The Structure and Evolution of the Nova Cygni 1992 Shell from HST Observations
Session 61 -- Novae and Stellar Accretion
Oral presentation, Tuesday, 10, 1995, 10:00am - 11:30am

## [61.01] The Structure and Evolution of the Nova Cygni 1992 Shell from HST Observations

W.J.Hack, F.Paresce,M.Livio,K.Korista (STScI)

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$\,${\sc III}, O$\,${\sc II}, H$\beta$, Lyman~$\alpha$, He$\,${\sc II}, Ne$\,${\sc III}, Ne$\,${\sc V}, Mg$\,${\sc II}, and N$\,${\sc IV}. Two objective prism images in the range 1200--5000\AA\ 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 22500~K black body between 1500--6000\AA\ but with a broad turnover below 1500\AA. A prominent 2170\AA\ 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 $\sim90$~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. Assuming the emitting material is expanding at $\sim830$--1500~km~s$^{-1}$ as deduced from IUE and optical observations, the ring is now at a distance of $\sim430$--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 yield 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~$M_{\sun}$. 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 of the nova shell.