Multiwavelength analyses of the extraordinary nova LMC 1991

LMC 91 was a very fast, classical nova and the brightest nova ever observed in the Large Magellanic Cloud. It was extensively observed during both its early optically thick and its nebular evolution in the optical and UV wavelength regions. We successfully fit all the optically thick spectra using a grid of spherically symmetric, non-LTE, line-blanketed, expanding synthetic spectra created with the model atmosphere code PHOENIX. The emission lines of the nebular spectra have been fitted using an optimization technique for the emission-line luminosities predicted by the photoionization code CLOUDY. Our analyses show the following: the bolometric luminosity was super-Eddington before visual maximum and reached L^max_Bol ∼ 6 × 10⁵ L_⊙, the ejected mass was ∼ 3 × 10^-4 M⊙, and nuclear burning on the white dwarf ceased after -100 days. The elemental abundances (by number) with respect to solar of the ejecta are He/H = 0.8 ± 0.2, C/H = 5^+2.5_-2, N/H = 85⁺²²_-17, O/H = 6.5^+2.5_-1.5, and all other elements ∼0.1. These abundances were determined from both the optically thick and nebular analyses. The extreme luminosity, high ejected mass, rapid turn-off time, and low metallicity (except for enhanced CNO) represent the extreme values of observational nova parameters and thus present interesting challenges for understanding the nature of the outburst of LMC 91.