Variability of massive stars in M31 from the Palomar Transient Factory
  Using data from the (intermediate) Palomar Transient Factory (iPTF), we
characterize the time variability of ~500 massive stars in M31. Our sample is
those stars which are spectrally typed by Massey and collaborators, including
Luminous Blue Variables, Wolf-Rayets, and warm and cool supergiants. We use the
high-cadence, long-baseline (~5 years) data from the iPTF survey, coupled with
data-processing tools that model complex features in the light curves. We find
widespread photometric (R-band) variability in the upper Hertzsprung Russell
diagram (or CMD) with an increasing prevalence of variability with later
spectral types. Red stars (V-I>1.5) exhibit larger amplitude fluctuations than
their bluer counterparts. We extract a characteristic variability timescale,
tch, via wavelet transformations that are sensitive to both continuous and
localized fluctuations. Cool supergiants are characterized by longer timescales
(>100 days) than the hotter stars. The latter have typical timescales of tens
of days but cover a wider range, from our resolution limit of a few days to
longer than 100 days timescales. Using a 60-night block of data straddling two
nights with a cadence of around 2 minutes, we extracted tch in the range
0.1--10 days with amplitudes of a few percent for 13 stars. Though there is
broad agreement between the observed variability characteristics in the
different parts of the upper CMD with theoretical predictions, detailed
comparison requires models with a more comprehensive treatment of the various
physical processes operating in these stars such as pulsation, subsurface
convection, and the effect of binary companions.
