A SAMPLE OF INTERMEDIATE-MASS STAR-FORMING REGIONS: MAKING STARS AT MASS COLUMN DENSITIES 1?g?cm–2

摘要

In an effort to understand the factors that govern the transition from low- to high-mass star formation, for the first time we identify a sample of intermediate-mass star-forming regions (IM SFRs) where stars up to (but not exceeding) ~8 M ☉ are being produced. We use IRAS colors and Spitzer Space Telescope mid-IR images, in conjunction with millimeter continuum and 13CO maps, to compile a sample of 50 IM SFRs in the inner Galaxy. These are likely to be precursors to Herbig AeBe stars and their associated clusters of low-mass stars. IM SFRs constitute embedded clusters at an early evolutionary stage akin to compact H II regions, but they lack the massive ionizing central star(s). The photodissociation regions that demarcate IM SFRs have typical diameters of ~1 pc and luminosities of ~104 L ☉, making them an order of magnitude less luminous than (ultra-)compact H II regions. IM SFRs coincide with molecular clumps of mass ~103 M ☉ which, in turn, lie within larger molecular clouds spanning the lower end of the giant molecular cloud mass range, 104-105 M ☉. The IR luminosity and associated molecular mass of IM SFRs are correlated, consistent with the known luminosity-mass relationship of compact H II regions. Peak mass column densities within IM SFRs are ~0.1-0.5?g?cm–2, a factor of several lower than ultra-compact H II regions, supporting the proposition that there is a threshold for massive star formation at ~1?g?cm–2.