A SAMPLE OF INTERMEDIATE-MASS STAR-FORMING REGIONS: MAKING STARS AT MASS COLUMN DENSITIES <1 g cm{sup -2}

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) {approx}8 M{sub sun} are being produced. We use IRAS colors and Spitzer Space Telescope mid-IR images, in conjunction with millimeter continuum and {sup 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 II regions, but they lack the massive ionizing central star(s). The photodissociation regions that demarcate IM SFRs have typical diameters of {approx}1 pc and luminosities of {approx}10{sup 4} L{sub sun}, making them an order of magnitude less luminous than (ultra-)compact H II regions. IM SFRs coincide with molecular clumps of mass {approx}10{sup 3} M{sub sun} which, in turn, lie within larger molecular clouds spanning the lower end of the giant molecular cloud mass range, 10{sup 4}-10{sup 5} M{sub sun}. 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 {approx}0.1-0.5 g cm{sup -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 {approx}1 g cm{sup -2}.