2021ApJ...911...28W

Yellowballs (YBs) were first discovered during the Milky Way Project (MWP) citizen science initiative. The MWP users noticed compact, yellow regions in Spitzer Space Telescope mid-infrared (MIR) images of the Milky Way plane and asked professional astronomers to explain these "yellow balls." Follow-up work by Kerton et al. determined that YBs likely trace compact photodissociation regions associated with massive and intermediate-mass star formation. The YBs were included as target objects in a version of the MWP launched in 2016, which produced a listing of over 6000 YB locations. We have measured distances, cross-match associations, physical properties, and MIR colors of ∼500 YBs within a pilot region covering the l = 30°-40°, b = ±1° region of the Galactic plane. We find that ∼20%-30% of YBs in our pilot region contain high-mass star formation capable of becoming expanding H II regions that produce MIR bubbles. A majority of YBs represent intermediate-mass star-forming regions whose placement in evolutionary diagrams suggest they are still actively accreting and may be precursors to optically revealed Herbig Ae/Be nebulae. Many of these intermediate-mass YBs were missed by surveys of massive star formation tracers; thus, this catalog provides information for many new sites of star formation. Future work will expand this pilot region analysis to the entire YB catalog.