Hubble Space Telescope imaging and ground-based spectroscopy of old nova shells - II. The bipolar shell of the slow nova HR Del

Abstract

We present Hubble Space Telescope (HST) WFPC2 imaging of HR Del in filters centred on the lines of Hα, [N ii] and [O iii], with corresponding William Herschel Telescope (WHT) ISIS slit spectroscopy for the lines of Hα, Hβ, [N ii] and [O iii]. The data show that the ejected shell of HR Del is composed of many small bright knots. These are distributed in a bipolar structure with an equatorial ring, and polar rings or caps. The [O iii] emission is significantly more bipolar than that seen in Hα/[N ii]. Diffuse material which extends radially outward from these knots may be further evidence for a fast wind from the central binary system. There are also [N ii] and [O iii] knots at both higher and lower velocity than that of the main shell which follow the general bipolar symmetry and may be associated with secondary mass ejections at outburst or interactions with the fast wind. Three-dimensional models of the shell that reproduce the observed emission-line images and longslit spectra in both the [O iii] and Hα morphologies are presented. We use these models to estimate an inclination for the shell of 35°± 3°, a polar expansion velocity of 560 ± 50 km s−1, an axial ratio of 1.75 ± 0.15 and a distance of 970 ± 70 pc. We discuss how these parameters can be used to improve the shell shape and maximum magnitude versus rate of decline (SSRD and MMRD, respectively) relationships for novae.