Near-infrared characterization of evolved massive stars in M31 and M33

ABSTRACT The upper region of the Hertzsprung–Russell diagram is populated by massive stars in a diversity of evolutionary stages, and the classification of these stars is often based on observed characteristics exclusively in the optical spectral range. The near-infrared regime provides useful complementary information that can help resolving ambiguities in stellar classification and add valuable information about circumstellar envelopes or late-type companions. We present new, near-infrared medium-resolution K-band spectra for a sample of six evolved massive stars, four in M31 and two in M33. The spectra are obtained with the Gemini Near-Infrared Spectrograph (GNIRS) at the Gemini North telescope. We detect CO band emission from the environment of two M31 objects, J004320.97+414039.6 and J004621.08+421308.2, which we classify as B[e] supergiants, with J004320.97+414039.6 being most likely in a post-red supergiant stage. Two objects have pure emission from the hydrogen Pfund series. Of these, we propose that J004415.00+420156.2 in M31 could also be a B[e] supergiant while J013410.93+303437.6 (Var 83) is a well-known luminous blue variable (LBV) in M33. The M31 star J004229.87+410551.8 has a featureless spectrum and its evolutionary stage remains inconclusive; it could be an LBV undergoing an S Dor cycle. The object J013242.26+302114.1 in M33 displays a pure absorption spectrum, including CO bands, consistent with its identification as a cool star. Radial velocity measurements of this red component, combined with modelling of the spectral energy distribution, suggest that J013242.26+302114.1 may be a binary system consisting of an LBV or B[e] supergiant primary and a red supergiant secondary. If confirmed, it would represent the first of its kind.