Optical and ultraviolet spectrophotometry of the ONeMg Nova V838 Herculis 1991

We present an analysis of optical and UV spectra of the fast nova V838 Herculis 1991. IUE observations began on 1991 March 25 and continued through 1991 August 20, while optical observations began on 1991 March 29 and continued through 1991 August 20. We determine a value for the reddening of E (B − V) ~ 0.5 ± 0.1 and a distance of ~ 3 ± l kpc based both on our own observations and on those of other groups. This nova was remarkable in being both fast and optically thick in the UV at maximum light. Our spectra show strong lines of helium, neon, sulphur and nitrogen. In contrast, oxygen lines were never present in our UV spectra and very weak in our optical spectra. We use a new optimization method to perform an abundance analysis of the spectra, and find that carbon, nitrogen, neon, silicon and sulphur are overabundant with respect to solar material by factors ranging from 5 to 37, while helium and oxygen are both underabundant by a factor of 0.3. Our best model is obtained with a luminosity for the central source of 5.1 × 1036 erg s−1, indicating that this nova may have turned off by two months after outburst. We also find an ejected mass for this nova of Mej ~ 1.8 × 10−4 M⊙. Based on both the characteristics of the outburst and our observed abundances, we suggest that this outburst occurred on a very massive ONeMg white dwarf.