| Abstract | We have analyzed the total sample of novae so far discovered by systematic surveys in the Andromeda Galaxy M31. The annual rate of outbursts has been determined for the Asiago 1.22 m and Ekar 1.82 m surveys (total of 142 objects-Rosino 1964, 1973; Rosino et al. 1989) by a statistical approach accounting for incomplete time coverage. The "observed" rates have been corrected for all known systematic errors, leading to a value 29 +/- 4 novae yr^-1^, consistent with Arp's (1956) revised rate. In particular, we have estimated the completeness corrections at the center for the five main nova surveys, and the mean internal absorption A^i^_pg_ ~0.20 mag. A composite surface-density profile for the nova system has been derived under rather general assumptions on the flattening curve of the isodensity contours, using all the available data. The comparison with the light-profile models for the bulge and the disk of the galaxy fully confirms that at least ~85% of the novae belong to the bulge (and the halo) of M31. The maximum magnitude versus rate-of-decline relation (MMRD) has been reviewed. By a subset of 17 reliable light curves we derive a robust fit in the linear regime 10 < t_2_ < 50 days: m_pg_ (max) - (16.88 +/- 0.15) = - (3.35 +/- 0.16) X [1.32 - log(t_2_)], proving that the slope computed for galactic novae (Cohen 1985) is largely underestimated. The best fit of M31 MMRD to a sample of galactic novae (Cohen 1985), the Buscombe-de Vaucouleurs (1955) law, and a new indicator based on the bimodal distribution of the maximum magnitude, consistently give a distance modulus of (m - M)_0_ = 24.27 +/- 0.20 mag, in very close agreement with other indicators. We discuss also the possible existence of a distinct family of fast and very bright novae, and the implications of our results on the understanding of how novae distribute in galaxies and galactic subsystems. |
