Spectroscopic evolution of nova LW serpentis 1978 during its early decline

Abstract

Optical spectroscopic data are presented on nova LW Serpentis 1978, obtained during its decline from V 9.0 to ≃10.2 (compared to a value of ~ 8.0 at recorded maximum). The spectrum and its evolution compare well with a typical nova, though the principal absorption (~ −750 km s^-1) was very weak in comparison with the diffuse-enhanced absorption (~ −1300 km s^-1). The principal absorption could be identified only in the lines of O I λ λ7774, 8446, and in moderate-resolution observations of Hα. The salient features of spectral evolution follow: The near-infrared triplet of Ca n continuously weakened. O I λ8446 was always brighter than O I λ 7774, indicating continued importance of Lyman Β fluorescence. The lines due to [O I], [N II] and N n brightened considerably near the end of our observations (37 days from maximum). The Hα emission line was asymmetric all through with more emission towards the red. Its emission profile showed considerable structure. Based on the individual peaks in the Hα line profile, a kinematical model is proposed for the shell of LW Ser. The model consists of an equatorial ring, and a polar cone on the side away from the earth. The nearer polar cone did not show significant emission of Hα during our observations. The polar axis of the shell is inclined at a small angle (~ 15‡) to the line of sight.