Massive Star Formation under Different Z & Galactic

Environment

Rosie Chen (University of Virginia)

Remy Indebetouw, You-Hua Chu, Robert Gruendl, Gerard Testor, & the SAGE team

Motivation

• Massive Stars -- energy source of the interstellar medium (ISM) -- affect evolution of their host galaxy

• Giant Molecular Clouds -- where most stars are formed -- how does stellar energy feedback affect GMC evolution?

• Quantifying Massive Star Formation in GMCs -- variations in star formation intensities in GMCs -- Kennicutt-Schmidt Relation (Kennicutt 1989)

SFR = A (Gas)N, N=1.0-1.4

SFR tracers: UV -- <SFR> in last 100 Myr (Leitherer et al. 1995)

H+24m -- <SFR> in last 10 Myr (Calzetti et al.2007)

GMC life time: <10 or >30 Myr (depends on whom you ask)

Kennicutt (1998)

Motivation

• Quantifying Massive Star Formation in GMCs SFR = A (Gas)N, N=1.0-1.4 (Kennicutt 1989)

SFR tracers: UV -- <SFR> in last 100 Myr (Leitherer et

al. 1995)

H+24m -- <SFR> in last 10 Myr (Calzetti et al.2007)

GMC life time: <10 or >30 Myr (depends on whom you ask)

SFI = SFI(SFE, time) K-S relation: <time> or <SFE>?

• Metallicity changes everything? lower Z & greater permittivity to UV of the ISM may affect dynamics of cloud formation, cooling & feedback from YSOs (Poglitsch

et al. 1995)

Studying massive YSOs in GMCs is the Studying massive YSOs in GMCs is the most direct way to connect MSF on GMC & most direct way to connect MSF on GMC &

kpc scales!kpc scales!

LMC+Bridge as a Lab for MSF

• Nearby, known distances YSOs can be resolved • Reduced metallicity; Tidal environment

Do these factors affect GMC/star formation?

• Large Spitzer surveys SAGE-LMC (88~7.27.2 kpc2), S3+SAGE-SMC (106~106 kpc2) (Meixner et al. 2006; Bolatto et al. 2007; Gordon et al. 2010)

LMC Bridge

kpc 50 55

Z 1/3 1/5-1/8

LMC+Bridge as a Lab for MSF

• Nearby, known distances YSOs can be resolved • Reduced metallicity; Tidal environment

Do these factors affect GMC/star formation?

• Large Spitzer surveys SAGE-LMC (88~7.27.2 kpc2), S3+SAGE-SMC (106~106 kpc2) (Meixner et al. 2006; Bolatto et al. 2007; Gordon et al. 2010)

LMC

SMC

Bridge

LMC Bridge

kpc 50 55

Z 1/3 1/5-1/8

Questions to be addressed

• What are massive YSOs? Can massive YSOs be reliably identified? What are their physical properties?

• Do they form in special conditions? What are the properties of massive YSOs among GMCs? Is there triggered star formation?

• How long will MSF continue in GMCs? How do massive stars/YSOs affect natal cloud and nearby YSOs?

• Does MSF happen the same everywhere? Is there dependence on metallicity, galactic environment?

Identification of massive YSOs

Step 1:Select YSO candidates w/ [8.0] vs [4.5]-[8.0] CMD:

[4.5]-[8.0] > 2.0 exclude normal stars & AGBs (Groenewegen 2006)

[8.0] < 14 - ([4.5]-[8.0]) exclude galaxies (Harvey et al.

2006), low-mass (≤ 4M)/evolved YSOs

Step 2: cull out contaminants w/ multi- SED (0.35-70 m) & high-res image examination crucial for Spitzer’s 2˝-resolution

YSOcandidates

Galaxies &Low-mass/evolvedYSOs

Sta

rs

AG

Bs,

pos

t-A

GB

s

~300 IRS obs of LMC YSOs confirm a > ~300 IRS obs of LMC YSOs confirm a > 95% correct rate w/ our method 95% correct rate w/ our method (Seale et al. (Seale et al.

2009)2009)

Infer YSO properties using SED fits

N44 N159 YSO properties inferred by comparing obs SEDs to large pre-calculated model grids (Robitaille et

al. 2007).

• YSOs mostly found in GMCs in 2 HII complexes (350x450 pc2,180x190 pc2) YSO mass (M): 4-45 evolutionary stage: I,II,III

• GMCs show a wide range of YSO mass, clustering.

YSO Properties in GMCs -- LMC cases

H imageCO contours

:17 Mo, :8-17Mo, :8 Mo, :unknown

(Chen et al. 2009, 2010)

YSO Properties v.s. GMC Properties

(Chen et al. 2009, 2010a)

X

Mvir/M

lum

Indebetouw et al. (2008)

High SFIUnst

able

• MYSO anti-correlates weakly w/ Mvir/Mlum (Indebetouw et al. 2008), ∆V small Eturb large-volume collapse (Klessen et al. 1998)

• SFEYSO ~2x10-4-9x10-3 ~1-25x Pipe Nebula (Forbrich et al. 2009)

• Massive YSOs preferentially found near ionized gas energy feedback significant in MSF. • YSOs distributed along superbubble rims, HII region edges likely triggered by the shell expanding into the GMC.

Energy Feedback vs. Triggered SF

H imageCO contours

:17 Mo, :8-17Mo, :8 Mo, :unknown

(Chen et al. 2009, 2010)

YSO Properties in the Bridge (Chen et al. 2009, 2010a)

• 23 embedded YSOs identified in the Bridge (in 2.9x1.3 kpc2 ): M*: 4-10 M v.s. LMC,SMC -- M*: 4-45 M

• Scarce CO coverage; most YSOs found in N(HI) > 8x1020 cm-2.

• All but 1 molecular cloud have YSOs SF starts quickly

:YSO, :fainter YSO, :HAeBe cand.

MSF Modes vs Triggered SF (Chen et al. 2009, 2010a)

• Clustering of massive YSOs: sparsely distributed, largest cluster in most massive cloud (7103M; Mizuno

et al. 2006) lower dust shielding, more difficult to form clusters/GMCs ? (Krumholz et al 2009)

• YSO found in edges of H blobs & shell rims maybe triggered -- needs high-res H+CO obs

:YSO, :fainter YSO, :HAeBe cand. :FUV-bright stars

• SFRH+24m vs SFRYSO: <SFR> in last 10 vs 1 Myr. • In GMCs w/ bright HII,

SFRYSO/SFRH+24~ 0.4-2.1 depend on GMC evolutionary stage

• In GMCs w/o bright HII, SFRH+24~0.02-0.1 HI+H2 SFRH+24m requires fully sampled IMF, not applicable to poor (small) clusters.

Resolved SFR in GMCs in LMC & Bridge

:17 Mo, :8-17Mo, :8 Mo, :unknown

(Chen et al. 2009, 2010a)

SFR not const. in 10 Myr.

reconsider SFRHa+24 for different modes of MSF

(Chen et al. 2010, 2011)

• SFRYSO : YSO ~ A Gas1.4 for GMCs in Bridge

YSO ~0.04 A Gas1.4 for entire Bridge (MHI

~5x107 M) • Low SFRFUV in outer disks of galaxies (Bigiel et al.

2010)

Inefficiency of MSF @ low N(HI)(Chen et al. 2009, 2010a)

small fraction forming GMCs/YSOs threshold (Z,n) to form H2 (Krumholz et al. 2009; Glover

et al. 2010)

Bigiel et al. 2010

1 10 100

SFEs across Z & galactic environments

:17 Mo, :8-17Mo, :8 Mo, :unknown

(Chen et al. 2009, 2010a)

• SFE: = N(YSO)/N[N(HI)]

• N(HI) > 8x1020 cm-2: Bridge < 1/2 LMC metallicity? (Krumholz et al. 2009)

N(HI) < 8x1020 cm-2: Bridge > LMC tidal effect? colliding flows? (Heitsch et al. 2006)

Bridge

comp (1020/cm2)

Conclusion

• What are massive YSOs? -- can be reliably identified w/ multi- SED+image examination Bridge LMC -- YSO mass range (M): 4-10 4-45 evolutionary stage: I,II,III I,II,III

• Do they form in special conditions? -- massive YSO formation may depend on GMC instability -- energy feedback important; promising cases of triggered MSF

• How long will MSF continue in GMCs? -- bright HII: SFRYSO/SFRH+24m ~ 0.4-2.1, depending on GMC stages -- faint HII: SFRYSO/SFRH+24m ~ 10-50 reconsider modes of MSF

• Does MSF happen the same everywhere? -- indication of metallicity & tidal effects in the Bridge

next: Herschel critical for studying youngest YSOs

next: Larger time & feedback strength coverage

next: ALMA critical for studying dense clumps

next: Mopra survey of MCs in Bridge; larger Z & gal. ranges