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The expansion of the envelope of Nova V 1974 Cygni and the distance problem
Received 22 September 1994 / Accepted 3 June 1996
Available optical and UV spectroscopic, HST and radio imaging observations of the Nova V 1974 Cygni were used to derive a kinematic model of the expansion of the nova shell. The nova shell consists of two major components: an outer fast tenuous low-mass envelope and an inner slow main high-mass envelope. The outer envelope, detected spectroscopically and on radio images, is spherical except for the polar region, where an outflow with twice as large a velocity is observed. The envelope is accelerated by the nova wind that consists of spherical and polar components, too. The most pronounced features of the inner envelope, detected spectroscopically and on the HST images, are an expanding dense equatorial ring and polar blobs. The ring is immersed in the expanding spherical lower density envelope. These structures are accelerated by the wind, too. The polar direction of the outer envelope coincides with the direction of expansion of the ejected blobs of the inner envelope. The HST images taken 818 days after the outburst show clearly the effect of the strong large-scale magnetic field, caused by the interplay of the dipole magnetic field of the white dwarf and the magnetized plasma, on the expanding prolate inner envelope. The "light charged particles" of the outflowing plasma in the polar region follow the magnetic field lines of force in about 10 meridional arcs (flux tubes), which resemble a water fountain. The particles moving in arcs interact with the expanding spherical inner envelope creating visible bright spots. HST images taken between days 467 and 818 show subsequent advance of the equatorial ring. Radial velocities of the components of the inner and outer envelopes and wind were derived from optical and UV spectroscopy. Expansion rates of the components of both envelopes in the plane of the sky were derived from HST images and radio images, respectively. These data were used to determine the inclination of the polar ejecta with respect to the observer. The resulting value of i = 38 7 2 1 enabled us to determine the true expansion velocities. Using this kinematic model we were able to review all published distances of the nova. The final value d = (1.77 0.11) kpc gives a good fit to all available data. It is shown that the white dwarf in Nova V 1974 Cyg is strongly magnetic.
Key words: stars: novae, cataclysmic variables stars: individual: Nova V 1974 Cygni stars: distances stars: circumstellar matter
Send offprint requests to: D. Chochol
Online publication: July 3, 1998