The Wolf-Rayet stars in M31: I. Analysis of the late-type WN stars
  Context: Comprehensive studies of Wolf-Rayet stars were performed in the past
for the Galactic and the LMC population. The results revealed significant
differences, but also unexpected similarities between the WR populations of
these different galaxies. Analyzing the WR stars in M31 will extend our
understanding of these objects in different galactic environments. Aims: The
present study aims at the late-type WN stars in M31. The stellar and wind
parameters will tell about the formation of WR stars in other galaxies with
different metallicity and star formation histories. The obtained parameters
will provide constraints to the evolution of massive stars in the environment
of M31. Methods: We used the latest version of the Potsdam Wolf-Rayet model
atmosphere code to analyze the stars via fitting optical spectra and
photometric data. To account for the relatively low temperatures of the late
WN10 and WN11 subtypes, our WN models have been extended into this temperature
regime. Results: Stellar and atmospheric parameters are derived for all known
late-type WN stars in M31 with available spectra. All of these stars still have
hydrogen in their outer envelopes, some of them up to 50% by mass. The stars
are located on the cool side of the zero age main sequence in the
Hertzsprung-Russell diagram, while their luminosities range from $10^5$ to
$10^6$ Lsun. It is remarkable that no star exceeds $10^6$ Lsun. Conclusions: If
formed via single-star evolution, the late-type WN stars in M31 stem from an
initial mass range between 20 and 60 Msun. From the very late-type WN9-11
stars, only one star is located in the S Doradus instability strip. We do not
find any late-type WN stars with the high luminosities known in the Milky Way.
