Classical nova outbursts are the result of thermonuclear explosions on the surface of a white dwarf star in a close binary system. Material from the other star in the system (one not unlike our own Sun) falls onto the surface of the white dwarf over thousands of years. The pressure at the base of this layer of accreted material builds up until thermonuclear reactions begin explosively. An earth's mass or more of material is ejected from the surface of the white dwarf at speeds of a few hundred to a few thousand kilometres per second. Old novae are therefore surrounded by shells of ejected matter illuminated by the ultraviolet light from the central binary system.
The following images of nova shells are taken from our ground-based imaging survey undertaken between 1993 and 1995 with the William Herschel and Anglo-Australian Telescopes (WHT & AAT, see Slavin, O'Brien & Dunlop 1995, Gill & O'Brien 1998).
(outburst 1901, image 1993 WHT)
|RR Pic (outburst 1927, image 1995 AAT)||
(outburst 1934, image 1993 WHT)
|T Pyx (outbursts 1890, 1902, 1920, 1944, 1966; image 1995 AAT)||HR Del (outburst 1967, image 1993 WHT)||V1500 Cyg (outburst 1975, image 1993 WHT)|
|MERLIN 6cm radio images of the expanding ejecta from the unusually slow nova V723 Cas (O'Brien et al 2002, in prep).|
|The shell of the nova FH Ser ejected in 1970 and imaged in 1997 with the Hubble Space Telescope|
The HST images of the shell of FH Ser combined with spatially resolved spectroscopy using WHT (Gill & O'Brien 2000) have allowed us to determine that it is encircled by an equatorial ring formed by an enhancement of [NII] emission. Our spectroscopy and MERLIN imaging of the ejecta of V705 Cas show evidence for coherent structures (Gill & O'Brien 1999, Eyres et al 2000). Of particular interest is the enhancement of [NII] in two of the line components revealing that, although still spatially unresolved optically, like FH Ser the shell appears to contain an [NII] enhanced ring. Similar features were present in our WHT imaging and spectroscopy of the shell of DQ Her. This newly discovered phenomenon must be due to one of two mechanisms: either, nitrogen enhancements in an extremely thin ring around the waist of the shell (a conclusion which would have important implications for the TNR process and mixing mechanisms on the surface of the white dwarf prior to outburst); or, significant changes in the photoionizing radiation field as one moves from the poles to the equator of the shell (perhaps a result of the changing line of sight to the accretion disc in the central CV system and hence providing important information on the nature of the central system on much smaller scales than the shell itself).
Over the last 4 or so years we have also been pursuing a campaign of multifrequency observations of V723 Cas (Nova Cas 1995; O'Brien et al, in preparation) combining several epochs of radio imaging with MERLIN with almost daily photometric monitoring and 19 epochs of optical spectroscopy. This is a truly international project with data obtained from observatories in the United Kingdom, La Palma, Slovakia, Russia, Italy and Israel. V723 Cas is an unusually slow nova with a decline characterised by several secondary maxima. Notably, the spectra indicate significant changes around the time of one of the secondary maxima suggesting these represent repeated mass ejections, a phenomenon not previously observed in classical nova eruptions. The MERLIN radio imaging reveal a brightening and expanding remnant, yet to become optically thin at 6cm. This project is continuing.
|Gas density in a 2.5D simulation of outflow from the binary during a nova explosion (click to get a 3MB mpeg movie of the simulation).|