Searching for Galactic red supergiants with Gaia RVS spectra

Abstract

Red supergiants (RSGs) are essential to understanding the evolution and the contribution to the interstellar medium of massive stars. However, the number of identified RSGs within the Milky Way is still limited, mainly due to the difficulty of measuring stellar extinction and distance. The release of approximately one million RVS spectra in Gaia DR3 presents a new opportunity for identifying Galactic RSGs because the equivalent width of the calcium triplet lines (EW(CaT)) in the spectra is an excellent indicator of stellar surface gravity. We used RVS spectra with a signal-to-noise ratio (S/N) greater than 100 to search for Galactic RSGs. We removed dwarf stars and red giants and selected RSG candidates based on their location in the EW(CaT) versus BP ‑ RP diagram. Early-type RSG candidates (K0-M2) were then identified using the criteria BP ‑ RP > 1.584 and EW(CaT) > 1.1 nm. The criteria of the average equivalent widths of TiO in the XP spectra (EW(TiO)) > 10 nm, the color index K ‑ W3 < 0.5, and the period-amplitude sequence from the Gaia DR3 LPV catalog were further applied to remove late-type red giants (after M2) and asymptotic giant branch stars. This method yielded 30 early-type (K0-M2) and 6196 late-type (after M2) RSG candidates, which is a significant increase compared to the previous Galactic RSG sample. The application of this approach to spectra with S/N > 50 resulted in 48 early-type and 11 491 late-type RSG candidates. This preliminary analysis paves the way for more extensive research with Gaia DR4, when larger spectral datasets are expected to significantly enhance our understanding of Galactic RSG populations.