THE STRUCTURE AND EVOLUTION OF THE NOVA V1974 CYGNI SHELL FROM HST OBSERVATIONS

The Faint Object Camera on the refurbished Hubble Space Telescope (HST) observed Nova Cygni 1992 (V1974 Cyg) on three separate occasions in the first five months of 1994 through a series of interference and narrow band filters sensitive to the emission lines of O III, O II, H beta, Lyman alpha, He II, Ne III, Ne V, Mg II, and N IV. Two objective prism images in the range 1200-5000 Angstrom were also taken on 14 February 1994 to complement the filter images. On this date, the central object had a dereddened continuum spectrum consistent with a power-law with an index of 1.64 for wavelengths below 1800 Angstrom and an index of 2.33 beyond 2000 Angstrom but with a broad turnover below 1500 Angstrom. A prominent 2170 Angstrom absorption trough is well fit by a dust extinction of E(B-V) = 0.35. The central object was fading rapidly during the January to May time frame with an e-folding time of similar to 90 days. The structure of the shell has changed considerably since the first HST observation in May 1993 reported by Paresce (1994) and is very wavelength dependent. In the low ionization emission lines of O III and H beta, an elliptical ring within a tenuous spherical outer structure is very evident while in the higher ionization lines of Ne III, Ne V, and N IV, the entire shell tends to be filled with relatively bright knots of emission. The two brightest knots lie along the minor axis of the ellipse. The distribution of light within the faint outer shell changes significantly over an interval of time as short as 37 days indicating that temporal variations may also be driven by very rapid changes in excitation conditions within the nebula. The bright elliptical ring has been expanding since May 1993 with a constant angular speed of 0.000297 ''/day along the major axis at 97 degrees PA and at 0.000218 ''/day along the minor axis. The ellipticity of the ring has remained at similar to 0.67 in the first 6 months of this year in contrast to the slightly lower value of 0.54 measured in May 1993. Assuming the emitting material is expanding at similar to 830-1500 km s(-1) as deduced from IUE and optical observations, the ring is now at a distance of similar to 430-800 AU from the central object which itself is at a distance of 1.8-3.2 kpc. A preliminary abundance analysis of one of the knots yields an overabundance of neon by about a factor 20-30 over the solar value. We use the available data to place theoretical constraints on the white dwarf mass, which is predicted to be in the range 0.75-1.1 N-circle dot. We show that the morphology and kinematics of the nova shell are consistent with the possibility that the common envelope phase played an important role in the shaping.