A Novel Survey for Young Substellar Objects with the W-band filter II. The Coolest and Lowest Mass Members of the Serpens-South Star-forming Region

Given its relative proximity (similar to 430 pc), compact size (<20 '), young age (similar to 0.5 Myr), and rich number of young stellar objects, the Serpens-South star-forming region is a promising site for studying young substellar objects, yet the low-mass members of this region remain largely undiscovered. In this article we report on a deep photometric survey using a custom 1.45 mu m filter (W band), as well as standard J and H near-IR filters, in order to identify candidate low-mass young brown dwarfs in the Serpens-South region. We constructed a reddening-insensitive index (Q) by combining J, H and W-band photometry for survey objects, in order to identify candidate low-mass members of Serpens based on the strength of the water-absorption feature at 1.45 mu m in the atmospheres of mid-M and later objects. We then conducted spectroscopic follow-up to confirm youth and spectral type for our candidates. This is the first survey to identify the very low-mass and coolest members of Serpens-South. We identify four low-mass candidate Serpens members, which all display IR excess emission, indicating the likely presence of circumstellar disks around them. One of the four candidate low-mass members in our list, SERP182918-020245, exhibits Pa beta and Br gamma emission features, confirming its youth and ongoing magnetospheric accretion. Our new candidate members have spectral types >M4 and are the coolest and lowest mass candidate members yet identified in Serpens-South.