Water Masers in the Andromeda Galaxy: II. Where Do Masers Arise?
  We present a comparative multi-wavelength analysis of water maser-emitting
regions and non-maser-emitting luminous 24 micron star-forming regions in the
Andromeda Galaxy (M31) to identify the sites most likely to produce luminous
water masers useful for astrometry and proper motion studies. Included in the
analysis are Spitzer 24 micron photometry, Herschel 70 and 160 micron
photometry, H$\alpha$ emission, dust temperature, and star formation rate. We
find significant differences between the maser-emitting and non-maser-emitting
regions: water maser-emitting regions tend to be more IR-luminous and show
higher star formation rates. The five water masers in M31 are consistent with
being analogs of water masers in Galactic star-forming regions and represent
the high-luminosity tail of a larger (and as yet undetected) population. Most
regions likely to produce water masers bright enough for proper motion
measurements using current facilities have already been surveyed, but we
suggest three ways to detect additional water masers in M31: (1) Re-observe the
most luminous mid- or far-IR sources with higher sensitivity than was used in
the Green Bank Telescope survey; (2) Observe early-stage star-forming regions
selected by mm continuum that have not already been selected by their 24 micron
emission, and (3) Re-observe the most luminous mid- or far-IR sources, and rely
on maser variability for new detections.
