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Spitzer View of Massive Star Formation in the Magelanic Bridge

Spitzer View of Massive Star Formation in the Magelanic Bridge. Rosie Chen (University of Virginia) Remy Indebetouw , Karl Gordon, Greg Sloan, & the SAGE-SMC team. Motivation. Kennicutt (1998). Massive Stars -- energy source of the interstellar medium (ISM)

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Spitzer View of Massive Star Formation in the Magelanic Bridge

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  1. Spitzer View of Massive Star Formation in the Magelanic Bridge Rosie Chen(University of Virginia) Remy Indebetouw, Karl Gordon, Greg Sloan, & the SAGE-SMC team

  2. Motivation Kennicutt (1998) • 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 MSF 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+24m -- <SFR> in last 10 Myr (Calzetti et al.2007) SFI = SFI(SFE, time)  K-S relation: <time> or <SFE>? Studying YSOs in GMCs is the most direct way to connect MSF on GMC & kpc scales!

  3. Bridge as a lab for MSF HI SMC LMC Bridge • Nearby, known distances  YSOs can be resolved Bridge: 55 kpc • Large Spitzer surveys SAGE-SMC (PI: Gordon) • reduced metallicity (Bridge: 1/5-1/8 Z),tidal environment  Do these factors affect GMC/star formation?  SF in early Universe

  4. 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?

  5. Identification of massive YSOs Step 1: Select YSO candidates with [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 YSO candidates Stars AGBs, post-AGBs Galaxies & Low-mass/evolved YSOs ~300 IRS obs of LMC YSOs confirm a > 95% correct rate w/ our method (Seale et al. 2009)

  6. Infer YSO properties using SED fits YSO properties inferred by comparing obs SEDs to large pre-calculated model grids (Robitaille et al. 2007). N44 N159

  7. :YSO, :evolved YSO, :HAeBe cand. YSO Properties in the Bridge (Chen et al. 2009, 2010a) • 21 embedded YSOs identified in the Bridge (in 1.3x2.9 kpc2 ):M*: 4-10 M v.s. LMC -- 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

  8. :YSO, :evolved YSO, :HAeBe cand. SF Modes vs Triggered SF (Chen et al. 2009, 2010a) • YSOs mostly found in “isolated” mode; “clustered” mode in Cloud C, the most massive cloud  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

  9. Bigiel et al. 2009 Resolved SFR in GMCs in LMC & Bridge • SFRH+24m 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+24m requires fully sampled IMF, not applicable to poor (small) clusters. (Chen et al. 2009, 2010a)  SFR not const. in 10 Myr. :17 Mo,:8-17Mo, :8 Mo, :unknown  reconsider SFRHa+24 w/ different modes of MSF

  10. comp (1020/cm2) SFEs across Z & galactic environments Bridge (Chen et al. 2009, 2010a) • SFE:  = N(YSO)/N[N(HI)] • N(HI)>12x1020 cm-2: Bridge ~1/3 LMC metallicity? (Krumholz et al. 2009) N(HI)=4-12x1020 cm-2: Bridge LMC tidal effect? colliding flows? (Heitsch et al. 2006) :17 Mo,:8-17Mo, :8 Mo, :unknown

  11. 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? -- SF in isolated mode; clustered mode in most massive CO cloud -- possible triggered SF  energy feedback important • How long will MSF continue in GMCs? -- bright HII: SFRYSO/SFRH+24m ~ 0.4-2.1, depending on GMC stages -- faint HII: SFRYSO/SFRH+24m ~ 10-50  reconsider modes of MSF • Does MSF happen the same everywhere? -- indication of metallicity & tidal effects in the Bridge

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