Wolf-Rayet stars in M81: Detection and Characterization using GTC/OSIRIS spectra and HST/ACS images

TL;DR

This study reports the detection and characterization of Wolf-Rayet stars in M81 using GTC/OSIRIS spectra and HST/ACS images, revealing their types, properties, and evolutionary implications.

Contribution

First detailed identification and classification of Wolf-Rayet stars in M81 combining spectroscopic and imaging data, highlighting discrepancies with evolutionary model predictions.

Findings

Identified 12 Wolf-Rayet stars of various types in M81

Transitional WN/C stars are more numerous than models predict

Spectroscopic and photometric properties align with Milky Way W-R stars

Abstract

We here report the properties of Wolf-Rayet (W-R) stars in 14 locations in the nearby spiral galaxy M81. These locations were found serendipitously while analysing the slit spectra of a sample of ~150 star-forming complexes, taken using the long-slit and Multi-Object spectroscopic modes of the OSIRIS instrument at the 10.4-m Gran Telescopio Canarias. Colours and magnitudes of the identified point sources in the Hubble Space Telescope images compare well with those of individual W-R stars in the Milky Way. Using templates of individual W-R stars, we infer that the objects responsible for the observed W-R features are single stars in 12 locations, comprising of 3 WNLs, 3 WNEs, 2 WCEs and 4 transitional WN/C types. In diagrams involving bump luminosities and the width of the bumps, the W-R stars of the same sub-class group together, with the transitional stars occupying locations intermediate between the WNE and WCE groups, as expected from the evolutionary models. However, the observed number of 4 transitional stars out of our sample of 14 is statistically high as compared to the 4% expected in stellar evolutionary models.