1996A&A...312..135M

A detailed photoionization model study of the shell ejected by Nova Muscae 1983 (GQ Mus) is presented for the period 1984-1990. The drastic time variation of the emission-line spectrum, including the transition from a nebular to a coronal stage in 1986, can be quantitatively understood in terms of a smooth evolution of both the expanding shell and the hot thermal source. This transition is due to a decrease of the shell density with time and not to an increase of the source effective temperature. The central-source power L_*=(1.75±0.2)x10³⁸x(v_exp/630km/s)²erg/s, with v_exp the mean expansion velocity of the shell, and the effective temperature T_*=(2.65±0.2)x10⁵K are uniquely determined in 1984 without reference to either the distance to GQ Mus or the precise reddening correction. The model shell comprises two components so that not all directions become optically thin to ionizing radiation simultaneously. The range of density and geometrical thickness of the emitting material is on the order of a factor 2. The total mass of the shell is M_shell=(12.5±1.5)x10^–5M_☉x(D/4kpc)²x10^3.1(EB-V_-0.5)^, D being the distance to the nova and E_B–V the interstellar differential extinction. This estimate includes some 20% of unseen mass. If the emitting mass of the shell were conserved after the end of the radiation-bounded phase, the gas density would have to decrease much faster than the inverse third power of time t since the explosion, implying a continuous differential acceleration of the material across a cloud and, according to models, a steady t^–1 decrease of L_*. Alternatively, assuming free expansion of the material, the rapid shrinking of H_β with time must be due to a decrease of the emitting mass and L_* is then found from photoionization models to remain nearly constant for at least five years. It is advocated from either model predictions or self-consistency arguments that the free-expansion alternative should be preferred. For the first time, a photoionization model analysis enables to disclose, on the basis of purely spectroscopic criteria, the constant bolometric luminosity phase predicted by the theory of novae. It is stressed that drawing firm conclusions from photoionization modelling demands scrutiny of uncertainties inherent to both atomic data and observation, full exploration of the free-parameter space, sufficient observational constraints, and explicit strategy of convergence to a solution. Collisional lines prove to be quite accurately predicted using abundances of He, C, N, and O previously determined from optical recombination lines only. This appropriateness is supporting evidence to the adequacy of the shell description. The mass fractions of Ne, Mg, Si, S, Ar, and Fe are well within a factor 2 of solar values. A factor 3 overabundance is suspected for calcium.