MODEST 6 -- Chicago, August 31, 2005

UVE UVE

by-products ofby-products ofbinary systemsbinary systems

study of the environment study of the environment effect oneffect on “canonical” “canonical”evolutionary sequencesevolutionary sequences

the “creation” of the “creation” of ARTIFICIALARTIFICIAL sequences as sequences as Blue Stragglers StarsBlue Stragglers Stars and exotic objectsand exotic objects

according to their position in the CMD, according to their position in the CMD, BSSBSS should be should be more massivemore massive than than

normal normal starsstars

BSS have been detected for the first time by Sandage (1953)BSS have been detected for the first time by Sandage (1953)

merger of 2 low-mass starsmerger of 2 low-mass stars unevolved, massive starunevolved, massive star

primordialprimordialbinariesbinaries

directdirectcollisionscollisions

BSS BSS crucial link between crucial link betweenstellar evolution & stellar dynamicsstellar evolution & stellar dynamics

BSS BSS are a normal population of GGCs, are a normal population of GGCs, found in each cluster properly observedfound in each cluster properly observed

BSSBSS are more concentrated than RGB starsare more concentrated than RGB stars exceptions: M3, M55exceptions: M3, M55,47 Tuc..,47 Tuc..

<1990<1990

loose GGCs natural habitat low c, low for BSS

>1990>1990

high resolution studies high resolution studies BSSBSS also in the inner regionalso in the inner region

of high density GGCsof high density GGCs

• NGC6397 NGC6397 Auriere et al. 1990 Auriere et al. 1990 • 47 Tuc 47 Tuc Paresce et al. 1991 Paresce et al. 1991 • M15 M15 Ferraro & Paresce 1993 Ferraro & Paresce 1993

Catalogs:Catalogs: Fusi Pecci et al. 1992Fusi Pecci et al. 1992Sarajedini et al. 1992Sarajedini et al. 1992

Ferraro, Fusi Pecci, Bellazzini 1995Ferraro, Fusi Pecci, Bellazzini 1995Guhathakurta et al. 1994, 1998 Guhathakurta et al. 1994, 1998

Piotto et al 2004Piotto et al 2004

UV sensitivityUV sensitivity , , high resolutionhigh resolution

systematic studies of hot SPs in the core of high density GGCs

HST

UV-planeUV-plane ideal to study ideal to study the photometric propertiesthe photometric properties of theof the BSSBSS population population::

- the distribution is almost vertical- the distribution is almost vertical- span more than 3 magnitudes- span more than 3 magnitudes

PHOT. PLATESPHOT. PLATES(Buonanno et al 1994)(Buonanno et al 1994)

HIGH RES. CCDHIGH RES. CCD(Ferraro et al 1993)(Ferraro et al 1993)

HST IMAGESHST IMAGES(Ferraro et al 1997(Ferraro et al 1997))

M3M3

M3 :M3 :The first surpriseThe first surprise

The very first complete The very first complete coverage of the entirecoverage of the entirethe cluster extensionthe cluster extension

BSS are more concentrated BSS are more concentrated in the central regionin the central region

BSS are less concentrated BSS are less concentrated in the external regionin the external region

For any not-segregated pop.For any not-segregated pop. R = 1R = 1

NNRGBRGB//NNTOTTOT

RRRGBRGB= =

LLSS//LLTOTTOT

NNBSSBSS//NNTOTTOT

RRBSSBSS= =

LLSS//LLTOTTOT

The BSS radial distributionThe BSS radial distribution is BIMODALis BIMODAL

Is this distribution Is this distribution really “peculiar”really “peculiar” & unique ?& unique ?

Highly peaked in the Highly peaked in the center rapidly decreasingcenter rapidly decreasingat intermediate radii and at intermediate radii and finally rising again at finally rising again at larger radiilarger radii

47 Tuc: another 47 Tuc: another surprise!!!!surprise!!!!

The BSS radial distribution inThe BSS radial distribution in47 Tuc is quite similar to that47 Tuc is quite similar to thatobserved in M3observed in M3

Mass segregationMass segregation &&dynamical frictiondynamical friction

Central peakCentral peak &&the Decreasing at the Decreasing at intermediate radiiintermediate radii

collisional BSS kicked off collisional BSS kicked off from the cluster corefrom the cluster core

BSS generated by BSS generated by primordial binariesprimordial binaries

oror

Dynamical simulations performedDynamical simulations performedin the case of 47 Tuc (Mapelli et in the case of 47 Tuc (Mapelli et al 2004) showed that a pop. of al 2004) showed that a pop. of PB is needed to reproduce thePB is needed to reproduce thebimodal distributionbimodal distribution

collisional BSS @ centercollisional BSS @ center& PB-BSS in the outer regions?& PB-BSS in the outer regions?

?

Ferraro et al (1993,1997)Ferraro et al (1993,1997)

M55M55

Zaggia et al (1997)Zaggia et al (1997)

47 Tuc47 Tuc

Ferraro et al (2004)Ferraro et al (2004)Sabbi et al (2004)Sabbi et al (2004)

NGC6752NGC6752

Is this the “natural” BSS radialIs this the “natural” BSS radial distribution?distribution?

Omega Centauri

Ferraro et al, (2004),ApJ, 603,L81

Centauri: NO evidence Centauri: NO evidence of mass segregationof mass segregation!!!!!!!!

A pure population of A pure population of non-collisional BSS?non-collisional BSS?

Piotto et al (2004) + Davies et al (2004)Piotto et al (2004) + Davies et al (2004)

The total number of BSS is independent of cluster The total number of BSS is independent of cluster mass and collision ratemass and collision rate

N(BSS) varies only a factor 10 !!!N(BSS) varies only a factor 10 !!!

New catalog containing 3000 BSS in 56 GGCs fromNew catalog containing 3000 BSS in 56 GGCs from HST optical observationsHST optical observations

BSS are produced by BSS are produced by both channels (collisions both channels (collisions & binary evolution)& binary evolution)

Accordingly with previous Accordingly with previous suggestions by Fusi Pecci et suggestions by Fusi Pecci et al (1993), Baylin (1995), etc…al (1993), Baylin (1995), etc…

Davies et al (2004)Davies et al (2004)

1-2 : A more massive MS star 1-2 : A more massive MS star exchanges into a MS-MS binaryexchanges into a MS-MS binary

3 : the primary evolves off the MS 3 : the primary evolves off the MS and fills its Roche lobeand fills its Roche lobe

4: the secondary gains mass from the 4: the secondary gains mass from the primary becoming a BSSprimary becoming a BSS

Davies et al (2004) suggested:Davies et al (2004) suggested:1. This process was particulary1. This process was particularyactive in the past in high-densityactive in the past in high-densityclusters clusters 2. most of the BSS generated 2. most of the BSS generated by this channel have now evolvedby this channel have now evolved

In more massive systemsIn more massive systemsexchange encounters with a third exchange encounters with a third more massive MS star “used up”more massive MS star “used up” PB early in the cluster historyPB early in the cluster history

Davies et al (2004)Davies et al (2004)

The number of Collisional BSS The number of Collisional BSS increases with the cluster increases with the cluster luminosityluminosity

The number of BSS produced The number of BSS produced from primordial binaries from primordial binaries decrease with clusterdecrease with clusterluminosity because they are luminosity because they are already evolved already evolved

The number of BSS produced The number of BSS produced over the last 1 Gyrover the last 1 Gyr

Interesting working hypothesisInteresting working hypothesisbut the scenario seemsbut the scenario seems more complex…. more complex…. Since the dynamical history Since the dynamical history of each cluster plays an of each cluster plays an important roleimportant role

NNBSSBSS must be normalized to the

cluster population

F = BSS specific frequency

F = NBSS / NHB

305 305 BSSBSS !! !! One of the largest population One of the largest population ever observed in a ever observed in a GGCGGC

The most concentrated The most concentrated

BSSBSS population population ever found in aever found in a GGCGGC

Could the dynamical evolution Could the dynamical evolution of the cluster play a role in the of the cluster play a role in the formation of formation of BSSBSS??

Are collisions delaying Are collisions delaying the core collapse and the core collapse and generating the generating the BSSBSS??

Why M80 has such a huge population of BSS ?Why M80 has such a huge population of BSS ?M80 is much more concentrated than M3

(Log = 5.8 M</pc3)

BUT other clusters with similar concentration like

47 Tuc (Log = 5.1 M</pc3)

NGC2808 (Log = 5.0 M</pc3)

NGC6388 (Log = 5.7 M</pc3)

have many fewer BSS (NBSS< 100)

M80M80 is not a is not a PCCPCC but it should be !!!! but it should be !!!! its dynamical time scaleits dynamical time scaleis much shorter than its age !is much shorter than its age ! BUT even the even the PCC PCC state cannot explain state cannot explain such a huge BSS population such a huge BSS population

M 3Log = 3.5 Ms/pc

3

Log =5.8 Ms

NBSS = 72

F = 0.28

M 13Log = 3.4 Ms/pc

3

Log =5.8 Ms

NBSS = 16

F = 0.07

twin clusters

different primordial different primordial binary population ?binary population ?

clusters in different clusters in different dynamical phases ?dynamical phases ? ??

Not-yet coll.

NBSS = 129

F = 0.44 – 1.0

Collapsing?

NBSS = 17

F = 0.16

binaries are preventing binaries are preventing core collapse ?core collapse ?

are binaries destroyedare binaries destroyed during the collapse ?during the collapse ???

Log = 2.1 Ms/pc3

NBSS = 24

F = 0.92 !!!the largest specific frequency

ever observed in one of the lowest density cluster

different types of different types of BSS BSS ??NGC288 has a large NGC288 has a large fraction of binaries !fraction of binaries ! Bellazzini et al. 2002 Bellazzini et al. 2002 AJ, 123, 1509AJ, 123, 1509

Log = 5.8 Ms/pc3

NBSS = 129

F = 0.44

F = 1 if only the PCis considered

??

M3 & M92• 2 GGCs without

HB tails

• similar b-BSS LF extending 2.5 mag

brighter than m255=19

are the are the BSSBSS photometric properties photometric properties and HB morphology linked?and HB morphology linked?

M3, M13 & M10• 3 GGCs with

long HB blue tails

• similar b-BSS LF extending <1.5 mag

brighter than m255=19

??

M80• b-BSS

distribution in the UV-CMD

BSSBSS distribution in the CMD depends on when the BSS are createddistribution in the CMD depends on when the BSS are created

Models by A. Sills:

All Collisional BSS: generated

by s-b interactions

Binary fraction 20%

BSS formation rate is constant or zero

BSSBSS formation has lasted over a relatively formation has lasted over a relatively long period (5-2 Gyr)long period (5-2 Gyr)

Models are still too Models are still too rough to properly rough to properly reproduce the reproduce the observationsobservations

The existence of bright BSS could indicateThe existence of bright BSS could indicate triple systems ?triple systems ?

Theoretical PredictionsModels give controversial

predictions on the resulting properties of a BSS formed via collision

process:

Collisional BSS show high rotational velocities(Benz & Hills 1987)

Collisional BSS are not fast rotators(Leonard & Livio 1995)

•Negligible mixing between inner cores and outer envelopes of colliding stars is expected (Lombardi et al. 1995)•Binary mass transfer is likely to create a fast rotating BSS and to lead an abundance pattern indicative of mixing with regions of incomplete CN-burning (Sarna and de Greve 1996)

These hypothesies These hypothesies have never been have never been checked with checked with observations!!!observations!!!

Looking for abundanceLooking for abundancesignatures of the formationsignatures of the formationprocess…… process……