Ultracool Spectroscopic Outliers in Gaia DR3

Abstract

Gaia DR3 provided a first release of RP spectra and astrophysical parameters for ultracool dwarfs (UCDs). We used these Gaia RP spectra and astrophysical parameters to select the most outlying UCDs. These objects have spectral types of M7 or later and might be young brown dwarfs or low-metallicity objects. This work aimed to find UCDs that have Gaia RP spectra significantly different to the typical population. However, the intrinsic faintness of these UCDs in Gaia means that their spectra were typically rather low signal-to-noise ratio in Gaia DR3. This study is intended as a proof of concept for future iterations of the Gaia data releases. Based on well-studied subdwarfs and young objects, we created a spectral type-specific color ratio, defined using Gaia RP spectra; this ratio is then used to determine which objects are outliers. We then used the objects kinematics and photometry external to Gaia to cut down the list of outliers into a list of ‘prime candidates’. We produce a list of 58 Gaia RP spectra outliers, seven of which we deem as prime candidates. Of these, six are likely subdwarfs and one is a known young stellar object. Four of six subdwarf candidates were known as subdwarfs already. The two other subdwarf candidates, namely 2MASS J03405673 + 2633447 (sdM8.5) and 2MASS J01204397 + 6623543 (sdM9), are new classifications.