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

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7. Discussion and problems

The most interesting observational fact - decrease of RV velocities of the ring and blobs after day 318 - can be explained either by an interaction of the expanding shell with the circumstellar medium or by other reasons connected with changes of the inclination angle or deviations of the ring from the circular shape.

Following arguments support the first interpretation:

1. Appearance of coronal lines of [Ar X], [Fe X], [Fe XI],[Fe XIV] around day 400 (Austin et al. 1996) with maximum of [Fe X] flux on day 519 (Rafanelli et al. 1995), which could be formed in a hot gas that is heated by interaction of the expanding shell with pre-existing circumstellar gas.

2. An increase of the X-ray flux in the range 0.5-0.9 keV detected on days 434 and 511 (see Fig. 2 of the work Krautter et al. 1996).

If the ejecta from a previous outburst were present, the high velocity wind and outer envelope could interact with them, too. In this case we have to detect hard X-rays when the wind as well as outer envelope reached the remnant material from a former nova outburst or preoutburst outflow. The inner envelope started to meet the circumstellar medium after day 318 at the distance of about 200 AU for blob and 160 AU for ring. Polar and spherical outflows of the outer envelope reached this distance around days 105 and 170, polar and spherical wind on days 56 and 82, respectively. The rise phase of the X-ray light curve was observed from day 63 to 147 and is characterized by a strong continuous increase of the count rate from 0.3 to 11.8 counts s-1. Spectral energy distributions were hard with no counts below 0.8 keV. During the plateau phase, which lasts from day 255 to 511 the counts rates increase to a peak of about 75 counts s-1. Hard energy excess was very weak compared to the soft component. Increase of hard X-ray flux was observed on days 434 and 511. Decline phase observed from day 511 to 653 was characterized by a strong continuum decrease down to 0.2 counts s-1. As it is clearly seen dates of increase of hard X-ray flux are in rough agreement with expected dates of encounters of the components of expanding shell with the circumstellar medium.

The arguments against this interpretation are as follows:

1. Austin et al. (1996) showed that the lines of [Fe VII] 608.7 nm and [Fe X] 647.4 nm can be well fitted by photoionization models, so the expanding gas could be heated by photoionization instead of interaction with circumstellar medium.

2. According to Krautter et al. (1996) the hard X-rays photons detected by the ROSAT satellite arose from the interaction of the expanding nova wind with density condensations within the shell and could not originate at the very outermost parts of the expanding envelope.

In this case the detected decrease of RV of the blob between days 318 and 480 could be explained purely by the change of its inclination angle from 38.7 to 44.7 due to the long time precession of the polar axis and corresponding change in the orientation of the magnetic field. For the ring the situation is more complex. The change of inclination angle included to the data in Fig. 18 does not explain the observed decrease of the velocity. If we reject the interaction with circumstellar medium there exists only one plausible explanation, that there were deviations from circular shape of the ring which have increased with time.

According to Krautter et al. (1996) soft X-rays (0.1 - 0.5 keV) were caused by the radiation of the hot white dwarf remnant on whose surface hydrostatic burning was going on until day 540, when the accreted hydrogen envelope was exhausted. Ivison et al. (1993) proposed from radio observations an instantaneous ejection and a continuous stellar wind as the major driving forces of the gas expansion in Nova V 1974 Cyg. Our results support this interpretation. The wind arising due to the TNR on the surface of white dwarf "snowplows" and accelerates the expanding envelope of Nova V 1974 Cyg. The polar wind could cause the "bar like structure" seen on the first HST image of Nova V 1974 Cyg (Paresce, 1994). Further HST images, which show only an expanding ring, were taken after cessation of the nuclear burning and substantial decrease of the wind density.

Possible existence of the magnetic fountain suggests that the X-rays can be produced by the interaction of light charged particles moving in meridional arcs with the expanding spherical inner lower density envelope. However, quantitative analysis has to be done to reveal the importance of the mechanism for the X-rays production. It is interesting to note that "fountains" or "anemone regions" have been previously found by Tousey et al. (1973) in the EUV observations of the solar active regions. Vourlidas et al. (1996) investigated solar active region NOAA 7124 with the peculiar soft X-ray morphology. Several discrete loops were visible emanating from the region and connecting to the surrounding area, giving it the appearance of a "sea anemone". Anemone regions are usually observed to emerge in magnetic unipolar areas associated with coronal holes. Sometimes they exhibit violent behaviour such as the soft X-ray jets (Shibata et al., 1994).

The orbital period of the underlying binary is 0.08123 days (De Young & Schmidt, 1994). According to Baptista et al. (1993) novae with orbital periods shorter than the period gap appear to be strongly magnetic. The magnetic fountain confirmed the presence of the strong magnetic field in the system. Semeniuk et al. (1995) found another period: 0.085-day, which is continuously decreasing and may be the spin period of the white dwarf component suggesting the presence of an AM Her star which was thrown off synchronism by the nova outburst and now evolves back into synchronization. Retter, Ofek & Leibowitz (1995) reported that the two periods obey exactly the linear relation of Stolz & Schoembs (1981) for SU UMa stars. In this case 0.085-day period is the superhump period and the spin period of white dwarf could be much faster. The very bright old nova V 603 Aql has been classified as a DQ Her type intermediate polar containing a rapidly rotating white dwarf (Schwartzenberg-Czerny, Udalski & Monier, 1992), and as a permanently superhumping extreme SU UMa system (Patterson & Richman, 1991). According to Scott (1995) the existence of an anisotropic wind with a high velocity polar component and prolate ejecta indicates a rapidly rotating white dwarf in the system.

In any case we can conclude that the magnetic field of the white dwarf (which is either a polar or an intermediate polar) play an important role in mass loss from the system and has a large influence in shaping of the nova shell, as well. The magnetic field can also be the cause for the large discrepancy in the rate of decline calculated from [FORMULA] and [FORMULA] time. Mass loss is clearly enhanced by magnetic field effects. Concerning the shaping, there are no doubts that the inner envelope of Nova V 1974 Cyg consists of an equatorial ring and polar blobs. Livio (1994) discussed the importance of the common envelope phase for shaping of nova shells. The mass loss which results from orbital energy deposition occurs preferentially in the orbital plane. Thus a density contrast can be generated between the equatorial (orbital plane) and polar directions. The equatorial ring formed during the common envelope phase expands in the orbital equatorial plane. The coupling of the nova wind and the magnetic field could also shape the nova shell (Orio et al. 1992). According to Chevalier & Luo (1994) the magnetic field in the wind from a magnetized, rotating star becomes increasingly toroidal with distance from the star. Toroidal magnetic tension can constrain the flow in the equatorial region, while not interfering with the flow in polar region. The advance of equatorial ring with time can be caused by complicated common influence of gravitational and magnetic forces on the expanding plasma.

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998