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Astron. Astrophys. 318, 908-924 (1997)

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3. Spectroscopic observations and data reduction

3.1. Optical spectra

Chochol et al. (1993) found in the high resolution tracings of the [FORMULA] line in the nebular stage of Nova V 1974 Cyg, taken on days 152 and 216 after the outburst, a splitting of the line profile into a few components corresponding to approaching and receding parts of an expanding equatorial ring and two polar blobs. In Fig. 1 we reproduce the emission profiles of [O III] lines taken on July 20 and Sept. 22, 1992 (152 and 216 days after outburst). They reveal the same behaviour. Detailed description of our spectroscopic observations was published by Chochol et al. (1993).

[FIGURE] Fig. 1. [O III] emission lines profiles of Nova V 1974 Cyg on July 20, 1992 (day 152 after outburst) and September 22, 1992 (day 216 after outburst)

We obtained further optical spectra using the CCD camera and main stellar spectrograph of the 6 m telescope of the SAO at Nizhnij Arkhyz in Russia. Spectrograms with dispersion 1.4 nm mm-1 covered the [FORMULA] spectral region. The spectral resolution was 0.07 nm. The journal of observations is given in Table 3. The spectra are depicted in Fig. 2. Resulting profiles on days 970 and 971 were obtained by summing up two and three spectra, respectively.


Table 3. The journal of spectroscopic observations

[FIGURE] Fig. 2. [FORMULA] emission line profiles

3.2. UV IUE spectra

Shore et al. (1993) published the evolution of the high resolution UV line profiles of Nova V 1974 Cyg from day 2 to 409 after the outburst using the IUE and HST GHRS spectra. To explain the complicated emission line profiles with many filaments during the nebular stage, they supposed a random spherical distribution of the emitting material around the central source. They simulated the structure of emission line profiles using a Monte Carlo method.

In the present paper we reanalyze IUE SWP spectra in the nebular stage of Nova V 1974 Cyg from day 80 to 202. The spectra were extracted from the IUE archive at Villafranca and further processing was done using standard ESO MIDAS software and the SPEFO code written by Horn (1992). The journal of observations is given in Table 4.


Table 4. The journal of IUE observations

Fig. 3 displays an evolution in the UV line profiles of N III] 175 nm and C III] 190.8 nm lines. The spectra were normalized to the maximum of the line intensity and filtered by a filter of 0.06 nm width. As in the optical spectra, the emission peaks, belonging to the expanding ring and blobs, are clearly visible and their radial velocities are very stable. Other UV lines are either very weak or blended, so they are not suitable for measurements of the radial velocities of emission peaks. The emission peak of the C III] line belonging to the approaching expanding ring is located exactly in the spectral region where different echelle orders of the spectrum overlap each other. As the line intensity in this region is very uncertain we removed this part of the line profile and designated it by the dashed line.

[FIGURE] Fig. 3. N III] and C III] line profiles

It is clear that the emission peaks are stable over several months and represent well the principal components of the expanding envelope, namely the very thin equatorial ring plus two polar blobs (visible also in the HST images). The central emission peak is caused by the movement of gaseous structures in the plane perpendicular to the line of sight.

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998