Young stellar objects (YSOs) represent the earliest stage of stellar evolution, providing a direct tracer of star formation in our Galaxy. The Spitzer/IRAC Candidate YSO (SPICY) catalog is one of the largest samples of such objects, containing ~120,000 sources from the Spitzer's GLIMPSE (and related) surveys that exhibit infrared (IR) excess. The YSOs were identified via model-based fitting to select stars with IR excess and statistical learning to separate YSOs from other classes of IR-excess sources. When cross-matched to the Gaia DR3 catalog, Gaia's astrometry can map their 3D distributions in the Galaxy. Highlights from several SPICY studies are summarized here:
1) The spatial distribution of YSOs reveals star formation in the Solar neighborhood (d<3 kpc) to be arranged in several dusty kpc-long filaments. We identified a new high-pitch angle filament containing several famous star-forming regions (e.g., Messier 8, 16, 17, 20). Although these star-forming regions have historically been used to underpin the Sagittarius Arm, this structure's length and high pitch angle better resemble a spur, similar to those identified in other spiral galaxies.
2) Isolated YSOs comprise a significant fraction (~50%) of the SPICY catalog. With Palomar/DBSP spectroscopy of two dozen such stars, we confirmed their YSO nature via placement on the Hertzsprung-Russell diagram, confirmation of IR excess, and spectral-line indicators of accretion. The isolated YSOs tend to be at the periphery of kpc-scale star-forming filaments and suggest that distributed star-formation contributes significantly to the star-formation rate of these structures.
3) Other highlights include the detection of an EX Lup-type outburst from a SPICY YSO and the identification of H2 outflows in JWST imaging of other SPICY YSOs.