A study of the expanding envelope of Nova V1974 Cyg 1992 based on IUE high resolution spectroscopy

We have carried out a detailed analysis of the IUE archival high resolution spectra of the classical nova V1974 Cyg 1992. The main UV resonance lines show P Cygni profiles in the first days, which change into symmetric pure emission lines, and then slowly become fainter and narrower. Lines of higher ionization species reach their peak luminosity later than those of low ionization. This can be explained by a fast wind which is optically thick in the early days, when the pseudo-photosphere is located inside the wind. As the mass loss decreases, the radius of the pseudo-photosphere schrinks. This has three effects that explain the observed changes: (1) the deeper accelerating layers of the wind become visible where the emission lines are formed by collisional excitation and/or recombination; (2) as the mass loss rate decreases the emission comes from deeper regions of the wind where the velocities are smaller; (3) the effective temperature and the degree of ionization increase. In addition to the P Cygni and emission lines, we could identify two shortward shifted absorption systems which originate in two separate expanding shells, outside the wind layers where the emission lines are formed. The velocity of both shells increase with time. The outer main shell, containing most of the matter ejected at the outburst, produces the so-called ``principal absorption line system'', and the inner faster moving second shell produces the so-called ``diffuse-enhanced absorption line system''. The acceleration of the two shells is the result of increasing line-radiation pressure due to the UV-brightening of the star as the effective radius decreases. Around day 60 the second shell has overtaken the slower moving principal system shell, and merged with it. This explains: the sudden disappearance of the diffuse line system near that date, the upward jump of Δv = 240 km s^-1 in velocity of the principal system and the first detection of hard X-ray emission on day 63. This velocity jump indicates that the main shell is ≈4 times more massive than the second shell. The deceleration suffered by the diffuse-enhanced system after the shock provides a shock temperature T_shock ≈ 1.6 keV, in fairly good agreement with the temperature of the observed hard X-ray emission. The UV observations are interpreted through an empirical model in which the pre-nova slow wind phase is followed by the ejection of two shells, where the principal and the diffuse-enhanced absorption systems are formed, and by a phase of fast continuous lower density wind. Our empirical expansion velocity law for the principal system, together with Hα interferometric observations of the angular radius on day 10 are used to determine the distance to the nova, which is found to be 2.9 ± 0.2 kpc, in agreement with HST imaging and with the absolute magnitude versus rate of decline relationship.

Tables in Appendix are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/420/571