The Physical Properties of Yellowballs Associated with Herschel Compact Sources

Yellowballs (YBs) were first discovered during The Milky Way Project citizen-science initiative (MWP; Simpson et al. 2012). Users noticed compact, yellow regions in Spitzer mid-infrared images of the Milky Way plane and asked professional astronomers to explain these “yellow balls.” Follow-up work by Kerton et al. (2015) determined that YBs likely trace compact photo-dissociative regions associated with massive and intermediate mass star formation. YBs were included as a target object in the second version of Milky Way Project (Jayasinghe et al. 2019), which produced a catalog of over 6000 YB locations. We have determined the physical properties of 368 of these YBs within a pilot region covering the 30 to 40-degree longitude range of the Galactic plane. Results from this pilot region study will be used to inform and automate the procedures required to analyze the entire YB catalog. LSR velocities of the YBs were determined using ¹³CO spectra from the Boston University Galactic Ring Survey (Jackson et al. 2006). Velocities were used to obtain distances to the YBs using a Bayesian distance calculator (Reid et al. 2016). Mass, bolometric luminosity, and surface densities were obtained by cross-matching the YB sample to the clumps presented in the Hi-Gal catalog and rescaling their reported values using our calculated distances. Most YB masses fall between 0<log(M_Solar)<4 and most YB luminosities fall between 1<log(L_Solar)<5. The L/M ratios indicate that ~24% of YBs are HII region candidates while surface densities indicate ~20% of the sources are potential sites of high-mass star formation. We performed photometry of the YBs at 8, 12, and 24 microns using GLIMPSE, WISE, and MIPSGAL data respectively (Benjamin et al. 2003, Churchwell et al. 2006, Wright et al. 2010, Carey et al. 2009). Initial analysis of the photometry gives an average 12/8 flux ratio of log(F12/F8) ~ -0.5, lower than the YB sample analyzed in Kerton et al. (2015) (-0.19). The more negative flux ratio value may indicate a larger percentage of compact sources in the new YB sample. The physical properties and colors indicate that most YBs have characteristics consistent with being accreting protostars and while YBs span a wide range of masses and luminosities, a substantial percentage of the YBs in our sample represent intermediate-mass star formation.