1996A&A...313..611M

Photoionization models for the shell ejected by Nova Muscae 1983 are obtained for the period 1984-1992, considering post-nova atmosphere continua computed by MacDonald & Vennes (1991). A fit to their grid of models is proposed in an appendix. All features of the model shell obtained by Morisset & Pequignot assuming a blackbody primary source are preserved. However the effective temperature T_* derived for the source is about 15% lower. Reliable collision strengths for coronal lines are needed before the late evolution of the post-nova can be accurately determined. An extensive exploration of possible photoionization-model solutions indicates that (1) a suitable effective collision strength {OMEGA}_X for [Fe X] 637.5nm at 2x10⁴K is 3 to 5 times the pioneering result of Blaha (1969) and less than one half recent elaborate values, which are therefore strongly suspected to be overestimated, (2) the collision strength for [S VIII] 991.1 is moderately larger than according to Blaha, (3) the decline of the bolometric luminosity L_* amounts to a factor ∼2 at the time of soft X-ray detection by ROSAT (day 3330), which may therefore nearly coincide with nuclear burning turnoff and maximum T_*, and (4) depending on the collision strength {OMEGA}_XI for [Fe XI] 789.2nm, the maximum T_* is either of the order of 5x10⁵K ({OMEGA}_XI close to Blaha's result) or smaller than 4x10⁵K (large {OMEGA}_XI). A way out of the conflict between theoretical and astrophysical determinations of {OMEGA}_X may be managed in the framework of present models if {OMEGA}_XI happens to be very large. Source parameters derived from nova shell models are relatively insensitive to details of stellar atmosphere models and may help to enhance the value of soft X-ray observations. Properties of GQ Mus disclosed by photoionization model analysis are shown to constitute a challenge to current understanding of novae and post-novae.