THE EXPANSION OF THE ENVELOPE OF NOVA V 1974 CYGNI AND THE DISTANCE PROBLEM

AvailableopticalandUVspectroscopic,HSTandra- dio imaging observations of the Nova V1974 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 enve- lope 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.Thesestructuresareacceleratedbythewind,too.The polar direction of the outer envelope coincides with the direc- tion 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 eld, caused by the interplay of the dipole magnetic eld 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 eld lines of force in about 10 meridional arcs (flux tubes), which resemble a water fountain. The particles moving in arcs interact with the expanding spher- ical 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 =3 8 : 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 Send offprint requests to: D. Chochol nova. The nal value d = (1.77 0.11) kpc gives a good t to all available data. It is shown that the white dwarf in Nova V1974 Cyg is strongly magnetic.