Spitzer view of massive star formation in the magelanic bridge
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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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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)

    -- 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!


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


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 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)


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


: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


: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


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


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


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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