Identification of late-type stars of class I

TL;DR

This study evaluates Gaia DR3 parameters for late-type stars of luminosity class I, identifying new red supergiants and classifying their spectral types with high accuracy, highlighting the importance of spectral inspection for accurate classification.

Contribution

It introduces a method combining Gaia DR3 data, infrared photometry, and spectral library comparison to accurately classify late-type supergiants and identify new RSGs.

Findings

15 S-type stars identified by ZrO and LaO features

20 new RSGs confirmed, 6 brighter than AGB limit

K- and M-type classification accurate within two subtypes

Abstract

Aims. Gaia DR3 GSP-Phot and GSP-Spec parameters of known K- and M-type stars with luminosity class I are examined and compared with parameters collected from the literature, to assess their accuracy and their potential for stellar classification of unknown intrinsically bright late-types. Gaia DR3 GSP-Phot and GSP-Spec parameters were generated by the Astrophysical Parameters Inference Software (Apsis). Methods. In the Gaia DR3 catalog, there are about 40,000 entries with Apsis parameters similar to those of known red supergiants, RSGs, good parallaxes, and infrared 2MASS and WISE data. By using parallactic distances, infrared photometry, and variability information, only 203 new entries are found with luminosities and temperatures similar to that of known RSGs and G-band amplitudes smaller than 0.5 mag. Their low-resolution BP/RP spectra are compared with an empirically made spectral library of BP/RP spectra of known bright late-type stars (C-rich, S-type, O-rich asymptotic giant branch stars (AGBs), and RSGs) to obtain their spectral types. Results. Among them, 15 S-type stars are identified by peculiar absorption features due to ZrO and LaO visible in their BP/RP spectra, one S/C star, and nine C-rich stars by their strong CN absorption bands. K- and M-types can be reproduced with an accuracy of two subtypes. 20 new RSGs are confirmed, of which six have bolometric magnitudes brighter than those of the AGB limit: 2MASS J21015501+4517205, 2MASS J16291280-4956384, 2MASS J10192621-5818105, 2MASS J20230860+3651450, 2MASS J17084131-4026595, and 2MASS J16490055-4217328. The flag for C-rich stars of the Gaia DR3 LPV pipeline is erroneously true for some RSGs and a visual inspection of the BP/RP spectra is mandatory.