The Vast Population of Wolf-Rayet and Red Supergiant Stars in M101. I. Motivation and First Results

Abstract

M101 is an ideal target in which to test predictions of massive star birth and evolution. The large abundance gradient across M101 (a factor of 20) suggests that many more WR stars must be found in the inner parts of this galaxy than in the outer regions. Many H ii regions and massive star-forming complexes have been identified in M101; they should be rich in WR stars, and surrounded by RSG stars. Finally, the Wolf-Rayet stars in M101 may be abundant enough for one to explode as a Type Ib or Ic supernova and/or GRB within a generation. The clear identification of the progenitor of a Type Ib or Ic supernova as a WR star would be a major confirmation of current stellar evolution theory.Motivated by these considerations, we have used the Hubble Space Telescope to carry out a deep, He ii optical narrowband imaging survey of the massive star populations in the ScI spiral galaxy M101. Combined with archival broadband images, we were able to image almost the entire galaxy with the unprecedented depth and resolution that only HST affords.We describe the extent of the survey and our images, as well as our data reduction procedures. A detailed study of a field east of the center of M101, containing the giant star-forming region NGC 5462, demonstrates how we find -2candidates, their properties and spatial distribution, and how we rule out most contaminants. The spatial distributions of the WR and RSG stars near a giant star-forming complex are strikingly different. WR stars dominate the complex core, while RSG dominate the complex halo. Future papers in this series will describe and catalog more than a thousand WR and RSG candidates that are detectable in our images, as well as spectra of many of those candidates.

Observational Clues to the Progenitors of Type Ia Supernovae

“…In any event, the X-ray data rule out the pre-explosion presence, at the location of SN 2011fe, of typical persistent supersoft X-ray sources, but allows the presence of somewhat fainter ones. Shara et al (2013) imaged M101 with HST, including the location of SN 2011fe, in 2010, about 1 year prior to the event, in a narrow band centered on the He II λ4686 line. The hard photon flux from a nuclear-burning accreting WD (see Section 3.1.3, should, in principle, produce a 1-30-parsec-sized (i.e.…”

Massive stars in the galaxies of the Local Group

“…If so, this could bring the WC to WN ratio down to 0.3, about as expected, but would require a surface density of WRs that is 20× greater than that of the LMC. Only two of the new WR candidates had been confirmed by Massey & Holmes (2002) (Shara et al 2013) among others. The problem is, of course, that these galaxies are found at distances ranging from 2.0 Mpc (NGC 300) to 7.0 Mpc (M83, M101), compared to, say, the distance to M33 (0.8 Mpc).…”

“…The former naturally associates the Ib/c's with Wolf-Rayet stars, but there are no cases where an actual progenitor of a Ib or Ic has been identified. (Shara et al 2013 argue that the identification of Wolf-Rayet stars in nearby galaxies serves the additional purpose of identifying Ib/c progenitors, so that when one of these stars explodes we'll be prepared. )…”

GalevNB: a conversion fromN-body simulations to observations

“…They are massive stars losing hydrogen envelopes, the socalled Wolf-Rayet stars in observations (Crowther 2007, Shara et al 2013). Since naked helium stars only appear at young age for a short time-scale, they cannot influence the UV-excess of the cluster in the long run.…”