Properties of YSOs - Cornell Universityastrosun2.astro.cornell.edu/academics/courses/a671... ·...

Properties of Properties of YSOsYSOsThomas OberstThomas Oberst

February 15, 2006February 15, 2006A671A671

Protostellar disks in Orion, HST imageC. R. O’Dell (Rice Univ.)

22February 15, 2006February 15, 2006 T. Oberst: Properties of YSOsT. Oberst: Properties of YSOs

Young Stellar Objects (Young Stellar Objects (YSOsYSOs))YSOsYSOs = = stars in the earliest stages of developmentstars in the earliest stages of developmentMajority Majority form from “clumps” or “cores” in Giant Molecular Cloudsform from “clumps” or “cores” in Giant Molecular Clouds((GMCsGMCs)) in the ISM and Galactic Nucleiin the ISM and Galactic Nuclei(Some (Some YSOsYSOs may also be found in may also be found in BokBok globules and diffuse (or translucent) globules and diffuse (or translucent) molecular clouds)molecular clouds)Typically Typically form in pairs or clustersform in pairs or clustersLife of YSO can be divided into two main stages:Life of YSO can be divided into two main stages:

1. 1. “Protostar”“Protostar”Embedded in Embedded in infallinginfalling envelope of gas & dust; optically thick diskenvelope of gas & dust; optically thick diskSED peak in SED peak in submmsubmm & FIR& FIR

2. 2. “Pre“Pre--main sequence star”main sequence star”Decrease in accretion; envelope shed/dissolved; optically thin dDecrease in accretion; envelope shed/dissolved; optically thin disk; isk; “T “T TauriTauri star”star”SED peak in visible & NIRSED peak in visible & NIR

Five Observational Classes Five Observational Classes (for low(for low-- to midto mid--mass YSO):mass YSO):


Class 0: massive infalling gas and dust envelope, deeply embedded protostar, main accretion phase, molecular outflows, FIR and submm emission

Class I: infalling envelope (~104 AU) + optically thick disk (~ a few 100 AU), luminosity from disk accretion shock, bipolar outflows, infrared emission

Class IIa: optically thick disk, residual envelope, strong stellar wind, disk accretion, outflows, NIR and MIR excess, strong Hα emission

Class III: naked T Tauri star (post T Tauriphase), very little gas and dust in disk, contraction towards main-sequence

Class IIb: “classical” T Tauri star (sometimes “weak-lined” T Tauri star), optically thin disk, decrease in accretion, stellar wind, strong surface magnetic activity; start of planet formation?

(diagram adapted from Wilking, 1989, PASP, 101, 229)(Slide adapted from a presentation by Elise Furlan)

a few to several Myr

tens of Myr

~ 105 yr

a few 104 yr

~ Pr

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~ Pr

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Seq.


Molecular Clouds (MCs)Molecular Clouds (MCs)Properties of Properties of GMCsGMCs

Optically thick (Optically thick (aavv > 1)> 1) ⇒⇒ shields H atoms from UV radiationshields H atoms from UV radiationDue to Due to dustdust (also provides good site for H atoms to bond)(also provides good site for H atoms to bond)Due to Due to HI shell (PDR)HI shell (PDR) (when N(when NHIHI ~ 10~ 102121 cmcm--22) )

Cold (10Cold (10--30K)30K)Atoms bond into moleculesAtoms bond into moleculesColdest objects in the UniverseColdest objects in the Universe

Chemical compositionChemical composition similar tosimilar to SunSun: : ~75% H~75% H22 (difficult to observe, CO used as tracer)(difficult to observe, CO used as tracer)~23% He~23% He~2% dust (including ~2% dust (including PAHsPAHs), CO, H), CO, H220, & many others (radio 0, & many others (radio observations have detected over 50 different molecules in ISM)observations have detected over 50 different molecules in ISM)

n ~ 100 n ~ 100 –– 300 cm300 cm--33

M ~ 10M ~ 1055 –– 101066 MM~ 50~ 50--100 pc 100 pc typical sizetypical size1000s of 1000s of GMCsGMCs knownknown to exist in Milky Way, mostly in spiral armsto exist in Milky Way, mostly in spiral arms


Molecular Clouds (MCs)Molecular Clouds (MCs)GMCsGMCs contain contain regions with hot dense coresregions with hot dense cores (10% of GMC mass)(10% of GMC mass)

r ~ 0.05 r ~ 0.05 –– 1 pc1 pcaavv ~ 50 ~ 50 –– 10001000T ~ 100 T ~ 100 –– 200 K200 Kn ~ 10n ~ 1044 –– 101099 cmcm--33

M ~ 10 M ~ 10 –– 1000 M1000 MProtostarsProtostars originate in such regionsoriginate in such regions

Diagram from Stacey A671 lecture, 1/25/06


Birth of Protostar: Class 0Birth of Protostar: Class 0No complete theory of stellar originsNo complete theory of stellar originsProtostar forms from Protostar forms from gravitational collapse of dense GMC coresgravitational collapse of dense GMC cores

Triggered by shock (?)Triggered by shock (?) from: supernova explosion; ionized gas from: supernova explosion; ionized gas expanding around hot star; galactic spiral density wave ?expanding around hot star; galactic spiral density wave ?“Jeans Criterion”“Jeans Criterion”::

Gravitational Energy > Thermal Energy of a volume of gasGravitational Energy > Thermal Energy of a volume of gas≡≡ Mass of cloud > Jeans MassMass of cloud > Jeans Mass≡≡ MMcc > M> MJJ ≈≈ (5kT / G(5kT / GµµmmHH))3/23/2 (3 / 4(3 / 4πρπρ00))1/21/2

[[ µµ = mean molecular weight; = mean molecular weight; mmHH = H atomic mass; = H atomic mass; ρρ00 = cloud density ] = cloud density ] (Carroll & (Carroll & OstlieOstlie 12.7)12.7)

Ignores effects of rotation (angular momentum), deviation from Ignores effects of rotation (angular momentum), deviation from spherical symmetry, and magnetic fields.spherical symmetry, and magnetic fields.Is still good “rough” estimate of conditions of gravitational coIs still good “rough” estimate of conditions of gravitational collapsellapse


Birth of Protostar: Class 0Birth of Protostar: Class 0Once Jeans Criterion is satisfied…Once Jeans Criterion is satisfied…

Initially MC is optically thin to FIR & Initially MC is optically thin to FIR & submmsubmm such that gravitational such that gravitational potential energy can be radiated awaypotential energy can be radiated away⇒⇒ IsothermalIsothermal dynamical collapsedynamical collapse

LLaccretionaccretion = GM= GM M / RM / R

M = mM = m00aa3 3 / G/ G

[ a = effective sound speed; m[ a = effective sound speed; m00 = constant ~ 1 ] (= constant ~ 1 ] (LadaLada, 2001), 2001)

Collapsing MC is in Collapsing MC is in freefall:freefall:

ttffff = (3= (3π π / 32G/ 32Gρρ00))1/2 1/2 = free= free--fall time fall time (Carroll & (Carroll & OstlieOstlie 12.16)12.16)

For typical mean density (n ~ 10For typical mean density (n ~ 1044 cmcm--33), ), ttffff ~ 4 x 10~ 4 x 1055 yryrCenter will collapse faster than exterior = Center will collapse faster than exterior = nonnon--homologous collapsehomologous collapse

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Birth of Protostar: Class 0Birth of Protostar: Class 0FragmentationFragmentation

Mass of Mass of infallinginfalling envelope within a GMC could well exceed Menvelope within a GMC could well exceed MJJlimit limit Initial Initial inhomogeneitiesinhomogeneities ⇒⇒ sections of collapsing envelope will sections of collapsing envelope will individually satisfy Mindividually satisfy MJJ limit and collapse locallylimit and collapse locallyExplains clusters of stars and lack of extremely highExplains clusters of stars and lack of extremely high--mass mass starsstars

Once core Once core ρρ ~ 10~ 10--1313 g cmg cm--33, the region become optically thick and , the region become optically thick and the the collapse becomes adiabaticcollapse becomes adiabatic rather than isothermal rather than isothermal (gravitational energy cannot be radiated away, and is conserved)(gravitational energy cannot be radiated away, and is conserved)

Rate of contraction in core slows & T risesRate of contraction in core slows & T risesCore is in hydrostatic equilibrium with radius ~ 5AUCore is in hydrostatic equilibrium with radius ~ 5AU

⇒⇒ ““PROTOSTARPROTOSTAR”” is bornis born


Birth of Protostar: Class 0Birth of Protostar: Class 0RotationRotation: as envelope collapses, the core begins to spin: as envelope collapses, the core begins to spin

Initial envelope has some total angular momentum (L) which Initial envelope has some total angular momentum (L) which = sum of individual L’s of constituent molecules= sum of individual L’s of constituent moleculesCollapse + conservation of L Collapse + conservation of L ⇒⇒ rotationrotation

Bipolar molecular outflowsBipolar molecular outflows (“jets”)(“jets”)Very energetic flows of cold molecular gas (discovered via Very energetic flows of cold molecular gas (discovered via mm observations in 1970s and 80s)mm observations in 1970s and 80s)Two spatially separate lobes Two spatially separate lobes moving diametrically away from moving diametrically away from an embedded YSO at hypersonic an embedded YSO at hypersonic speeds, speeds, ⊥⊥ to rotation planeto rotation planeOrigin uncertain: manifestation Origin uncertain: manifestation of underlying primary wind of underlying primary wind generated by generated by protostellarprotostellar corecore

Class 0 YSO


Birth of Protostar: Class 0Birth of Protostar: Class 0ProtostellarProtostellar core is extremely extinguished and embeddedcore is extremely extinguished and embedded

SED has width similar to SED has width similar to singlesingle--temperature BB functiontemperature BB function with with very low temperature (~30K); peaks in very low temperature (~30K); peaks in submmsubmm

Class 0 Class 0 YSOsYSOs are extremely rare (~10% of embedded sources) are extremely rare (~10% of embedded sources) ⇒⇒ lifetime ~ 10lifetime ~ 1044 yryr. (consistent with dynamical estimates of . (consistent with dynamical estimates of their associated outflows)their associated outflows)

Class 0 SED


Class IClass IAbove adiabatic Above adiabatic protostarprotostar core, core, isothermal freeisothermal free--fall accretion fall accretion continues; core continues; core ρρ and T continue to increaseand T continue to increaseWhen freeWhen free--falling material meets hydrostatic core, a falling material meets hydrostatic core, a supersonic supersonic shock front developsshock front developsPrimary stellar winds increasePrimary stellar winds increase

Bipolar JetsBipolar Jets mass > YSO mass mass > YSO mass ⇒⇒ Jet consists of sweptJet consists of swept--up material, up material, not not ejectaejecta; Jets extend ~ 1 pc, hypersonic velocity; Jets extend ~ 1 pc, hypersonic velocityHerbigHerbig--HaroHaro objectsobjects: clumps of shock: clumps of shock--excited gas from collision of excited gas from collision of primary wind with dense ambient cloud material; can extend up toprimary wind with dense ambient cloud material; can extend up to 1 pc1 pcCircumstellarCircumstellar JetsJets: very close to : very close to protostellarprotostellar object (<50AU); most object (<50AU); most common manifestation of primary wind; contain hot ionized gas, scommon manifestation of primary wind; contain hot ionized gas, shaped haped like extended bow shocks that frequently terminate at like extended bow shocks that frequently terminate at HerbigHerbig--HaroHaroobjectsobjectsHH22O masersO masers: small dense areas where microwave transitions in H: small dense areas where microwave transitions in H22O are O are nonlinearly amplified by stimulated emissionnonlinearly amplified by stimulated emission


Class IClass IRotation increases Rotation increases ⇒⇒ circumcircum--protostellarprotostellar disk formsdisk forms

Optically thickOptically thick~ few 100 AU~ few 100 AU

Class I YSO Herbig-Haro object in HH555 in the Pelican Nebula (NOAO: http://www.noao.edu/outreach/press/pr03/pr0308.html#sb-return)


Class IClass ISED isSED is broaderbroader than singlethan single--T BB function and T BB function and shows MIR & NIR shows MIR & NIR excessexcess

Excess IR comes from large amounts of Excess IR comes from large amounts of circumcircum--protostellarprotostellar dustdust10 um silicate absorption feature10 um silicate absorption feature clearly present clearly present ~50% of Class I sources exhibit atomic emission lines in NIR, bu~50% of Class I sources exhibit atomic emission lines in NIR, but t otherwise spectra are featureless and heavily veiled.otherwise spectra are featureless and heavily veiled.

ClassClass I sources are rare among I sources are rare among YSOsYSOs in MCs in MCs ⇒⇒ statistical statistical arguments suggest arguments suggest ages ~ 1 ages ~ 1 -- 5 x 105 x 1055 yryr

Class I SED


Class Class IIaIIaDisk accretion continuesDisk accretion continuesOptically thick disk is fully developedOptically thick disk is fully developedStellar winds very strongStellar winds very strong

Causes envelope to shed/dissipateCauses envelope to shed/dissipateNIR & Optical excessNIR & Optical excess

10um feature still visible10um feature still visible

Class IIa YSOClass IIa SED


Carroll & Ostlie, Figure 12.7

HR-diagram for 1M Protostar (Classes 0, I, & IIa)


ProtostarProtostar to Preto Pre--Main Main SeqSeq Star: Class Star: Class IIbIIbT reaches ~1000K T reaches ~1000K →→ Dust vaporizes Dust vaporizes →→ opacity drops opacity drops →→ T rises T rises T reaches ~2000K T reaches ~2000K →→ HH22 dissociates dissociates →→ absorbs energy necessary absorbs energy necessary for hydrostatic for hydrostatic equilequil. . →→ Second CollapseSecond Collapse (R drops ~30%) (R drops ~30%) →→ Second Second supersonic shock supersonic shock →→ hydrostatic hydrostatic equilequil. reestablished. reestablishedStrong surface Strong surface magnetic activity magnetic activity →→ slows rotationslows rotation rate through rate through interaction with diskinteraction with diskBy now accretion and stellar winds haveBy now accretion and stellar winds have removed what is left of removed what is left of envelopeenvelope →→ Accretion rate slows due to lack of Accretion rate slows due to lack of infallinginfalling mattermatterOptically thick circumOptically thick circum--protostellarprotostellar disk disk →→ Optically thin Optically thin protoplanetaryprotoplanetary diskdisk ((ProplydProplyd); planet formation starts (?)); planet formation starts (?)Optical observations possibleOptical observations possible

ProtostarProtostar becomes a Classical T becomes a Classical T TauriTauri Star (CTTS)Star (CTTS)((ProtostarProtostar →→ PrePre--Main Sequence Star)Main Sequence Star)


ProtostarProtostar to Pre Mainto Pre Main--SeqSeq Star: Class Star: Class IIbIIbCTTSsCTTSs show emission from Hshow emission from H--BalmerBalmer series, Ca II, iron, absorption series, Ca II, iron, absorption lines in Li, and forbidden lines of [OI] and [SII] (lines in Li, and forbidden lines of [OI] and [SII] (⇒⇒ low gas densities)low gas densities)SEDSED

peaks in visible or NIR, with some UV excesspeaks in visible or NIR, with some UV excessbroaderbroader than single BB functionthan single BB function>2>2µµm m →→ falls in falls in powerpower--lawlaw fashionfashion

If disk has concentric annuli with area 2If disk has concentric annuli with area 2ππRR∆∆R, spectrum will be R, spectrum will be series of BB curves with n ~ T(R) ~ Rseries of BB curves with n ~ T(R) ~ R--nn

Luminosity radiated by an annulus is Luminosity radiated by an annulus is LLννddνν = 2= 2ππRdRRdRσσT(R) ~ RT(R) ~ R22--3n3nddνν~~ νν33--2/n2/n ⇒⇒ ννLLνν ~ ~ νν44--2/n2/n

⇒⇒ IR excess arises in IR excess arises in circumstellarcircumstellar diskdisk, slope directly related to T , slope directly related to T gradient in disk (values ~ gradient in disk (values ~ --0.7 to 0.7 to --1.3).1.3).

Age ~ few 10Age ~ few 1066 yryr


ProtostarProtostar to Preto Pre--Main Main SeqSeq Star: Class Star: Class IIbIIb

Power law dist of SED (Lada, 2001)

Class IIb SEDClass IIb YSO


Class IIIClass III

Called PostCalled Post--T T TauriTauri Star (PTTS) or WeakStar (PTTS) or Weak--lined T lined T TauriTauri Star (WTTS)Star (WTTS)SED SED

peaks in visible and NIR and > 2peaks in visible and NIR and > 2µµm drops more steeply than Class II.m drops more steeply than Class II.~ single~ single--T BB T BB ⇒⇒ ~ ~ photoshperephotoshpere of young starof young starHowever light could still be substantially extinguished by dustHowever light could still be substantially extinguished by dust

Little or no HLittle or no Hαα emission (optical astronomers: equiv. width > 10emission (optical astronomers: equiv. width > 10ÅÅ = = CTTS; equiv. width < 10CTTS; equiv. width < 10ÅÅ = WTTS)= WTTS)XX--ray emission: strong variable sourceray emission: strong variable sourceAge > 5 x 10Age > 5 x 1066 yryrT T →→ 10MK 10MK ⇒⇒ H fusion H fusion ≡≡ MainMain--Sequence StarSequence Star

Class III SED Class III YSO


Evolutionary TracksEvolutionary Tracks

Evolutionary tracks of pre-main- sequence stars in the Hertzsprung-Russell diagram for stars from 0.6 to 6 M (from Palla &Stahler, 1993, ApJ, 418, 414).

stellar birth line

zero-age main-sequence (ZAMS)

Observed distribution of low- and intermediate-mass pre-main-sequence stars in the H-R diagram (from Palla &Stahler, 1993, ApJ, 418, 414).

Herbig Ae/Be stars T Tauri stars


Class 0: massive infalling gas and dust envelope, deeply embedded protostar, main accretion phase, molecular outflows, FIR and submm emission

Class I: infalling envelope (~104 AU) + optically thick disk (~ a few 100 AU), luminosity from disk accretion shock, bipolar outflows, infrared emission

Class IIa: optically thick disk, residual envelope, strong stellar wind, disk accretion, outflows, NIR and MIR excess, strong Hα emission

Class III: naked T Tauri star (post T Tauriphase), very little gas and dust in disk, contraction towards main-sequence

Class IIb: “classical” T Tauri star (sometimes “weak-lined” T Tauri star), optically thin disk, decrease in accretion, stellar wind, strong surface magnetic activity; start of planet formation?

(diagram adapted from Wilking, 1989, PASP, 101, 229)(Slide adapted from a presentation by Elise Furlan)

a few to several Myr

tens of Myr

~ 105 yr

a few 104 yr

~ Pr

otos

tar

~ Pr

e-M

ain

Seq.


A Note on HighA Note on High--Mass StarsMass Stars

Previous discussion applies to stars < 7Previous discussion applies to stars < 7--8 M8 MStars > Stars > 77--8 M8 M have very different evolution:have very different evolution:

timescale for Htimescale for H--burning = Kelvinburning = Kelvin--HelmholtzHelmholtz time = time = ttKHKH = = GMGM 22/R/R LL ((LadaLada, 2001), 2001)

~ 10~ 1044 yr for Myr for M = 50 = 50 MM~ 3 x 10~ 3 x 1077 yr for Myr for M = 1 = 1 MM

Recall typical freeRecall typical free--fall time fall time ttffff ~ 4 x 10~ 4 x 1055 yryr⇒⇒ reach mainreach main--sequence before termination of collapse of sequence before termination of collapse of protostellarprotostellar envelopeenvelope


ReferencesReferencesAllen, L.E. et. al., “IRAC Colors of Young Stellar Objects,” Allen, L.E. et. al., “IRAC Colors of Young Stellar Objects,” ApJApJ 2004, 2004, 154:363154:363--366. 366. Carroll & Carroll & OstlieOstlie, , Modern AstrophysicsModern Astrophysics, Addison, Addison--WeselyWesely 19961996LadaLada, Charles J., “Young Stellar Objects,” , Charles J., “Young Stellar Objects,” Encyclopedia of Encyclopedia of Astronomy & AstrophysicsAstronomy & Astrophysics, 2001, 2001((pdfpdf article available on NASA ADS)article available on NASA ADS)

Palla, F., Stahler, S.W., “The Pre-Main-Sequence Evolution of Intermediate-Mass Stars,” ApJ 1993, 418, 414.WilkingWilking, Bruce A., “The Formation of Low Mass Stars,” Astronomical , Bruce A., “The Formation of Low Mass Stars,” Astronomical Society of the Pacific, March 1989, vol. 101, No. 637.Society of the Pacific, March 1989, vol. 101, No. 637.

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