IRAC/MIPS view of the ISM: PAHs dominate in IRAC

Li & Draine (2001) dust emissivity model; only very strong (> 103 x avg.) radiation fields contribute to thermal emission at 8 m

Our entire mosaic is remarkably constant in CH4/Ch3 ratio (~ 3) => PAH-dominated with a few exceptions near HII regions

Massive stars are rare, but once a single massive star forms, the game changes. •L ~ 105 – 106 L

• Intense EUV, FUV radiation

• Powerful winds

• Supernova

This energy input quickly reshapes the environment, dominating all that goes on there, perhaps including low-mass star formation.

Star formation near massive stars is NOTNOT Taurus-Auriga writ large!!!!

Disks in Orion

Disks in Taurus-Auriga

Structure of H II region carved by radiation from massive stars.

Dark – Molecular gas

Blue – Hot (~104 K), tenuous (~30 cm-3), ionized cavity. The H II region interior.

Massive stars

Red – Ionization front pushing into and evaporating dense gas

Dense gas compressed in advance of the ionization front

• Isolated star formation, such as that seen in Taurus-Auriga is atypical. The majority of low-mass, Sun-like stars form instead in rich clusters and in proximity to massive stars.

• Short-Lived Radionuclides (SLRs) require that the Sun formed near a massive star.

• Solar mass stars form in dense gas, then are soon uncovered by the ionization front, leaving young disks sitting in a hot, tenuous region near one or more massive stars.

Direct countsE.g., Lada & Lada, 2003, Ann. Rev. Astron. Astrophys, 41, 57

Complete survey of embedded clusters out to 2 kpc:- 70 – 90% of stars form in dense clusters- Of these, 75% currently near massive stars

• Triggered Star Formation may be the standard mode of star formation, even in turbulent environments.

Embedded YSOs - 4 kpc*

< 2 Msun2 < Msun < 5 > 5 Msun

*AssumingGLIMPSEsensitivities

Limitations of Color-Color Analysis

• You need to detect the source in all four IRAC bands

• Does not work well in crowded regions• There is significant ambiguity in

interpreting the results– Does not discriminate different masses well– Although the effects of extinction are much

smaller at these wavelengths, there are still degeneracies typical for stars at the GC

• Solution: Fit ALL the SEDs

Tests on M16 data

Embedded Disk Embedded or disk

M16: SED Fitter at Work (Whitney et al. 2003a,b,2004; Indebetouw et al. 2006; Robitaille et al. 2006)

Stage I: YellowStage II: GreenStage III: Blue

• A by-product of the fitting program is a best-fit extinction to each source• Includes the MS and Giant stars