1. 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

2. 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 -- in last 100 Myr (Leitherer et al. 1995) H  +24  m -- in last 10 Myr (Calzetti et al.2007) GMC life time: 30 Myr (depends on whom you ask) Kennicutt (1998)

3. Motivation Quantifying Massive Star Formation in GMCs  SFR = A (  Gas ) N, N=1.0-1.4 (Kennicutt 1989)  SFR tracers: UV -- in last 100 Myr (Leitherer et al. 1995) H  +24  m -- in last 10 Myr (Calzetti et al.2007) GMC life time: 30 Myr (depends on whom you ask) SFI = SFI(SFE, time)  K-S relation: or ? 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 most direct way to connect MSF on GMC & kpc scales!

4. 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 (8  8  ~7.2  7.2 kpc 2 ), S 3 +SAGE-SMC (10  6  ~10  6 kpc 2 ) (Meixner et al. 2006; Bolatto et al. 2007; Gordon et al. 2010) LMCBridge kpc5055 ZZ 1/31/5-1/8

5. 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 (8  8  ~7.2  7.2 kpc 2 ), S 3 +SAGE-SMC (10  6  ~10  6 kpc 2 ) (Meixner et al. 2006; Bolatto et al. 2007; Gordon et al. 2010) LMC SMC Bridge LMCBridge kpc5055 ZZ 1/31/5-1/8

6. 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?

7. 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 YSO candidates Galaxies & Low-mass/evolved YSOs Stars AGBs, post-AGBs ~300 IRS obs of LMC YSOs confirm a > 95% correct rate w/ our method (Seale et al. 2009)

8. Infer YSO properties using SED fits N44N159 YSO properties inferred by comparing obs SEDs to large pre-calculated model grids (Robitaille et al. 2007).

9. YSOs mostly found in GMCs in 2 HII complexes (350x450 pc 2,180x190 pc 2 ) 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  image CO contours :  17 Mo, :8-17Mo, :  8 Mo, :unknown (Chen et al. 2009, 2010)

10. YSO Properties v.s. GMC Properties (Chen et al. 2009, 2010a) X M vir /M lum Indebetouw et al. (2008)  High SFI Unstable   M YSO anti-correlates weakly w/ M vir /M lum (Indebetouw et al. 2008), ∆V  small E turb  large-volume collapse (Klessen et al. 1998) SFE YSO ~2x10 -4 -9x10 -3 ~1-25x Pipe Nebula (Forbrich et al. 2009)

11. 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  image CO contours :  17 Mo, :8-17Mo, :  8 Mo, :unknown (Chen et al. 2009, 2010)

12. YSO Properties in the Bridge (Chen et al. 2009, 2010a) 23 embedded YSOs identified in the Bridge (in 2.9x1.3 kpc 2 ): M * : 4-10 M  v.s. LMC,SMC -- M * : 4-45 M  Scarce CO coverage; most YSOs found in N(HI) > 8x10 20 cm -2. All but 1 molecular cloud have YSOs  SF starts quickly :YSO, :fainter YSO, :HAeBe cand.

13. MSF Modes vs Triggered SF (Chen et al. 2009, 2010a) Clustering of massive YSOs: sparsely distributed, largest cluster in most massive cloud (7  10 3 M  ; 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

14. SFR H  +24  m vs SFR YSO : in last 10 vs 1 Myr. In GMCs w/ bright HII, SFR YSO /SFR H  +24 ~ 0.4-2.1  depend on GMC evolutionary stage In GMCs w/o bright HII, SFR H  +24 ~0.02-0.1  HI+H2  SFR H  +24  m 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 SFR Ha+24 for different modes of MSF (Chen et al. 2010, 2011)

15. SFR YSO :  YSO ~ A  Gas 1.4 for GMCs in Bridge  YSO ~0.04 A  Gas 1.4 for entire Bridge (M HI ~5x10 7 M  ) Low SFR FUV 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 H 2 (Krumholz et al. 2009; Glover et al. 2010) Bigiel et al. 2010 1 10 100

16. 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) > 8x10 20 cm -2 :  Bridge  LMC  tidal effect? colliding flows? (Heitsch et al. 2006) Bridge  comp (10 20 /cm 2 )

17. 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: SFR YSO /SFR H  +24  m ~ 0.4-2.1, depending on GMC stages -- faint HII: SFR YSO /SFR H  +24  m ~ 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