Discovery of Candidate Luminous Blue Variables in M31

Abstract

Luminous blue variables (LBVs) constitute a short-lived, eruptive phase in the evolution of some of the most massive stars. Only a handful have yet been identified in the Galaxy and in each of the nearby galaxies; there are four known LBVs in M31. We have found an efficient method to identify candidate LBV stars in nearby galaxies. The candidates are identified in a pair of deep, continuum-subtracted narrowband Hα and [S II] images as objects with extremely low [S II] to Hα ratios, and with coincident stellar objects in continuum images. Five of the most promising new candidates identified by these criteria in the northeastern half of M31 were subsequently confirmed by optical spectroscopy to show spectra similar to the previously identified M31 LBV, HS var 15. These five candidates also have much in common with B[e] stars, of which none were known to exist in M31. They are bright Hα sources, (120 L☉ < LHα < 1300 L☉) with no detectable [S II] emission, large Hα equivalent widths (-60 to -400 Å), and broad wings on the Hα profiles (FWZI = 1200-2000 km s-1). Most candidates have Fe II emission. We discuss the environments of the candidates and find that many objects are likely older than several million years because they tend not to be located inside bright H II regions. We predict, based on the current results, that at least 20-25 LBV/B[e] candidates may be present in M31.