Research on novae at Jodrell Bank

Introduction

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).

GK Per (outburst 1901, image 1993 WHT)	RR Pic (outburst 1927, image 1995 AAT)	DQ Her (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)

Current work

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

Our ground-based imaging survey led to Hubble Space Telescope (HST) observations of 10 extended nova shells which have recently been completed (Gill & O'Brien 2000). When combined with ground-based spectroscopy these observations allow detailed kinematical models of the shells to be constructed enabling accurate distance estimates and investigation of nebular shaping mechanisms.

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).

On the theoretical front we have used hydrodynamical simulations to investigate the ejection and shaping of matter following the thermonuclear runaway. We have included the angular momentum of the accreted envelope and the effects of the material sweeping past the binary companion (Porter, O'Brien & Bode 1998; Lloyd, O'Brien & Bode 1997). These have been shown to result in the development of prolate ellipsoidal shells with density enhancements in the form of tropical rings. We have also begun an investigation of the clumping properties of the ejecta by considering the development of Rayleigh-taylor instabilities in the shell formed by the sweeping up of material ejected earlier in the outburst (e.g. Lloyd, O'Brien & Bode 1997). These clumps, revealed in our high-resolution HST images, are likely to be the sites of dust formation in the nova ejecta. This interaction was also shown to be the probable cause of prompt hard X-ray emission which we discovered in the outburst of V838 Her in 1991 (O'Brien, Lloyd & Bode 1994, Lloyd et al 1992). In a number of subsequent observations through the 1990's using ROSAT, ASCA and BeppoSAX this has been shown to be a relatively common feature of novae in outburst. The unified model for X-ray emission from classical novae we proposed in 1994 has been further strengthened by ASCA, RXTE and BeppoSAX observations of Nova Vel 1999 (V382 Vel, Orio et al 1999, Mukai 1999) and will undoubtedly be tested and refined by observations with the latest generation of X-ray telescopes including Chandra and XMM. Most recently, prompted by our observations of [NII]-enhanced equatorial rings in a number of nova shells, we have begun development of aspherical photoionization models and are investigating mechanisms for producing latitudinal abundance variations in the accreted evelope on the white dwarf prior to outburst.

The research group & collaborators