Analysis of Three Fast Oxygen‐Neon‐Magnesium Novae

Abstract

Since astronomers have only known about the existence of oxygen-neon-magnesium (ONeMg) novae for the last decade, little work has been done on them as a group. The purpose of this work was to study three fast ONeMg novae: V838 Nova Herculis 1991 (Her 91), V693 Nova Coronae Australis 1981 (CrA 81), and Nova LMC 1990 No. 1 (LMC 90a). Each of these novae has been studied in detail, and a comparison of the results is provided in order to better understand ONeMg novae as a class. My analyses have provided values of the elemental abundances, ejection velocities, ejected masses, white dwarf (WD) masses, maximum outburst luminosities, and white dwarf turn-off times for these novae. All three novae have abundances enhanced relative to solar values, with the oxygen abundance of Her 91 being the exception (see Table 1). They show high N/O ratios, indicating high-mass WD progenitors. We find CrA 81 and LMC 90a to have almost identical abundances, even though the initial metal abundances for the LMC novae are one-third of solar values. Each of the novae exhibited a super-Eddington luminosity phase at maximum light. Our analysis of the Her 91 spectrum taken 2 months after outburst, however, required a luminosity almost 2 orders of magnitude lower than the Eddington limit for a 1.4 M, WD. The best fits for CrA 81 and LMC 90a, on the other hand, were consistent with a constant luminosity of ∼1038 ergs s for at least 2 months after outburst. While we were unable to accurately determine ejected masses for either CrA 81 or LMC 90a owing to the lack of optical data, we found an ejected mass of ∼1024 M, for Her 91. This is in agreement with the values found by other groups using different data and different techniques. The results of this study can be used in many ways to help further the understanding of novae and stellar evolution. The elemental abundances that have been determined can be used as input to theoretical hydrodynamic models. This will provide better constraints on the models in order to determine the energetics of the outburst and the mass of the underlying white dwarf, properties that are difficult to obtain observationally. Knowledge of the abundances will also help determine the contribution of fast ONeMg novae to the heavy-element content of the interstellar medium. In particular, with the abundance results, the mass fraction of Al in the nova ejecta can be estimated. These estimates will help to determine if novae are important for Galactic nucleosynthesis. Analysis of LMC 90a provides the means to compare Galactic with extragalactic novae. For example, the LMC novae can be used to calibrate the luminosities of (and therefore distances to) Galactic novae. This calibration will allow a better determination of the fast end of the absolute-magnitude decay-rate relationship or perhaps develop a separate relationship for this class of novae.