A Spitzer Survey of Novae in M31
  We report the results of the first infrared survey of novae in the nearby
spiral galaxy, M31. Both photometric and spectroscopic observations of a sample
of 10 novae (M31N 2006-09c, 2006-10a, 2006-10b, 2006-11a, 2007-07f, 2007-08a,
2007-08d, 2007-10a, 2007-11d, and 2007-11e) were obtained with the Spitzer
Space Telescope. Eight of the novae were observed with the IRAC (all but M31N
2007-11d and 2007-11e) and eight with the IRS (all but 2007-07f and 2007-08a),
resulting in six in common between the two instruments. The observations, which
were obtained between ~3 and ~7 months after discovery, revealed evidence for
dust formation in two of the novae: M31N 2006-10a and (possibly) 2007-07f, and
[Ne II] 12.8 micron line emission in a third (2007-11e). The Spitzer
observations were supplemented with ground-based optical photometric and
spectroscopic data that were used to determine the speed classes and
spectroscopic types of the novae in our survey. After including data for
dust-forming Galactic novae, we show that dust formation timescales are
correlated with nova speed class in that dust typically forms earlier in faster
novae. We conclude that our failure to detect the signature of dust formation
in most of our M31 sample is likely a result of the relatively long delay
between nova eruption and our Spitzer observations. Indeed, the two novae for
which we found evidence of dust formation were the two "slowest" novae in our
sample. Finally, as expected, we found that the majority of the novae in our
sample belong to the Fe II spectroscopic class, with only one clear example of
the He/N class (M31N 2006-10b). Typical of an He/N system, M31N 2006-10b was
the fastest nova in our sample, not detected with the IRS, and just barely
detected in three of the IRAC bands when it was observed ~4 months after
eruption.
