The SPICY Milky Way: Evaluating the Reliability of an Inner Galactic Plane YSO Catalog

Young stellar objects (YSOs) are stars in the early stages of their evolution that frequently exhibit infrared-excess emission from dusty, protoplanetary disks. The Spitzer IRAC Candidate YSO (SPICY) Catalog is the most comprehensive list of YSOs in the Galactic midplane compiled to date, containing ~120,000 objects identified from GLIMPSE source lists. We selected a combined 876 candidate YSOs in the SPICY Catalog in two massive star-forming regions within 10° (projected) of the Galactic center, NGC 6334 and NGC 6357, to evaluate contamination rates in one of the most crowded sightlines of the Galactic midplane. We supplement SPICY with Chandra/ACIS X-ray-identified YSOs, which expands the area of mid-IR color space to include stars with weaker IR excess. The most likely contaminating sources would be highly-reddened background giants. We find that the H – K_S versus K_S – [4.5] color-color diagram is the most useful to evaluate SPICY contamination. The sources separate into two loci: a larger, lower locus containing YSOs showing intrinsic infrared excess and an upper locus strongly affected by foreground reddening that hosts a mix of X-ray YSOs and likely contaminating sources. The upper locus reveals a higher ratio of sources with E(H – K_S) > 1.5 compared to the lower locus, representing an estimated 3% contamination rate in the SPICY Catalog for these regions. We also examine areas of the mid-plane without massive star-forming regions, where the YSOs are more widely distributed on the sky, and contamination rates are expected to be higher. Finally, we construct a comprehensive membership list of ~1,600 sources for NGC 6334, including both SPICY and Chandra/Gaia-identified young stars and YSOs.