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Massive Star Formation under Different Z & Galactic Environment

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. Kennicutt (1998). Massive Stars -- energy source of the interstellar medium (ISM)

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Massive Star Formation under Different Z & Galactic Environment

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  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 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 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+24m -- <SFR> in last 10 Myr (Calzetti et al.2007) GMC life time: <10 or >30 Myr (depends on whom you ask)

  3. 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+24m -- <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 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 kpc2), S3+SAGE-SMC (106~106 kpc2) (Meixner et al. 2006; Bolatto et al. 2007; Gordon et al. 2010)

  5. LMC+Bridge as a Lab for MSF SMC LMC Bridge • 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 kpc2), S3+SAGE-SMC (106~106 kpc2) (Meixner et al. 2006; Bolatto et al. 2007; Gordon et al. 2010)

  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 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)

  8. 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

  9. YSO Properties in GMCs -- LMC cases (Chen et al. 2009, 2010) H image CO contours :17 Mo,:8-17Mo, :8 Mo, :unknown • 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.

  10. Mvir/Mlum Unstable   High SFI Indebetouw et al. (2008) YSO Properties v.s. GMC Properties (Chen et al. 2009, 2010a) X • 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)

  11. :17 Mo,:8-17Mo, :8 Mo, :unknown Energy Feedback vs. Triggered SF (Chen et al. 2009, 2010) H image CO contours • 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.

  12. :YSO, :fainter YSO, :HAeBe cand. 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

  13. :YSO, :fainter YSO, :HAeBe cand. :FUV-bright stars MSF Modes vs Triggered SF (Chen et al. 2009, 2010a) • Clustering of massive YSOs: sparsely distributed, largest cluster in most massive cloud (7103M; 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

  14. 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 for different modes of MSF (Chen et al. 2010, 2011)

  15. Inefficiency of MSF @ low N(HI) (Chen et al. 2009, 2010a) 1 10 100 Bigiel et al. 2010 • 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) • small fraction forming GMCs/YSOs • threshold (Z,n) to form H2(Krumholz et al. 2009; Glover et al. 2010)

  16. comp (1020/cm2) SFEs across Z & galactic environments Bridge (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) :17 Mo,:8-17Mo, :8 Mo, :unknown

  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: 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 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

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