Properties of M31. III: Candidate Beat Cepheids from PS1 PAndromeda Data
  and Their Implication on Metallicity Gradient
  We present a sample of M31 beat Cepheids from the Pan-STARRS 1 PAndromeda
campaign. By analyzing three years of PAndromeda data, we identify seventeen
beat Cepheids, spreading from a galactocentric distance of 10 to 16 kpc. Since
the relation between fundamental mode period and the ratio of fundamental to
the first overtone period puts a tight constraint on metallicity we are able to
derive the metallicity at the position of the beat Cepheids using the relations
from the model of Buchler (2008). Our metallicity estimates show subsolar
values within 15 kpc, similar to the metallicities from HII regions (Zurita &
Bresolin 2012). We then use the metallicity estimates to calculate the
metallicity gradient of the M31 disk, which we find to be closer to the
metallicity gradient derived from planetary nebulae (Kwitter et al. 2012) than
the metallicity gradient from HII regions (Zurita & Bresolin 2012).
