[ITAL]Hubble Space Telescope[/ITAL] NICMOS Observations of Classical Nova Shells

Abstract

We present results of a campaign to obtain near-infrared images of evolved classical nova shells by using the Near-Infrared Camera and Multi-Image Spectrometer (NICMOS) on the Hubble Space Telescope. Images were obtained in six filters from 1.87 to 2.37 μm. Three of our four targets, QV Vulpeculae (Nova Vul 1987), QU Vulpeculae (Nova Vul 1984 No. 2), and V1974 Cygni (Nova Cyg 1992) exhibit pronounced shell emission in the narrowband Paα (F187N) filter. In the other five filters, the shells are only marginally detected with the exception of QU Vul, which exhibits spatially extended low surface brightness emission in the medium-band filter (F222M) centered near 2.22 μm. Spatially extended emission from V723 Cassiopeiae (Nova Cas 1995) was not detected above the image noise in any filter. The observed ejecta geometries lend further support to the suggestion that the speed class of a nova may correlate with the shape of the shells. The NICMOS images, in combination with ground-based spectroscopic determination of the ejecta expansion velocities, enable us to estimate the expansion parallaxes for V1974 Cyg, QU Vul, and QV Vul. Of note, the expansion velocity (in milliarcseconds per day) determined for V1974 Cyg has remained constant over the 4 year period (1993-1998) after onset of the optical thin phase. Lastly, more structures in the shell of V1974 Cyg have appeared since 1993.