5.5 Other Distance Indicators using Novae
Several other methods for using novae as distance indicators have been proposed in the literature. Here we give a brief discussion of some of these methods, and their limitations.
<M15>: Buscombe and de Vaucouleurs (1955) first showed that the mean magnitude of an ensemble of novae 15 days after maximum light was a constant; from the most recent data on Galactic novae, Cohen (1985) has shown that <M15> -5.60 ± 0.14, where the quoted error is the 1 error in the mean. The rough constancy of <M15> is a consequence of the MMRD relation, and has been explained theoretically by Shara (1981a). For Virgo cluster novae, <B15> +26 if (m - M) = 31.5; hence the <M15> distance indicator will be strongly affected by Malmquist bias at the distance of the Virgo cluster unless the observational completeness limit goes somewhat fainter than B > 26. Note that there are exceptional objects (e.g., M31 novae Arp 1, 2, and 3) that suggest this method be used with caution.
Luminosity Function of Novae: The luminosity function of novae at maximum light is approximately Gaussian in the data of Rosino (1973); de Vaucouleurs (1978) used the mean magnitude of this Gaussian to determine the distance to M31. More recently, Pritchet and van den Bergh (1987a) used this technique to determine the distance to the Virgo cluster, and obtained a result that was consistent with the MMRD relation. The most recent compilation of M31 nova data by Capaccioli et al. (1989) appears to show a double-peaked luminosity function; Capaccioli et al. use the magnitude of the minimum between the two peaks as a distance indicator. Yet another method is to use the integral luminosity function of novae at maximum light; this function is linear over a wide range of magnitude, and possesses a well-defined intercept (van den Bergh and Pritchet 1986).
The use of the luminosity function of novae at maximum light as a distance indicator demands large samples of novae that are essentially complete at the faintest magnitudes; it is therefore unlikely that this method will be useful for any but the nearest luminous galaxies (i.e., those with high nova rates). The Capaccioli et al. (1989) technique of using the dip between two peaks in the luminosity function is not completely reliable, because, as Capaccioli et al. demonstrate, different samples of M31 novae have luminosity functions with very different structure. (The very existence of this dip is in some question - cf. Rosino 1973, Capaccioli et al. 1989.)
Finally, it should be mentioned that the luminosity function of all M31 nova observations (i.e., random phases) possesses no useful information that can be used in determining extragalactic distances (van den Bergh and Pritchet 1986; Ford and Ciardullo 1988).
Period of Visibility: Van den Bergh and Pritchet (1986) show that there exists a strong correlation between the mean period of visibility of novae (down to some limiting magnitude mlim, and the absolute magnitude that this mlim corresponds to. Application of this correlation (calibrated using M31 data) to the Virgo elliptical observations of Pritchet and van den Bergh (1987a) yields a distance modulus that is similar to that obtained using the MMRD relation. This method needs complete samples of novae down to some chosen mlim, but the samples do not have to be large. (With a large sample of novae, it would in principle be possible to apply this technique at several different mlim values.)