2010AJ....140..462A

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/cm2, 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/cm2.