On the Spatial Distribution, Stellar Population, and Rate of Novae in M31
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Abstract
As part of a program to study the stellar population of novae, multiepoch Hα surveys of M31 have been undertaken. In the first survey, conducted between 1990 and 1993, a total of 53 novae were discovered in 11 CCD fields oriented generally along the major axis of M31. The second survey was conducted between 1995 and 1997, yielding a total of 29 novae in five fields including the outer regions of M31's disk. In both surveys, the spatial distribution of novae is found to follow the background galactic light. However, after decomposing the background B light into its bulge and disk components, the nova distribution is found to follow the bulge light better than the overall light, with the disk light providing a significantly poorer fit. As a check on the possibility that extinction in the disk of M31 is biasing the results, an analysis of the spatial distribution of M31's planetary nebulae—the space density of which is expected to be only weakly affected by stellar population—was also considered. Unlike the nova distribution, the planetary nebula distribution is found to follow the overall background B light remarkably well, establishing that, regardless of extinction, the nova distribution is more centrally concentrated than the planetary nebula distribution. To quantify the results, a maximum likelihood analysis was conducted, and it suggests that ~70% of the novae arise from the bulge, although a bulge fraction as low as 50% cannot be ruled out by the data. A Monte Carlo simulation that makes use of the properties of observed Hα light curves was employed in order to translate the observed nova rate in the central region of M31 to a total bulge nova rate. The procedure was applied to the bulge field of our survey as well as to the Hα bulge survey of Ciardullo and coworkers and to a bulge subset of Arp's extensive B-band survey. In all three cases, the results are remarkably consistent, suggesting a total bulge nova rate of ~25 ± 4 yr-1. Based on the bulge nova rate, an Hα mean lifetime versus Hα absolute magnitude relation is derived, which can be used to estimate the nova rate in galaxies of known distance that have been surveyed in Hα. A second-order relation given by log τc(days) = -(4.78 + 2.10Mc + 0.162M) provides the best fit to the data. Finally, the bulge rate is extrapolated to the entire galaxy assuming the disk-to-bulge nova ratio determined in the maximum likelihood analysis. A global nova rate of ~37 is found, which is somewhat higher than previous estimates of ~30 yr-1 for the overall nova rate in M31. If the luminosity-specific disk and bulge nova rates are comparable, the overall nova rate in M31 could approach 50 yr-1.