-
perpanel),D’Antona & M azzitelli(1997)(centralpanel)and Palla & Stahler(1999)(lower
panel).
{ 23 {
6.2. T he star form ation e� ciency
Based on 12CO and 13CO colum n density m aps,Ram esh (1995)estim ated thatthem ass
ofL1616 aloneisin therange169� 193 M � ,depending on theused tracer.Yonekura etal.
(1999)m apped both L1615 and L1616 in theCO J=1-0 transition line.They inferred that
the m assofthe cloud system asa whole is� 530 M � ,� 350 M � and � 440 M � based on12CO,13CO and 18CO observationsrespectively.They alsoderived them assofthe13CO and18CO cores(i.e.L1616only)�nding avalueof� 146M � and � 161M � ,in good agreem ent
with theRam esh (1995)’sdeterm ination.
Alcal�a etal.(2004) estim ated the SFE in L1616 to be � 14% ,i.e. higher than the
average value m easured in otherlow-m assstarform ing region (. 3% ). The SFE reported
by Alcal�a etal.(2004) for L1616 is based on the cloud m ass reported by Ram esh (1995)
and the totalstellarm assofthe 32 m em bersofthe cloud investigated by the authors(i.e.
� 30 M � ).
Since the starform ation history ofL1615 and L1616 isintim ately connected,we have
re-calculated the SFE considering the system L1615 plus L1616 asa whole. By using the
NICER colorexcess m ethod by Lom bardi& Alves (2001),we m apped the dustextinction
ofthecloud com plex and derived itsm ass.In theNICER techniquetheJ � H and H � K s
coloursofthe�eld starsarecom pared tothecoloursofstarsin anearby reference�eld.The
colour-excessesofthe�eld starsarethen com bined and transform ed to thevisualextinction
A V ,�xing the form ofthe reddening curve. The norm alinterstellarextinction law derived
by (Cardellietal.1989)hasbeen adopted by usforthispurpose.W eretrieved the2M ASS
colours ofallthe point-like sources located in a 500� 500 region centered on NGC 1788
and in a reference �eld close to the L1615/L1616 cloud6. The latter�eld was selected by
using the nearby low-intensity regions ofthe IRAS 100�m dust em ission m ap,according
to prescription by Kainulainen etal.(2006). Finally,the visualextinction m ap resulting
from theNICER m ethod wasconverted tocloud m assby assum ing thestandard gas-to-dust
ratio (Bohlin etal.1978).W efound a valueof� 550 M � ,in good agreem entwith the one
inferred by Yonekura etal.(1999)on the basisof12CO observations. Based on thisvalue
and on thetotalm assofthe56 present-day known m em bersofthecom plex (i.e.41{46M � ,
depending on theadopted evolutionary tracks),wederived a SFE in L1615/L1616of7{8% ,
i.e. signi�cantly lowerthan the previousestim ate by Alcal�a etal.(2004),butstillin good
agreem entwith the one generally found in giantm olecularcloudshosting OB associations
(5{10% )and predicted by theoreticalcalculationson theform ation ofOB associations(see
6TheSO FIsurvey isonly centered on thedensestpartofthecloud and can notbeused to m ap thecloud
asa whole.
{ 24 {
Clark etal.2005,and referencetherein).
6.3. D ensity ofstar form ation and star form ation rate
An interesting question is whether L1615/L1615 can be considered as a cluster. Ac-
cording to the de�nition suggested by Lada & Lada (2003),a cluster is a group ofsom e
35 m em bers with a totalm ass density larger than 1.0 M � pc�3 . To estim ate the density
ofPM S objectsin L1615/L1615,we considered the region spectroscopically surveyed by us
(Figure 1). This region covers an area ofapproxim ately 0.25 square degrees and includes
about40 PM S objects. Assum ing a distance of450 pc,the resulting area is about4 pc2,
which m eans a surface density ofabout10-11 PM S objects perpc2 and a volum e density
on the orderof7 PM S objectsperpc3. In the lattercalculation we estim ated the volum e
asV = 0:752�Area1:5 (see J�rgensen etal.2007),assum ing a locally sphericaldistribution
ofsources. The average m ass ofthe 40 PM S objects,as determ ined from the results in
Section 5.4,ison theorderof0.7M � ,which im pliesa volum edensity ofabout4-5M � pc�3 .
Therefore,thegroup of40 PM S objectscon�ned in thespectroscopically surveyed area can
beconsidered asa clusteraccording to thecriterion by Lada & Lada (2003).
Now,itisinteresting to estim ate the rate atwhich the starsin thissm allcluster are
form ed. According to the resultsofSection 5.4,we estim ated thatthe totalm assin PM S
objectsin thatregion ison the orderof41{46 M � . Therefore,considering the average age
of2 M yrfortheseobjects(seeFigure5),wefound thatthecom etary cloud isturning som e
20-23 M � into PM S objectsevery M yr,which islowerthan thestarform ation ratein other
clusterslike those in Serpens(Harvey etal.2007),buthigherthan in otherT associations
like Cham aeleon II(Alcal�a etal.2007)and Lupus(M er��n etal.2007). Thuswe concluded
thatL1615/L1616 isa sm allclusterwith a m oderatestarform ation rate.
6.4. T he InitialM ass Function
Though based on a low-num ber statistics,the �rst clue on the InitialM ass Function
in L1615/L1616 was given by Alcal�a etal.(2004). Based on this study,the IM F in this
region appearsroughly consistent with thatofthe solarneighbourhood in the m assrange
0:3 < M < 2:5 M � (M iller& Scalo 1979). The authors also found severalcandidates for
young BDsand estim ated theexpected num berofBDsrelativeto m orem assivePM S stars
in L1615/L1616 to be interm ediate between Taurus(� 13% ,Brice~no etal.2002)and the
Trapezium cluster(� 26% ,Luhm an etal.2000).
{ 25 {
The depth of the spectroscopic survey conducted in this work goes down to IC �
19:0 m ag (i.e. M � 0:03� 0:05 M � ,see Section 6.5),leading to the discovery ofyoung
objects with a m ass close to the Hydrogen burning lim it. W e have thus attem pted a fair
IM F determ ination down to thelow-m assstellarregim e(i.e.M & 0:1 M � ).
Allthepresent-day known m em bersofL1615/L1616 havem assesin therangebetween
�0.1and 5.5M � ;theirm ean m assvariesbetween 0.6and 0.8M � depending on theadopted
evolutionary tracks (see Table 7 and Fig 5,left panels). However, although our survey
recovers m ost ofthe previously known young objects in L1615/L1616,the com pleteness
ofour PM S sam ple is di�erent from the one of Alcal�a etal.(2004) sam ple. The latter
includesbrightand solar-likeobjectslocated in areasnotcovered by thedeeperW FIsurvey.
Therefore,we restriced ourIM F analysisto the sam ple ofobjectsthatlie on the sky-area
covered with W FI(Figure4,rightpanel).
The shape ofthe IM F in the cloud m ay then be constrained using the usualapproxi-
m ation forthe low-m ass function (dNdM
/ M �� ),asreported by (M oraux etal.2003). W e
divided them assrangeintom assbinsofabout0:2M � ;thisvalueislargerthan theaccuracy
on m assestim atesasderived from theuncertaintieson tem peraturesand lum inositiesand,
atthesam etim e,allowsusto havea statistically signi�cantnum berofobjectsin each bin.
{ 26 {
Fig.7.| TheL1615/L1616IM F between 0.1and5.5M � asderived byusingtheevolutionary
tracksbyBara�eetal.(1998)& Chabrieretal.(2000)(upperpanel),D’Antona & M azzitelli
(1997)(centralpanel)and Palla & Stahler(1999)(lowerpanel).
{ 27 {
Figure7showstheIM F in L1615/L1616asobtained byusingthethreesetsofevolution-
ary tracksby Bara�eetal.(1998)& Chabrieretal.(2000),D’Antona & M azzitelli(1997)
and Palla & Stahler(1999). Valuesof� in the three casesagree within the errorsand we
can derivea weighted m ean valueof:
� = 0:84� 0:07
asthebestapproxim ationoftheIM F slopeinL1615/L1616inthem assrange0:1�M � 5:5M � .
W enotice,however,thattheevolutionary m odelsby D’Antona & M azzitelli(1997)tend to
produceasteeperIM F with respecttothosebyBara�eetal.(1998)& Chabrieretal.(2000)
and Palla & Stahler(1999),asalready found by Spezzietal.(2008).
In Table 8 the � slope obtained forthe IM F in L1615/L1616 iscom pared with those
obtained in other star form ing regions with di�erent ages and environm entalconditions.
Thesevaluesindicateacom m on shapeofIM F in them assrangefrom � 0:1to� 1M � .This
providesfurtherevidenceofauniversalIM F alsoin thelow-m assregim eand isin agreem ent
with theresultsalready established fortheinterm ediateand m assivestardom ains(Kroupa
2002).
6.5. N o sub-stellar objects in L1615/L1616?
Priorto oursurvey,no con�rm ed BDsin L1615/L1616 werereported in theliterature.
However,Alcal�a etal.(2004) photom etrically selected som e 30{40 candidates for BDs in
this region and estim ated a fraction ofsub-stellar objects relative to PM S stars (R SS) in
the range from 18% to 25% ,i.e. sim ilar to the value m easured in the Trapezium cluster
(Hillenbrand & Carpenter 2000;Brice~no etal.2002;M uench etal.2002)and in otherOB
associations.
Our spectroscopic observations,which investigated about 50% ofthe BD candidates
with IC . 19:0 m ag reported by Alcal�a etal.(2004),revealed 3{4 young objectswith m ass
M � 0:1 M � (seeTable7),closeto theHydrogen burning lim it,and no objectswith lower
m ass.However,takingintoaccounttheaccuracy with which weestim ated them asses,these
objectscould be BDswith a m assjustbelow the Hydrogen burning lim it. The R SS in the
cloud would be then around 5{7% . Though the very low-m ass BDs(M . 0:03 M � )and
som e deeply em bedded low-m ass objects m ight have escaped detection in oursurvey,this
value should be slightly lower than the one estim ated for other T associations (12{15% ;
Brice~no etal.2002;L�opezM art�� etal.2004).
The photom etric survey conducted by usin L1615/L1616 isspatially and photom etri-
cally com plete down to IC � 21:5 m ag at10� level. However,only objects brighterthan
{ 28 {
IC � 19:0 m ag could be observed spectroscopically with a S/N ratio su�cientforspectral
type classi�cation purpose. A 1{3 M yrold object(i.e. the typicalage ofthe L1615/L1616
population) at the distance of450 pc (i.e. a distance m odulus of8.27 m ag) with a m ass
M & 0:03 M � would have IC . 19:0 m ag in theabsence ofextinction (Bara�eetal.1998).
Thus,in the o�-cloud regionsofL1615/L1616,where A V � 0 m ag,oursurvey issensitive
down to � 0:03 M � . The visualextinction occurring in the dense head ofthe cloud varies
in the range 1{5 m ag which displacesthe lim iting m assofthe objectswith IC . 19:0 m ag
to 0.05{0.50 M � (Bara�eetal.1998). W e conclude thatwe are probably m issing the very
low-m assobjectsonly in thesm alldensecoreofthecloud.Given thepeculiarstarform ation
history in thiscloud (Section 6.1),theknowledgeofitssub-stellarm assspectrum could give
crucialinsightsinto theprocessform ing BDsunderparticularenvironm entalconditions.
{ 29 {
Fig.8.| IC versus(R C � IC )diagram forthepoint-likeobjectsin L1615/L1616detected in
ourW FIsurvey above3� level(grey littledots).ThetheoreticalisochronesbyBara�eetal.
(1998) & Chabrieretal.(2000) forthe Cousins photom etric system ofBessel(continuous
line)and fortheW FI-Cousinssystem (dashed lines,Spezzietal.2007)areoverplotted.The
PM S stars with both R C -and IC -band m easurem ents (see Table 3) are overplotted with
black big dots.TheA V = 2 m agreddening vectorisalso shown.Thecon�rm ed PM S object
falling wellbelow the10 M yrisochroneistheveiled starTTS050649.8�032104
.
{ 30 {
The BD candidates sam ple by Alcal�a etal.(2004) was selected by inspection ofIC
versus (R C � IC )colour-m agnitude diagram in com parison with the theoreticalisochrones
by Bara�eetal.(1998)& Chabrieretal.(2000).Thisapproach to search forPM S starand
young BD candidatesisthem ostdirectway totackletheproblem using opticalphotom etric
data;however,inordertoselectclusterm em bersonthebasisoftheirm agnitudesandcolours,
isochronesthatadequately m odelthe data m ustbe used,since the shape ofthe isochrones
m ay vary signi�cantly from onephotom etric system to another.The theoreticalisochrones
forlow-m ass starsand BDsare provided by Bara�eetal.(1998)& Chabrieretal.(2000)
in the Cousinsphotom etric system (Cousins1976),while the observationsby Alcal�a etal.
(2004)wereperform ed byusingtheW FIcam eraattheESO 2.2m telescope.TheW FI�lters
transm ission curvesaresom ewhatdi�erentfrom theoriginalCousinsones,in particularfor
the I-band. Spezzietal.(2007) have com puted ad hoc isochrones for the W FI-Cousins
system . In Figure 8 these isochrones are com pared with those by Bara�eetal.(1998) &
Chabrieretal.(2000).Thetwosetsofisochronesappearsigni�cantly di�erent,in particular
forthecoolestobjects,i.e.(R C � IC )> 1:7,wheretheuseoftheisochronesin theCousins
system would produced m any spurious m em ber candidates. This consideration,together
with thecontam ination bybackground/foreground starsinthe�eld taken intoaccountbythe
authors,could explain why Alcal�a etal.(2004)overestim ated thenum berofBD candidates.
TheuseoftheadequateisochronesfortheW FI-Cousinssystem would producem uch lessBD
candidates,in agreem entwith thenum berofPM S objectswith m asscloseto theHydrogen
burning lim itcon�rm ed by ourspectroscopicfollow-up (seeTable 7).
Though theR SS estim ated by usforL1615/L1616needsfurtherre�nem ents,theregion
seem stobepoorofBDs,withaR SS ratioclosetothatobserved inT associationsor,perhaps,
even lower.Though in L1615/L1616 theradiation from them assivestarsin theOrion OB1
associationsislikely responsibleforthestarform ation (Section 6.1),thisprim ordialprocess
does not seem s to favour the form ation ofBDs. As reported in Alcal�a etal.(2004),the
radialvelocity (RV)distribution ofthe starsin L1615/L1616 showsa wellde�ned peak at
22:3Km /swith thestandard deviation of4:6Km /swhich isconsistentwith theaverageRV
error(i.e. � 5 Km /s). Thism ay indicate thatthe velocity dispersion ofthe starsm ustbe
lessthan 5 Km /s.Considering a velocity dispersion ofa few Km /s,2 M yrold objectswould
disperseoveradistanceofabout2pc,which istheapproxim ateprojected sizeofthehead of
thecom etary cloud and hasbeen com pletely covered by ourphotom etricand spectroscopic
observations.
Thus,the results ofour study in L1615/L1616 does not play in favour ofdynam ical
ejection orphotoevaporation byionisingradiation from m assivestarsbeingtriggeringfactors
oftheBDsform ation m echanism .
{ 31 {
7. Sum m ary
W ehaveundertaken astudy ofthestarform ation processin theL1615/L1616com etary
cloud,a sm allstarform ing region located in theoutskirtoftheOrion OB associations.W e
aim ed atcharacterising itsPM S population and studying both itsIM F and starform ation
history,in orderto assesstheroleofthetriggered starform ation scenario.
Our study is based on opticaland near-infrared photom etric observations,as wellas
m ulti-objectfollow-up spectroscopy,both carried outusing ESO and OAC facilities. Com -
plem entary JH K sphotom etricdata from the2M ASS and DENIS surveyshasbeen used as
well.
Oneofthem ajorgoalofthiswork hasbeen thephysicalparam etrisation oftheyoung
stellarpopulation in L1615/L1616. The spectraltype classi�cation hasbeen perform ed by
using a grid ofreference spectra ofgiants,dwarfs,and interm ediate tem plates,constructed
by averaging spectra ofgiantand dwarfstarsofthesam espectraltype.W ehavecom puted
stellar lum inosities by m eans ofa SED �tting procedure properly developed by us,which
also evaluatesthe interstellar/circum stellar reddening. Thisallowed usto derive the m ass
and ageofeach m em berofL1615/L1616by com paringthelocation oftheobjecton theH-R
diagram with di�erentsetsoftheoreticalPM S evolutionary tracks.
Ouranalysisoftheyoung population in L1615/L1616 yielded thefollowing results:
� By using theH� em ission lineintensity,aswellasthestrength ofthelithium �6708�A
absorption lineasm ain diagnosticsofthePM S natureoftheobjects,weidenti�ed 25
new m em bersofL1615/L1616,alm ostdoublingthenum berofpreviously known young
objectsin thiscloud.
� The age distribution ofthe L1615/L1616 population peaks between 1 and 3 M yr;
however,the m em bers ofthe cloud span a wide range in age,from �0.1 M yr up to
30 M yr,suggesting m ultipleeventsofstarform ation.
� Theevidenceofm ultiplestarform ation events,thespatialdistribution oftheclassical
and weak T Tauripopulations,aswellasthe cloud shape,supportthe hypothesisof
thestarform ation in L1615/L1616 being triggered by them assivestarsin thenearby
Orion OB associations.
� TheslopeoftheIM F in L1615/L1616in them assrange0:1�M � 5:5M� is0:84� 0:07,
i.e. sim ilarto thatobtained in otherstarform ing regionsby di�erentauthors. This
provides further support ofa universalIM F in the stellar dom ain,regardless ofthe
environm entalconditions.
{ 32 {
� The star form ation e�ciency is about 7{8% as expected for m olecular clouds in
the vicinity ofOB associations. According to the criterion by Lada & Lada (2003),
L1615/L1616 isa sm allclusterwith m oderatestarform ation.
� A very low-fraction ofpossiblesub-stellarobjects(� 5{7% )isfound in L1615/L1616;
thus,thedynam icalinteraction with theclosem assive starsin theOrion OB associa-
tionsand/orthe photoevaporation induced by theirionising radiation do notappear
ase�cienttriggering factorsforBD form ation.
W e thank the anonym ous referee for his/her carefulreading, usefulcom m ents, and
suggestionswhich helped to im prove the m anuscript. Thispaperisbased on observations
carried out at the European Southern Observatory,La Silla and Paranal(Chile), under
observing program snum bers64.I-0355,70.C-0629,70.C-0536,074.C-0111 and 076.C-0385.
The authors are gratefulto M enadora Barcellona and Salvatore Spezzifor their supports
during the preparation ofthispaper. W e thank J.Hernandez forproviding the calibration
curves for spectralclassi�cation,and for discussions concerning the spectralclassi�cation
ofearly-type stars. W e are also gratefulto M atula forherassiduous and warm assistance
during the writing ofthis m anuscript. This work was partially �nanced by the Istituto
Nazionale diAstro�sica (INAF) through PRIN-INAF-2005. D.G.acknowledges �nancial
supportfrom PRIN-INAF-2005(\Stellarclusters:abenchm arkforstarform ation and stellar
evolution"). This publication m akes use ofdata products from the Two-M icron All-Sky
Survey,which isajointprojectoftheUniversityofM assachusettsandtheInfraredProcessing
and AnalysisCenter/California InstituteofTechnology,funded by NASA and theNational
Science Foundation. This research has m ade use ofthe SIM BAD database,operated at
CDS,Strasbourg,France. This paper includes data from the DENIS project,which has
been partly funded by the SCIENCE and the HCM plans ofthe European Com m ission
undergrantsCT920791 and CT940627.
A . SpectralType C lassi� cation
In order to �nd the tem plate spectrum which better reproduces the targetspectrum ,
weproceed asfollows:
� W ehave�rstperform ed a splineinterpolation ofeach standard spectra onto thewave-
length pointsofthetargetspectrum .Thisallowed ustogetahom ogeneouswavelength
{ 33 {
grid with thesam estep astheobserved spectrum .M oreover,thespectralregionsen-
com passing both the O 3 and H 2O telluric bands near 6900,7250,and 7600 �A,have
been excluded in the�tting procedure.
� The standard spectra have been shifted in wavelength to the radialvelocity ofthe
target;a cross-correlation algorithm was used to �nd the velocity shift between the
tem platesand targetspectrum and superim pose each other.
� The extinction law derived byCardellietal.(1989)have been adopted to redden the
wavelength-shifted standard spectra before com paring them to the observed one. W e
assum ed a norm alslope ofthe reddening law,i.e. we adopted a value ofthe ratio of
total-to-selectiveextinction R V = A V =E (B � V )equalto3.1.Each standard spectrum
hasbeen progressively reddened assum ing an increasing valueoftheextinction in the
V -band (A V );weletitvary from 0 to 15 m ag,with a step of0.05 m ag.
� Aftereach reddening step,thestandard spectrum wassubtracted to theobserved one.
The sum ofthe residuals was taken as indicator for the goodness ofthe \�t". The
reddened standard spectrum which gavethelowestvalueofthisindicatorprovided the
spectraltypeofthetargetaswellasa �rstestim ation oftheextinction valueA V .
As an exam ple,the FORS2 spectra oftwo PM S stars,nam ely RX J0507.6�0318 and
RX J0507.4�0320,as wellas the low-and the interm ediate-resolution VIM OS spectra of
TTS050752.0�032003 are shown in Figure 9 and Figure 10,respectively;the best-�tting
tem plates(thick lines)aresuperim posed on theobserved spectra (thin lines).Notethatthe
two VIM OS spectra yielded thesam espectraltypeand aconsistentvalueofA V ,wellwithin
theerrors,regardlessofthedi�erentresolving powerand wavelength coverage.
{ 34 {
Fig. 9.| FORS2 spectra (thin lines) of RX J0507.6�0318 (upper panel) and
RX J0507.4�0320 (lower panel). Overplotted with thick lines are the best-�tting K7 and
M 4 tem plates,reddened with A V = 0:1 and 0:2 m ag,respectively,asderived by the spec-
trum �ttingprocedure.Thespectra arearbitrarily norm alised tothe ux at6500 �A.TheO 2
absorption telluricbandsnear�6900 �A arem arked.Thisspectralregion hasbeen excluded
in the �tting procedure.The Lii�6708 �A absorption and the H� em ission linesare shown
in m ore detailin the insetofeach panel. Note the di�erences in H� and Liibetween the
tem plate and the observed spectra,con�rm ing thePM S nature ofthetwo stars.Also note
the good agreem ent in the two independent determ inations ofA V ,as obtained from the
spectraltypeclassi�cation and theSED �tting procedure(Table 5).
{ 35 {
Fig.10.| Low-(upperpanel)and interm ediate-resolution (lowerpanel)VIM OS spectra of
TTS050752.0�032003. The observed spectra are displayed with thin lines,while the best-
�ttingM 5.5tem plate,reddened with A V = 0:6and 0:5m ag,respectively,isoverplotted with
a thick line. The spectra have been arbitrarily norm alised to the ux at6760 �A.The O 2
absorption telluricbandsnear�6900and �7600�A,aswellastheH 2O onenear�7250�A are
m arked.Notethatthese spectralregionshave been excluded in the �tting procedure.The
m ain TiO an VO bandsarem arked also.TheLii�6708�A absorption and theH� em ission
lines are shown in m ore detailin the inset ofthe lower panel. Note the di�erences in H�
and Liibetween thetem plate and theobserved spectra,con�rm ing thePM S natureofthe
star.Also notethegood agreem entbetween thetwo valuesofA V and theonederived from
theSED �tting procedure(Table 5).
{ 36 {
B . SED � tting procedure
Them ethod weem ployed todeterm ineboth thereddeningparam eters(i.e.A V and R V )
and thestellarintrinsicproperties(i.e.R ? and L?)ofeach PM S star,isbased on theuseof
appropriatestellaratm ospherem odels.Taking into accountthee�ectivetem peraturerange
oftheL1615/L1616m em bers(seeTable5),aswellasthewidewavelength coveragerequired
to encom pass their SEDs (see Table 6), we m erged three distinct sets of low-resolution
synthetic spectra,adopting thefollowing criteria:
� forthetwo interm ediate-m assstars,nam ely theB9V-typeHD 293815 and theHerbig
B3e-typestarKisoA-097415,wechosethegrid ofstellaratm ospherem odelscom puted
by Kurucz(1979);
� forsourcescovering the e�ective tem perature intervalbetween 4000 and 10000 K we
used the low-resolution synthetic spectra calculated by Hauschildtetal.(1999)with
theirNextGen m odel-atm ospherecode;
� forobjectscoolerthan 4000 K weused thesyntheticlow-resolution StarDusty spectra
for low-m ass stars and BDs by Allard etal.(2000). The StarDusty spectra,which
take into account dust grain form ation in the m odelatm osphere due to an e�cient
gravitationalsettling process,arethe m ostsuitable forsim ulating coolobjects,down
to the very low-m assend. Indeed,Allard etal.(2001)found thatsilicate dustgrains
can form abundantly in theouteratm osphericlayersofthelatestM dwarfsand BDs.
Each set of atm osphere m odels contains ux density spectra at the stellar surface,
com puted fordi�erent value ofgravity and m etallicity. W e kept the gravity ofthe atm o-
spherem odelsto a �xed valueoflog(g)= 4:0 (cgsunits),which isappropriateforlow-m ass
starsand BDsyoungerthan 10 M yr(Chabrieretal.2000).Furtherm ore,thisvalueclosely
m atches the one expected for HAeBe and early type m ain sequence stars (Kurucz 1979;
Schm idt-Kaler1982). W e also adopted a solarm etallicity. However,we rem ind the reader
thatRom aniello etal.(2002)have analysed the in uence ofgravity and m etallicity on the
�tting-procedureand found thatneitherofthem a�ectsthevaluesofthederived param eters
signi�cantly.
A grid ofm odels m atching the adopted e�ective tem perature scale (see Section 5.2)
was�rstderived by perform ing a linearinterpolation ofthem erged setofsyntheticspectra.
In orderto com pare the observed SEDswith theoreticalones,synthetic m agnitudesin the
sam e photom etric system s in which the observations were perform ed are required. Thus,
for each adopted tem perature we derived a look-up table ofsynthetic UB V R C IC JH K s
{ 37 {
absolutem agnitudesbyreddeningthecorrespondingtheoreticalspectrum with an increasing
value ofA V (from 0 to 15 m ag with a step of0.05 m ag)and R V (from 1 to 9 with a step
of0.1),and subsequently integrating the reddened spectrum over each photom etric pass-
band. The extinction law by Cardellietal.(1989)wasadopted to perform thisreddening
procedure. Since the m odelspectra are given as ux density at the star’s surface, the
syntheticm agnitudeswerecom puted forastar7 with onesolarradius,asseen from adistance
of10pc.TheJohnson-CousinsUB V R C IC and 2M ASS JH K stransm ission curvesfrom The
Asiago Database on Photom etric System s(M oro & M unari2000;Fiorucci& M unari2002),
aswellastheabsolute ux calibration constantsreported in Table 6 wereused to integrate
each progressively reddened theoreticalspectra.
Finally,thevaluesofA V and R V ofeach PM S star(seeTable5)werederived sim ultane-
ously by �tting thecorresponding observed SED to thelook-up tableofreddened synthetic
m agnitudeshaving thesam ee�ectivetem peratureasthegiven object.In orderto elim inate
the m agnitude shift introduced by the unknown star’s radius and distance,the observed
and the synthetic m agnitudes were previously norm alised to the IC -band. ForPM S stars
showing clearevidence ofnear-infrared excessassociated with thepresence ofcircum stellar
m aterial,the�twasperform ed to theshort-wavelength portion (� � �J)oftheSED,which
islesscontam inated by near-infrared excess. The erroron A V and R V hasbeen evaluated
taking into accountboth the accuracy on the spectraltype classi�cation (Section 5.2)and
thephotom etricerrorson theobserved m agnitudes.
An exam ple ofthe outputofthe �tting procedure isshown in the upperpanelofFig-
ure 11 ,where a countourplotofthe �2 value in the R V -A V plane isreported. Asalready
stated in Section 5.3,ourtwo-param eter�ttingprocedurecan beapplied only tostarswhich
su�era visualextinction largerthan about0.5 m ag.Forlow-reddened starsthevalueofA V
isfairly independentofchangesin R V .Asclearly evidentin thelowerpanelofFigure11,in
thiscase a unique solution could notbe found sim ultaneously in the R V -A V plane. There-
fore,the value ofR V = 3:1,typicalofthe di�use interstellarm edium ,wasassum ed forall
theobjectwith A V . 0:5 m ag.
Oncetheextinction param eterswere determ ined,thevalueofR ? foreach PM S object
wasobtained from the m agnitude shiftsrequired to m atch the observed m agnitudesto the
theoreticalones,reddened with the best-�tting A V and R V . Only the photom etric bands
actually used in the �tting procedure for each given object were em ployed to determ ine
its stellar radius. As suggested by Alcal�a etal.(2004),we m ade the assum ption that all
7This choice is only conventional,since at this step we were only interested in the shape ofthe ux
distribution.
{ 38 {
the m em bers of L1615/L1616 are located at 450 � 20 pc. Finally the value ofL? was
derived assum ing a black body em ission atthestar’se�ectivetem peratureand radius.The
associated uncertainties were derived taking into account the errors on the distance and
e�ective tem perature.
{ 39 {
Fig. 11.| �2 contour plots produced by the SED �tting procedure for the two stars
RX J0506.9�0320E and RX J0506.8�0327.
{ 40 {
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{ 45 {
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Thispreprintwasprepared with the AAS LATEX m acrosv5.2.
{46{
Table1. JournaloftheFORS2 and VIM OS spectroscopicobservations.
Spectrograph Period Grism Ordersorting �=�� � �range Texp N m ask N object
�lter (�A) (sec) perm ask
FORS2@VLT Feb{M ar2003 GRIS-1200R GG435 2140 5700{7300y 960 5 18{19
VIM OS@VLT Dec2004{M ar2005 HR red GG475 2500 6300{8700y 2000 3 � 80
VIM OS@VLT Dec2004{M ar2005 M R GG475 580 5000{8000 900 6 � 120
�Achieved at� = 6500�A with a 100slitwidth.
yCorresponding to a slitprojected onto thecentreofthedetector.
{ 47 {
Table2. PM S starsin L1615/L1616.Coordinatesarefrom the2M ASS catalogue.The
PM S objectsidenti�ed in thiswork and thepreviously known onesarelabelled with
\NEW " and \P.K.",respectively.Foreach star,additionaldesignationsfound in literature
arelisted in thelastcolum n.
Star �(2000) �(2000) Note O therDesignations
(hh:m m :ss) (dd:m m :ss)
1RXS J045912.4� 033711 04:59:14.59 � 03:37:06.3 P.K .
1RXS J050416.9� 021426 05:04:15.93 � 02:14:50.5 P.K .
TTS050513.5� 034248 05:05:13.47 � 03:42:47.8 NEW
TTS050538.9� 032626 05:05:38.85 � 03:26:26.4 NEW
RX J0506.6� 0337 05:06:34.95 � 03:37:15.9 P.K .
TTS050644.4� 032913 05:06:44.42 � 03:29:12.8 NEW
TTS050646.1� 031922 05:06:46.05 � 03:19:22.4 P.K .
RX J0506.8� 0318 05:06:46.64 � 03:18:05.6 P.K .
TTS050647.5� 031910 05:06:47.45 � 03:19:09.7 NEW
RX J0506.8� 0327 05:06:48.32 � 03:27:38.2 P.K .
RX J0506.8� 0305a 05:06:48.98 � 03:05:42.9 NEW
TTS050649.8� 031933 05:06:49.77 � 03:19:33.1 NEW
TTS050649.8� 032104 05:06:49.78 � 03:21:03.6 NEW 1RXSJ050651.9� 032031
TTS050650.5� 032014 05:06:50.50 � 03:20:14.3 NEW
TTS050650.7� 032008 05:06:50.74 � 03:20:08.0 NEW
RX J0506.9� 0319NW 05:06:50.83 � 03:19:35.2 P.K .
RX J0506.9� 0319SE 05:06:50.99 � 03:19:38.0 P.K . L1616 M IR5
HD 293815 05:06:51.05 � 03:19:59.9 P.K .
RX J0506.9� 0320W 05:06:52.86 � 03:20:53.2 P.K . L1616 M IR2
RX J0506.9� 0320E 05:06:53.32 � 03:20:52.6 P.K . L1616 M IR1
TTS050654.5� 032046 05:06:54.53 � 03:20:46.0 NEW
LkH� 333 05:06:54.65 � 03:20:04.8 P.K . HBC 82,K isoA-0974 14,CSI-03-05045
L1616 M IR4b 05:06:54.93 � 03:21:12.7 P.K .
K isoA-0974 15 05:06:55.52 � 03:21:13.2 P.K . NSV 1832,IRAS 05044� 0325,L1616 M IR3
RX J0507.0� 0318 05:06:56.94 � 03:18:35.5 P.K .
TTS050657.0� 031640 05:06:56.97 � 03:16:40.4 NEW
TTS050704.7� 030241 05:07:04.71 � 03:02:41.0 NEW
TTS050705.3� 030006 05:07:05.32 � 03:00:06.2 NEW
RX J0507.1� 0321 05:07:06.10 � 03:21:28.2 P.K . K isoA-0974 16
TTS050706.2� 031703 05:07:06.22 � 03:17:02.9 NEW
RX J0507.2� 0323 05:07:10.95 � 03:23:53.4 P.K . K isoA-0974 18
TTS050713.5� 031722 05:07:13.52 � 03:17:22.1 NEW
RX J0507.3� 0326a 05:07:14.99 � 03:26:47.3 NEW
TTS050717.9� 032433 05:07:17.85 � 03:24:33.1 P.K .
RX J0507.4� 0320 05:07:22.28 � 03:20:18.5 P.K .
RX J0507.4� 0317a 05:07:25.93 � 03:17:12.3 NEW
{ 48 {
Table2| Continued
Star �(2000) �(2000) Note O therDesignations
(hh:m m :ss) (dd:m m :ss)
TTS050729.8� 031705 05:07:29.80 � 03:17:05.1 NEW
TTS050730.9� 031846 05:07:30.85 � 03:18:45.6 NEW
TTS050733.6� 032517 05:07:33.58 � 03:25:16.7 NEW
TTS050734.8� 031521 05:07:34.83 � 03:15:20.7 NEW
RX J0507.6� 0318a 05:07:37.67 � 03:18:15.6 NEW
TTS050741.0� 032253 05:07:41.00 � 03:22:53.0 NEW
TTS050741.4� 031507 05:07:41.35 � 03:15:06.7 NEW
TTS050752.0� 032003 05:07:51.95 � 03:20:02.8 NEW
TTS050801.4� 032255 05:08:01.43 � 03:22:54.5 P.K .
TTS050801.9� 031732 05:08:01.94 � 03:17:31.6 P.K .
TTS050804.0� 034052 05:08:04.00 � 03:40:51.7 P.K .
TTS050836.6� 030341 05:08:36.55 � 03:03:41.4 P.K . K isoA-0974 21
TTS050845.1� 031653 05:08:45.10 � 03:16:52.5 P.K .
RX J0509.0� 0315 05:09:00.66 � 03:15:06.6 P.K . 1RXSJ050859.6� 031503
RX J0510.1� 0427 05:10:04.60 � 04:28:03.7 P.K . 1RXSJ051004.9� 042757
1RXSJ051011.5� 025355 05:10:10.86 � 02:54:04.9 P.K . V1011 O ri,IRAS 05076� 0257
RX J0510.3� 0330 05:10:14.78 � 03:30:07.4 P.K . 1RXSJ051015.7� 033001
1RXSJ051043.2� 031627 05:10:40.50 � 03:16:41.6 P.K . RXJ0510.7� 0316
RX J0511.7� 0348 05:11:38.93 � 03:48:47.1 P.K .
RX J0512.3� 0255 05:12:20.53 � 02:55:52.3 P.K . V531 O ri,1RXSJ051219.9� 025547
aX-ray sourceby Alcal�a etal.(2004)
bM id-infrared sourceby Stankeetal.(2002)
{49{
Table3. Optical(UB V R C IC )and near-infrared (JH K s)photom etry ofthePM S starsin L1615/L1616.Labelsare
asexplained in thefootnotes.
PM S Star U B V R C IC J H K s N otes
1R X S J045912.4�033711 12.74�0.05 12.45�0.04 11.73�0.03 N O 10.72�0.06 10.07�0.02 9.62�0.02 9.47�0.02 a,b,c
1R X S J050416.9�021426 14.84�0.09 13.99�0.08 12.96�0.02 N O 11.59�0.03 10.66�0.02 10.10�0.02 9.98�0.03 a,c,d
TTS050513.5�034248 N O N D N D 18.97�0.02 17.03�0.02 15.12�0.04 14.60�0.05 14.20�0.06 c
TTS050538.9�032626 N O 18.88�0.02 17.30�0.02 16.21�0.02 14.67�0.01 13.29�0.02 12.60�0.02 12.39�0.02 c
R X J0506.6�0337 13.12�0.04 12.93�0.02 12.23�0.01 N O 11.24�0.05 10.66�0.02 10.30�0.03 10.20�0.02 a,b,c
TTS050644.4�032913 N O 19.33�0.01 17.69�0.02 16.68�0.02 15.08�0.01 13.64�0.03 13.04�0.03 12.78�0.03 c
TTS050646.1�031922 N O 18.88�0.01 17.34�0.02 16.09�0.01 14.81�0.01 12.60�0.01 11.09�0.01 10.22�0.01
R X J0506.8�0318 N O 16.26�0.01 14.86�0.02 13.91�0.01 13.01�0.01 11.95�0.01 11.22�0.01 10.97�0.01
TTS050647.5�031910 N O 22.55�0.12 20.51�0.03 19.01�0.03 16.75�0.02 14.30�0.01 13.38�0.01 13.04�0.01
R X J0506.8�0327 N O 17.59�0.01 15.96�0.02 14.90�0.02 13.30�0.01 11.74�0.01 10.92�0.01 10.59�0.01
R X J0506.8�0305 N O 18.99�0.01 17.33�0.02 16.23�0.02 14.59�0.01 13.02�0.01 12.39�0.01 12.13�0.01
TTS050649.8�031933 N O 19.42�0.03 17.63�0.02 16.32�0.02 14.48�0.01 12.44�0.01 11.52�0.01 11.18�0.01
TTS050649.8�032104 N O 19.91�0.02 18.59�0.02 17.30�0.02 15.56�0.01 12.69�0.01 11.43�0.01 10.51�0.01
TTS050650.5�032014 N O 23.58�0.77 21.11�0.05 19.18�0.03 16.51�0.02 13.78�0.01 13.09�0.01 12.60�0.01
TTS050650.7�032008 N O 21.50�0.13 19.54�0.03 18.14�0.02 16.03�0.02 13.78�0.01 13.07�0.01 12.59�0.01
R X J0506.9�0319N W N O 18.18�0.01 16.73�0.02 15.70�0.02 14.31�0.01 12.46�0.01 11.60�0.01 11.33�0.01
R X J0506.9�0319SE N O 16.34�0.01 14.82�0.02 13.80�0.02 12.67�0.01 11.09�0.01 10.16�0.01 9.97�0.01
H D 293815 10.33�0.01 10.28�0.01 10.08�0.01 9.94�0.01 9.77�0.01 9.50�0.02 9.48�0.03 9.37�0.03 c,e
R X J0506.9�0320W N O 17.36�0.01 15.33�0.02 13.94�0.02 12.39�0.01 10.66�0.01 9.51�0.01 8.94�0.01
R X J0506.9�0320E N O 17.66�0.01 15.74�0.02 14.36�0.02 12.87�0.01 10.75�0.01 9.51�0.01 8.76�0.01
TTS050654.5�032046 N O N D N D 20.98�0.08 18.92�0.03 15.92�0.02 14.92�0.01 14.23�0.01
LkH � 333 16.55�0.02 15.74�0.01 14.19�0.01 S S 10.34�0.02 9.24�0.02 8.44�0.03 c,f
L1616 M IR 4 N O 21.12�0.13 18.99�0.04 17.59�0.02 15.94�0.01 12.78�0.02 10.83�0.01 9.66�0.01
K isoA -0974 15 13.54�0.01 13.65�0.01 12.84�0.01 12.18�0.01 11.42�0.01 9.89�0.03 9.13�0.03 8.49�0.03 c,g
R X J0507.0�0318 N O 16.89�0.01 15.21�0.02 14.08�0.02 12.91�0.01 11.48�0.01 10.48�0.01 10.13�0.01
TTS050657.0�031640 N O 18.80�0.01 17.69�0.02 16.48�0.02 14.88�0.01 13.12�0.01 12.48�0.01 12.11�0.01
TTS050704.7�030241 N O 21.87�0.04 20.04�0.02 18.81�0.02 16.75�0.02 14.94�0.01 14.32�0.01 14.00�0.01
TTS050705.3�030006 N O 16.95�0.01 15.50�0.02 14.55�0.02 13.60�0.01 12.63�0.01 11.75�0.01 11.63�0.01
R X J0507.1�0321 N O 17.56�0.01 16.13�0.02 15.08�0.02 13.87�0.01 12.40�0.01 11.53�0.01 11.20�0.01
TTS050706.2�031703 N O 21.83�0.04 19.99�0.02 18.71�0.02 16.66�0.02 14.69�0.01 14.18�0.01 13.84�0.01
R X J0507.2�0323 N O 15.09�0.01 13.95�0.01 S 12.61�0.02 11.79�0.01 11.01�0.01 10.95�0.01 d
TTS050713.5�031722 N O 19.09�0.01 17.57�0.02 16.55�0.01 15.61�0.01 14.05�0.01 13.02�0.01 12.61�0.01
R X J0507.3�0326 N O 15.72�0.01 14.31�0.01 13.42�0.02 12.58�0.02 11.45�0.02 10.78�0.02 10.63�0.02 c,d
TTS050717.9�032433 N O 18.24�0.01 16.67�0.02 15.64�0.01 14.40�0.01 13.17�0.01 12.48�0.01 12.23�0.01
R X J0507.4�0320 N O 18.37�0.01 16.71�0.02 15.58�0.01 13.98�0.01 12.42�0.01 11.73�0.01 11.49�0.01 h
{50{
Table3| Continued
PM S Star U B V R C IC J H K s N otes
R X J0507.4�0317 N O 18.68�0.01 17.19�0.02 16.17�0.02 14.70�0.01 13.25�0.01 12.61�0.01 12.34�0.01
TTS050729.8�031705 N O 22.77�0.08 20.83�0.03 19.39�0.02 17.16�0.02 14.94�0.04 14.23�0.04 13.96�0.05 c
TTS050730.9�031846 N O N D 22.12�0.08 21.09�0.04 18.93�0.02 16.49�0.03 15.94�0.03 15.40�0.02
TTS050733.6�032517 N O 21.36�0.03 19.62�0.02 18.39�0.02 16.49�0.02 14.79�0.01 14.21�0.01 13.90�0.01
TTS050734.8�031521 N O 20.59�0.02 19.05�0.02 17.85�0.02 16.10�0.01 14.51�0.01 13.87�0.01 13.60�0.01
R X J0507.6�0318 N O 16.52�0.01 15.16�0.02 14.27�0.01 13.42�0.01 12.44�0.01 11.71�0.01 11.50�0.01
TTS050741.0�032253 N O 19.16�0.01 17.55�0.02 16.51�0.02 14.96�0.01 13.52�0.03 12.81�0.03 12.57�0.03 c
TTS050741.4�031507 N O 19.43�0.01 17.92�0.02 16.86�0.02 15.13�0.01 13.60�0.01 12.90�0.01 12.66�0.01
TTS050752.0�032003 N O 21.30�0.03 19.55�0.02 18.27�0.02 16.32�0.02 14.52�0.03 13.98�0.04 13.51�0.04 c
TTS050801.4�032255 N O N O N O N O 13.82�0.04 12.51�0.02 11.63�0.02 11.23�0.02 c,d
TTS050801.9�031732 N O N O N O N O 13.84�0.04 12.39�0.02 11.66�0.02 11.33�0.03 c,d
TTS050804.0�034052 N O N O N O N O 14.28�0.04 13.00�0.03 12.33�0.03 12.07�0.02 c,d
TTS050836.6�030341 N O N O N O N O 13.46�0.03 11.93�0.02 11.16�0.03 10.73�0.02 c,d
TTS050845.1�031653 N O N O N O N O 13.88�0.04 12.37�0.04 11.82�0.04 11.46�0.03 c,d
R X J0509.0�0315 12.26�0.04 12.09�0.03 11.39�0.02 N O 10.51�0.04 9.91�0.02 9.53�0.02 9.41�0.02 a,b,c
R X J0510.1�0427 13.36�0.01 12.65�0.01 11.73�0.01 N O 10.41�0.05 9.68�0.02 9.14�0.03 8.99�0.02 a,b,c
1R X S J051011.5�025355 14.27�0.24 13.43�0.23 12.42�0.23 11.81�0.23 11.24�0.23 10.45�0.02 9.95�0.02 9.73�0.02 c,i
R X J0510.3�0330 N O 12.53�0.04 11.74�0.03 11.29�0.02 10.87�0.02 10.04�0.15 9.81�0.06 9.75�0.05 l,m ,n
1R X S J051043.2�031627 12.11�0.04 12.01�0.03 11.38�0.03 N O 10.57�0.04 10.08�0.03 9.73�0.02 9.65�0.03 b,c,p
R X J0511.7�0348 13.13�0.07 12.77�0.06 12.02�0.04 N O 11.01�0.04 10.34�0.03 9.87�0.02 9.81�0.02 a,b,c
R X J0512.3�0255 14.10�0.06 13.56�0.04 12.61�0.03 12.05�0.04 11.49�0.04 10.43�0.02 9.69�0.02 9.14�0.02 c,i
S Saturated object.
N D O bjectnot detected.
N O O bjectnotobserved.
a U B V from observations carried outatthe Catania A strophysicalO bservatory (Section 2.1).
b IC from TA SS M ark IV catalogue (D roege etal.2006).
c JH K s from 2M A SS pointsource catalogue (Cutriet al.2003).
d IC from D EN IS catalogue (Epchtein etal.1997).
e U and B from Lee (1968);V ,R C ,and IC from Tayloret al.(1989).
f U from M undt& Bastian (1980).
{51{
g U B V R C IC from V ieira etal.(2003).
h Source blended with another starofsim ilarbrightness:the W FIB V R C IC photom etry corresponds to the com bined lightofboth stars.
i W eighted m ean opticalphotom etry from Cieslinskiet al.(1997).
l B V R C IC from A lcal�a etal.(1996).
m J from D EN IS catalogue (Epchtein et al. 1997, converted to the 2M A SS photom etric system using the transform ation equations provided by
Carpenter (2001)).
n H K s from 2M A SS pointsource catalogue (Cutrietal.2003).
p U B V from Frasca et al.(2003).
{ 52 {
Table4. Spectraltype,H� and LiiequivalentwidthsofthePM S starsin L1615/L1616.
Additionalem ission linesobserved in thespectra arealso tabulated.
Star SpT W (H�) W (Li) Type O therem ission lines Notes
(�A) (m �A)
1RXS J045912.4� 033711 G 8 1:80� 0:20 355� 10 W TTS a
1RXS J050416.9� 021426 K 3 0:03� 0:20 475� 10 W TTS a
TTS050513.5� 034248 M 5.5 � 9:80� 0:40 800� 100 W TTS
TTS050538.9� 032626 M 3.5 � 4:90� 0:20 350� 20 W TTS Hei
RX J0506.6� 0337 G 9 1:00� 0:20 299� 10 W TTS a
TTS050644.4� 032913 M 4.5 � 4:70� 0:30 560� 40 W TTS
TTS050646.1� 031922 K 4 � 41:00� 2:00 560� 20 CTTS Hei,[N ii],[Sii]
RX J0506.8� 0318 K 8.5 � 3:80� 0:20 570� 30 W TTS Hei
TTS050647.5� 031910 M 5.5 � 10:20� 0:80 560� 50 W TTS
RX J0506.8� 0327 M 3.5 � 6:75� 0:20 470� 10 W TTS
RX J0506.8� 0305 M 4.5 � 4:80� 0:30 590� 50 W TTS Hei
TTS050649.8� 031933 M 3.5 � 15:50� 1:00 540� 50 W TTS Hei,[O i]
TTS050649.8� 032104 M 1: � 195:00� 5:00 190� 20 CTTS Hei,Nai,[O i] b
TTS050650.5� 032014 M 6.5 � 14:00� 1:00 680� 40 W TTS
TTS050650.7� 032008 M 4.5 � 26:00� 0:50 620� 50 CTTS Hei,[O i],[N ii],[Sii]
RX J0506.9� 0319NW M 2.5 � 3:50� 0:30 590� 50 W TTS
RX J0506.9� 0319SE K 5 � 4:80� 0:50 470� 10 CTTS [O i],[N ii],[Sii]
HD 293815 B9V 8:60� 0:50 c
RX J0506.9� 0320W K 8.5 � 2:90� 0:50 500� 20 W TTS
RX J0506.9� 0320E K 0 � 1:50� 0:50 380� 10 W TTS
TTS050654.5� 032046 M 4 � 60:00� 5:00 CTTS
LkH� 333 K 4 � 60:00� 2:00 430� 10 CTTS Hei,[O i],[Sii]
L1616 M IR4 K 1 � 60:00� 6:00 370� 50 CTTS [O i],[N ii],[Sii]
K isoA-0974 15 B3e � 67:50� 1:00 HAeBe e
RX J0507.0� 0318 M 0 � 2:00� 0:50 570� 30 W TTS
TTS050657.0� 031640 M 4.5 � 94:00� 2:00 430� 40 CTTS Hei,[O i]
TTS050704.7� 030241 M 6 � 17:50� 1:00 740� 50 W TTS
TTS050705.3� 030006 M 0 � 2:00� 0:40 400� 20 W TTS
RX J0507.1� 0321 M 1 � 38:50� 3:00 590� 20 CTTS Hei,[O i]
TTS050706.2� 031703 M 6 � 13:50� 0:50 730� 50 W TTS
RX J0507.2� 0323 K 4 � 1:10� 0:10 480� 20 W TTS
TTS050713.5� 031722 K 8.5 � 10:50� 0:50 520� 30 CTTS
RX J0507.3� 0326 M 0 � 1:90� 0:20 550� 10 W TTS
TTS050717.9� 032433 M 2.5 � 3:50� 0:50 470� 10 W TTS
RX J0507.4� 0320 M 4 � 5:00� 0:50 530� 50 W TTS Hei
RX J0507.4� 0317 M 3 � 14:50� 0:50 510� 50 W TTS Hei
{ 53 {
Table4| Continued
Star SpT W (H�) W (Li) Type O therem ission lines Notes
(�A) (m �A)
TTS050729.8� 031705 M 6.5 � 12:00� 0:50 450� 20 W TTS
TTS050730.9� 031846 M 5.5 � 290:00� 30:00 CTTS Hei
TTS050733.6� 032517 M 5.5 � 2:30� 0:20 610� 20 W TTS
TTS050734.8� 031521 M 5 � 6:80� 0:40 610� 50 W TTS Hei
RX J0507.6� 0318 K 7 � 1:60� 0:20 520� 50 W TTS
TTS050741.0� 032253 M 4 � 4:30� 0:10 540� 20 W TTS
TTS050741.4� 031507 M 4.5 � 6:80� 0:40 550� 20 W TTS
TTS050752.0� 032003 M 5.5 � 9:50� 0:20 530� 30 W TTS
TTS050801.4� 032255 M 0.5 � 21:50� 0:50 510� 25 CTTS Hei,[Sii] a
TTS050801.9� 031732 M 1 � 15:50� 0:50 410� 25 CTTS Hei,[Sii] a
TTS050804.0� 034052 M 2.5 � 2:20� 0:20 410� 20 W TTS a
TTS050836.6� 030341 M 1.5 � 68:50� 1:00 560� 30 CTTS Hei a
TTS050845.1� 031653 M 3.5 � 6:70� 1:00 530� 50 W TTS
RX J0509.0� 0315 G 8 1:10� 0:20 320� 10 W TTS a
RX J0510.1� 0427 K 4 � 0:20� 0:20 230� 10 W TTS a
1RXS J051011.5� 025355 K 0 � 0:10� 0:20 415� 10 W TTS a
RX J0510.3� 0330 G 8 1:50� 0:20 320� 10 W TTS a
1RXS J051043.2� 031627 G 2 2:40� 0:20 235� 10 W TTS a
RX J0511.7� 0348 K 1 1:80� 0:20 370� 15 W TTS a
RX J0512.3� 0255 K 2 � 6:50� 0:50 440� 10 CTTS a
aW (H�)and W (Li)from Alcal�a etal.(2004).
bVeiled CTTS.
cSpectraltype from Sharpless(1952).
eSpectraltype from Vieira etal.(2003).
{ 54 {
Table5. Stellarparam etersofthePM S starsin L1615/L1616.
PM S Star logTeff R V A V R ? logL=L�
(m ag) (R � )
1RXS J045912.4� 033711 3:746� 0:009 3:1� 0:1? 0:30� 0:12 2:63� 0:12 0:78� 0:05
1RXS J050416.9� 021426 3:675� 0:014 3:1� 0:1? 0:35� 0:22 2:35� 0:11 0:40� 0:07
TTS050513.5� 034248 3:494� 0:010 3:1� 0:1? 0:49� 0:20 0:55� 0:03 � 1:60� 0:06
TTS050538.9� 032626 3:531� 0:009 3:1� 0:1? 0:05� 0:10 1:08� 0:05 � 0:85� 0:05
RX J0506.6� 0337 3:733� 0:013 3:1� 0:1? 0:05� 0:10 1:92� 0:09 0:45� 0:06
TTS050644.4� 032913 3:513� 0:009 3:1� 0:1? 0:00+ 0:05 1:00� 0:05 � 0:99� 0:05
TTS050646.1� 031922 3:662� 0:026 4:2� 0:4 4:33� 0:35 1:91� 0:11 0:17� 0:11
RX J0506.8� 0318 3:597� 0:008 3:1� 0:1? 0:00+ 0:05 1:62� 0:07 � 0:24� 0:05
TTS050647.5� 031910 3:494� 0:010 3:0� 0:2 2:30� 0:17 1:03� 0:05 � 1:04� 0:05
RX J0506.8� 0327 3:531� 0:009 3:1� 0:1? 0:44� 0:10 2:43� 0:11 � 0:15� 0:05
RX J0506.8� 0305 3:513� 0:009 3:1� 0:1? 0:05� 0:10 1:28� 0:06 � 0:78� 0:05
TTS050649.8� 031933 3:531� 0:009 4:1� 0:4 1:67� 0:10 2:04� 0:09 � 0:30� 0:05
TTS050649.8� 032104 3:573� 0:008 3:1� 0:1? 4:72� 0:12 2:29� 0:10 � 0:03� 0:05
TTS050650.5� 032014 3:476� 0:008 1:7� 0:1 2:93� 0:19 1:30� 0:06 � 0:91� 0:05
TTS050650.7� 032008 3:513� 0:009 3:4� 0:2 2:20� 0:17 1:28� 0:06 � 0:78� 0:05
RX J0506.9� 0319NW 3:549� 0:008 3:1� 0:1? 0:00+ 0:05 1:22� 0:05 � 0:68� 0:05
RX J0506.9� 0319SE 3:622� 0:025 4:1� 0:4 1:62� 0:49 2:78� 0:14 0:33� 0:11
HD 293815 4:021� 0:049 3:5� 0:5 0:76� 0:05 2:03� 0:09 1:65� 0:20
RX J0506.9� 0320W 3:597� 0:008 2:4� 0:2 2:24� 0:12 3:98� 0:18 0:54� 0:05
RX J0506.9� 0320E 3:720� 0:014 3:3� 0:1 4:72� 0:15 3:65� 0:17 0:96� 0:07
TTS050654.5� 032046 3:522� 0:009 3:1� 0:1? 4:33� 0:10 0:58� 0:03 � 1:42� 0:05
LkH� 333 3:662� 0:026 3:1� 0:1? 2:60� 0:44 3:92� 0:29 0:79� 0:12
L1616 M IR4 3:706� 0:015 4:1� 0:1 7:09� 0:15 2:45� 0:12 0:56� 0:07
K isoA-0974 15 4:272� 0:058 5:5� 0:1 5:20� 0:20 2:59� 0:15 2:87� 0:24
RX J0507.0� 0318 3:589� 0:008 4:2� 0:5 1:08� 0:15 2:56� 0:12 0:13� 0:05
TTS050657.0� 031640 3:513� 0:009 3:1� 0:1? 0:44� 0:12 1:30� 0:06 � 0:76� 0:05
TTS050704.7� 030241 3:484� 0:009 2:5� 0:4 0:74� 0:47 0:71� 0:08 � 1:41� 0:10
TTS050705.3� 030006 3:589� 0:008 3:1� 0:1? 0:00+ 0:05 1:25� 0:06 � 0:49� 0:05
RX J0507.1� 0321 3:573� 0:008 3:1� 0:1? 0:49� 0:15 1:55� 0:07 � 0:37� 0:05
TTS050706.2� 031703 3:484� 0:009 3:0� 0:1 0:88� 0:22 0:78� 0:04 � 1:33� 0:06
RX J0507.2� 0323 3:662� 0:026 3:1� 0:1? 0:25� 0:22 1:53� 0:07 � 0:03� 0:11
TTS050713.5� 031722 3:597� 0:008 3:1� 0:1? 1:18� 0:15 0:79� 0:04 � 0:86� 0:05
RX J0507.3� 0326 3:589� 0:008 3:1� 0:1? 0:00+ 0:05 2:23� 0:10 0:01� 0:05
TTS050717.9� 032433 3:549� 0:008 3:1� 0:1? 0:00+ 0:05 1:09� 0:05 � 0:77� 0:05
RX J0507.4� 0320 3:522� 0:009 3:1� 0:1? 0:20� 0:10 1:68� 0:08 � 0:51� 0:05
RX J0507.4� 0317 3:540� 0:009 3:1� 0:1? 0:00+ 0:05 1:04� 0:05 � 0:85� 0:05
{ 55 {
Table5| Continued
PM S Star logTeff R V A V R ? logL=L�
(m ag) (R � )
TTS050729.8� 031705 3:476� 0:008 2:5� 0:1 1:32� 0:19 0:73� 0:03 � 1:42� 0:05
TTS050730.9� 031846 3:494� 0:010 3:1� 0:1? 1:81� 0:15 0:33� 0:01 � 2:02� 0:05
TTS050733.6� 032517 3:494� 0:010 3:1� 0:1? 0:54� 0:20 0:69� 0:03 � 1:39� 0:06
TTS050734.8� 031521 3:504� 0:009 3:1� 0:1? 0:05� 0:10 0:67� 0:03 � 1:37� 0:05
RX J0507.6� 0318 3:604� 0:008 3:1� 0:1? 0:00+ 0:05 1:26� 0:06 � 0:43� 0:05
TTS050741.0� 032253 3:522� 0:009 3:1� 0:1? 0:05� 0:10 1:02� 0:05 � 0:94� 0:05
TTS050741.4� 031507 3:513� 0:009 3:1� 0:1? 0:05� 0:07 1:00� 0:05 � 0:99� 0:05
TTS050752.0� 032003 3:494� 0:010 3:1� 0:1? 0:64� 0:20 0:74� 0:03 � 1:33� 0:06
TTS050801.4� 032255 3:581� 0:008 3:1� 0:1? 0:54� 0:12 1:47� 0:07 � 0:39� 0:05
TTS050801.9� 031732 3:573� 0:008 3:1� 0:1? 0:39� 0:12 1:46� 0:07 � 0:42� 0:05
TTS050804.0� 034052 3:549� 0:008 3:1� 0:1? 0:00+ 0:05 1:15� 0:05 � 0:73� 0:05
TTS050836.6� 030341 3:565� 0:008 3:1� 0:1? 0:64� 0:10 1:97� 0:09 � 0:20� 0:05
TTS050845.1� 031653 3:531� 0:009 3:1� 0:1? 0:00+ 0:05 1:61� 0:07 � 0:51� 0:05
RX J0509.0� 0315 3:746� 0:009 3:1� 0:1? 0:00+ 0:05 2:62� 0:12 0:78� 0:05
RX J0510.1� 0427 3:662� 0:026 3:1� 0:1? 0:00+ 0:05 3:74� 0:17 0:75� 0:11
1RXS J051011.5� 025355 3:720� 0:014 3:1� 0:1? 0:60� 0:22 2:37� 0:11 0:59� 0:07
RX J0510.3� 0330 3:746� 0:009 3:1� 0:1? 0:10� 0:08 2:16� 0:10 0:61� 0:05
1RXS J051043.2� 031627 3:768� 0:004 3:1� 0:1? 0:00+ 0:05 2:25� 0:10 0:73� 0:04
RX J0511.7� 0348 3:706� 0:015 3:1� 0:1? 0:00+ 0:10 2:40� 0:11 0:54� 0:07
RX J0512.3� 0255 3:690� 0:016 3:1� 0:1? 0:00+ 0:10 1:99� 0:10 0:31� 0:08
?Assum ed (seeSection 5.3).
{ 56 {
Table 6:E�ective wavelengths and absolute ux calibration constants for the Johnson-
Cousinsand 2M ASS passbands.Them ain referencepapersarelisted in thelastcolum n.
Filter �eff F 0� Reference
(�m ) (erg=s2=cm 2=�A)
UJ 0.36 4.350�10�9 Johnson (1965)
B J 0.44 7.200�10�9 Johnson (1965)
VJ 0.55 3.920�10�9 Cousins(1976)
R C 0.64 2.254�10�9 Cousins(1976)
IC 0.79 1.196�10�9 Cousins(1976)
J 1.24 3.129�10�10 Cohen etal.(2003)
H 1.66 1.133�10�10 Cohen etal.(2003)
K s 2.16 4.283�10�11 Cohen etal.(2003)
{ 57 {
Table7. M assesand agesforthePM S starsin L1615/L1616 asinferred by the
evolutionary m odelsby Bara�eetal.(1998)& Chabrieretal.(2000),
D’Antona & M azzitelli(1997)and Palla & Stahler(1999).
PM S Star M B a98+ C h00 AgeB a98+ C h00 M D M 97 AgeD M 97 M P S99 AgeP S99
(M � ) (M yr) (M � ) (M yr) (M � ) (M yr)
1RXS J045912.4� 033711 > 1:40 � 2.00 5.00 1.70 3.50
1RXS J050416.9� 021426 > 1:40 � 0.95 0.70 1.60 3.00
TTS050513.5� 034248 0.12 6.00 0.17 10.00 0.14 10.00
TTS050538.9� 032626 0.35 4.04 0.25 2.20 0.26 2.80
RX J0506.6� 0337 1.35 14.00 1.65 5.50 1.40 8.00
TTS050644.4� 032913 0.25 3.17 0.20 2.50 0.19 2.70
TTS050646.1� 031922 > 1:40 � 0.75 1.00 1.35 4.00
RX J0506.8� 0318 1.05 5.00 0.50 1.00 0.68 3.00
TTS050647.5� 031910 0.18 2.00 0.16 2.00 0.13 2.00
RX J0506.8� 0327 0.55 < 1:00 0.20 0.10 0.25 0.40
RX J0506.8� 0305 0.26 1.78 0.19 1.50 0.18 1.50
TTS050649.8� 031933 0.47 1.00 0.22 0.20 0.25 0.70
TTS050649.8� 032104 0.87 1.30 0.35 0.40 0.50 0.80
TTS050650.5� 032014 0.09 < 1:00 0.13 1.00 < 0:10 0.70
TTS050650.7� 032008 0.26 1.60 0.19 1.50 0.18 1.50
RX J0506.9� 0319NW 0.50 4.50 0.33 2.00 0.35 2.50
RX J0506.9� 0319SE 1.40 1.30 0.45 0.30 0.93 0.80
HD 293815 > 1:40 � 2.50 3.00 2.50 3.00
RX J0506.9� 0320W 1.25 < 1:00 0.32 0.07 0.68 0.40
RX J0506.9� 0320E > 1:40 � 2.30 0.90 2.30 1.50
TTS050654.5� 032046 0.26 14.20 0.25 14.00 0.22 10.00
LkH� 333 > 1:40 � 0.75 0.20 1.70 0.50
L1616 M IR4 > 1:40 � 1.70 1.70 1.75 3.00
K isoA-0974 15 > 1:40 � > 3:00 � 5.50 0.30
RX J0507.0� 0318 1.10 1.45 0.35 0.30 0.60 0.80
TTS050657.0� 031640 0.27 1.60 0.19 1.50 0.18 1.50
TTS050704.7� 030241 0.11 3.00 0.15 4.50 0.10 3.00
TTS050705.3� 030006 0.90 10.05 0.55 3.00 0.63 4.50
RX J0507.1� 0321 0.80 4.30 0.40 1.00 0.50 2.50
TTS050706.2� 031703 0.12 2.55 0.15 4.00 0.10 2.50
RX J0507.2� 0323 1.25 11.30 0.85 2.00 1.20 7.50
TTS050713.5� 031722 0.73 35.80 0.70 20.00 0.60 20.00
RX J0507.3� 0326 1.05 2.00 0.37 0.40 0.60 0.80
TTS050717.9� 032433 0.50 6.32 0.34 2.80 0.35 3.00
RX J0507.4� 0320 0.36 1.13 0.20 0.70 0.22 0.80
RX J0507.4� 0317 0.42 5.50 0.28 2.80 0.31 3.00
{ 58 {
Table7| Continued
PM S Star M B a98+ C h00 AgeB a98+ C h00 M D M 97 AgeD M 97 M P S99 AgeP S99
(M � ) (M yr) (M � ) (M yr) (M � ) (M yr)
TTS050729.8� 031705 0.09 2.53 0.13 4.00 < 0:10 3.00
TTS050730.9� 031846 0.11 20.11 0.15 30.00 0.13 30.00
TTS050733.6� 032517 0.14 4.00 0.17 5.50 0.13 4.00
TTS050734.8� 031521 0.17 5.65 0.20 6.50 0.16 5.00
RX J0507.6� 0318 1.00 11.27 0.62 3.00 0.75 7.00
TTS050741.0� 032253 0.30 3.60 0.23 2.50 0.22 2.50
TTS050741.4� 031507 0.25 3.17 0.20 2.50 0.18 2.50
TTS050752.0� 032003 0.15 3.56 0.17 4.50 0.13 3.00
TTS050801.4� 032255 0.88 5.67 0.47 1.60 0.56 3.00
TTS050801.9� 031732 0.80 5.02 0.44 1.50 0.50 2.70
TTS050804.0� 034052 0.50 5.67 0.33 2.50 0.35 2.80
TTS050836.6� 030341 0.75 2.01 0.32 0.60 0.44 1.00
TTS050845.1� 031653 0.40 1.61 0.23 0.90 0.26 1.00
RX J0509.0� 0315 > 1:40 � 2.00 4.50 1.75 3.50
RX J0510.1� 0427 > 1:40 � 0.70 0.20 1.70 0.50
1RXS J051011.5� 025355 > 1:40 � 1.85 2.50 1.65 4.00
RX J0510.3� 0330 > 1:40 � 1.70 7.00 1.50 6.00
1RXS J051043.2� 031627 > 1:40 � 1.65 8.00 1.50 6.50
RX J0511.7� 0348 > 1:40 � 1.70 2.00 1.70 4.00
RX J0512.3� 0255 > 1:40 � 1.30 2.00 1.50 5.00
{ 59 {
Table8. � slopesoftheIM F in di�erentclustersand associations.
Cluster Age M assrange � Ref.
(M yr) (M � )
M 35 150 0.1-0.4 0.58 a
� Per 80 0.06-0.2 0.56 b
Pleiades 120 0.03-0.45 0.60�0.11 c
� Orionis 5 0.02-1.2 0.60�0.06 d
� Orionis 5 0.013-0.2 0.8�0.4 e
Trapezium 0.4 0.035-0.56 �0.3 f
0.25-3 �0.7
� Oph 0.1-1 0.02-0.4 �0.5 g
IC 348 3 0.035-0.5 0.7�0.2 h
Taurus-Auriga 1-2 0.08-1 1.35 i
Cha I � 3 0.08-1.2 0.6-1.1 j,k
Cha II 2-5 0.1-1 0.5-0.6 l
L1615/L1616 1-3 0.1-5.5 0.84�0.07 Thiswork
References. | a) Barrado Y Navascu�esetal. (1998); b)
Barrado Y Navascu�esetal.(2002); c) M oraux etal.(2003); d)
Barrado Y Navascu�esetal. (2004b); e) B�ejaretal. (2001); f)
Luhm an etal.(2000); g) Luhm an & Rieke (1999); h) Tejetal.
(2002);i) Brice~no etal.(2002);j) L�opezM art�� etal.(2004);k)
Com er�on etal.(2000);l)Spezzietal.(2008).