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

Abstract

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 ∆v = 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 diffuseenhanced 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.

The Massive CO White Dwarf in the Symbiotic Recurrent Nova RS Ophiuchi

“…Similar resonance-line profiles, including that of Al III, were observed in other ONe, e.g. in IUE spectra of V1974 Cyg (Cassatella et al 2004, their fig. 3), and HST/STIS spectra of V382 Vel (Shore et al 2003, their fig.…”

“…All spectra correspond to the epoch when the emission line spectrum was strongest, and roughly represent the same phase of nova outburst development. We note that the nova evolution, including spectroscopic characteristics, scales with t 3 (Cassatella et al 2004;Vanlandingham et al 2001). Here we adopted t 3 = 14 days (Schaefer 2010) for RS Oph, and the values from Cassatella et al (2004), and Vanlandingham et al (2001) 2669 line is also present in the long wavelength IUE spectrum (not shown here).…”

“…Such broad P Cyg absorption components on the UV resonance lines were not seen in any of the CO novae observed with the IUE (e.g. Cassatella et al 2004), and they are a phenomenological characteristic of the ONe novae (Shore et al 1994).…”

A THREE-DIMENSIONAL VIEW OF THE REMNANT OF NOVA PERSEI 1901 (GK Per)

“…We favor the latter hypothesis, which is supported by the observations of nova shells soon after outburst. It has been argued in the literature that two distinct outflow components of different origin are present at early times: an earlier, slow one from the circumbinary gas, and a later, fast one resulting from the outburst (see, e.g., Cassatella et al 2004;Williams & Mason 2010). In the case of Nova Cyg 1992, Cassatella et al (2004) show that the main shell ejected at the moment of the outburst contains the bulk of the ejected matter and is located in the outermost region of the ejecta region, while a less massive impulsive event forms a faster-expanding shell that eventually will catch up with the main shell, generating a region of shock interaction where the two shells meet.…”

Spectroscopic monitoring of the transition phase in nova V4745 Sgr

“…Interestingly, they found evidence for two distinct shell ejections, the second one of which was ejected with a higher velocity, containing significantly less material. Cassatella et al (2004) suggested that the second ejection was a reaction to the first one as follows. After the ejection of the main shell, the envelope is out of balance and will try to restore equilibrium, but if this does not happen immediately (e.g.…”