Exploring the MMRD Relation for Novae in M31

Abstract: The results of a two decade long $R$-band photometric survey of novae in M31 are presented. From these data, $R$-band light curves have been determined for 180 novae with data sufficient for estimating peak brightness and subsequent rate of decline. The data show a weak correlation of peak brightness with fade rate consistent with the well-known Maximum Magnitude versus Rate of Decline (MMRD) relation. As generally appreciated for Galactic novae, the large scatter in the MMRD relation precludes its use in determining distances to individual novae. The novae at maximum light are distributed with standard deviation $\sigma=0.89$ mag about a mean $R$-band absolute magnitude given by $\langle M_R \rangle=-7.57\pm0.07$. The overall M31 luminosity distribution is in excellent agreement with that found for Galactic novae suggesting that the nova populations in M31 and the Galaxy are quite similar. The notion that all novae can be characterized by a standard luminosity 15 d after maximum light ($M_{15}$) is also explored. Surprisingly, the distribution of $M_{15}$ values is characterized by a standard deviation only slightly smaller than that for novae at maximum light and thus offers little promise for precise extragalactic distance determinations. A dozen faint and fast novae that are likely to be previously unidentified recurrent novae have been identified from their position in the MMRD plot and in the $M_{15}$ distribution.