Nova Sagittarii 1994 1 (V4332 Sagittarii): The discovery and evolution of an unusual luminous red variable star

We report photometry and spectroscopy of the evolution of Nova Sagittarii 1994 1 (V4332 Sagittarii) during outburst. We compare the photometric and spectral evolution of this outburst with known classes of outbursts-including classical novae and outbursts occurring on symbiotic stars-and fi this object does not conform to any known class of outburst. The closest match to the behavior of this unusual object is M31 RV, an extremely luminous and red variable object discovered in the bulge of M31 in 1988. However, the temporal behavior and maximum luminosity of the two events differ by several orders of magnitude, requiring substantial intrinsic variation if these two events are members the same type of outburst. Our model of the spectroscopic evolution of this outburst shows that the effective temperature cooled from 4400 to 2300 K over the 3 month span of our observations. In combination with line diagnostics in our later spectra, including [O I] lambda 5577 and the dramatic increase in the H alpha-to-H beta ratio, we infer the existence of a cool, dense (N-e similar to 10(8)-10(9) cm(-3)) envelope that is optically thick in the hydrogen Balmer recombination lines (case C). We suggest that a nuclear event in a single star, in which a slow shock drove the photosphere outward, can power the observed luminosity evolution and the emission spectrum.