The first transition Wolf-Rayet WN/C star in M31

TL;DR

This paper reports the discovery of the first transition Wolf-Rayet star in M31, providing spectral analysis and contextual comparison with similar stars in other galaxies, highlighting its significance in stellar evolution studies.

Contribution

The paper presents the serendipitous discovery and spectral classification of M31 WR 84-1 as the first transition WN/C star in M31, expanding understanding of WR star diversity.

Findings

First transition WR star identified in M31

Spectral classification as WN5/WC6

Presence of surrounding nebula with solar metallicity

Abstract

Three decades of searches have revealed 154 Wolf-Rayet (WR) stars in M31, with 62 of WC type, 92 of WN type and zero of transition type WN/C or WC/N. In apparent contrast, about two percent of the WR stars in the Galaxy, the LMC and M33 simultaneously display strong lines of carbon and nitrogen, i.e. they are transition type WN/C or WC/N stars. We report here the serendipitous discovery of M31 WR 84-1, the first transition star in M31, located at RA = 00:43:43.61 DEC = +41:45:27.95 (J2000). We present its spectrum, classify it as WN5/WC6, and compare it with other known transition stars. The star is unresolved in Hubble Space Telescope narrowband and broadband images, while its spectrum displays strong, narrow emission lines of hydrogen, [NII], [SII] and [OIII]; this indicates a nebula surrounding the star. The radial velocity of the nebular lines is consistent with that of gas at the same position in the disc of M31. The metallicity at the 11.8 kpc galactocentric distance of M31 84-1 is approximately solar, consistent with other known transition stars. We suggest that modest numbers of reddened WR stars remain to be found in M31.