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 ~(13)CO 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 ∏ 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 ～ 10~4 L⊙, making them an order of magnitude less luminous than (ultra-)compact ∏ regions. IM SFRs coincide with molecular clumps of mass ～10 ~3 M⊙ which, in turn, lie within larger molecular clouds spanning the lower end of the giant molecular cloud mass range, 10~4-10 ~5 M⊙. The IR luminosity and associated molecular mass of IM SFRs are correlated, consistent with the known luminosity-mass relationship of compact Hu regions. Peak mass column densities within IM SFRs are ～0.1-0.5 g cm~(-2), a factor of several lower than ultracompact H∏ regions, supporting the proposition that there is a threshold for massive star formation at ～1 g cm~(-2).