Radio observations of the classical nova Cygni 92 eighty days after outburst

Abstract

CATACLYSMIC variables are interacting binary stellar systems containing a main-sequence secondary component and a collapsed primary component, usually a white dwarf. A subset of these systems give rise to classical novae, in which an explosive outburst caused by 'runaway' thermonuclear processes on the surface of the white dwarf produces a large increase in visual magnitude and the ejection of substantial quantities of stellar material¹. Radio emission has been detected from several classical novae¹⁷, and is attributed to the thermal bremsstrahlung mechanism operating within an isothermal, expanding shell of ionized gas. Such observations offer a means of directly probing the shells during the early stages of their evolution — long before they can be resolved by optical telescopes. Here we report radio observations of one of the brightest novae in recent years, Nova Cygni 92, using the MERLIN radiotelescope array. The structure of the shell is first resolved eighty days after the initial outburst, revealing nonspherical expansion and thermal electron temperatures that rapidly exceed the expected value for a radiatively excited hydrogen plasma. We explain the observed temperatures by means of a model in which the gas is heated in shocks produced by the thermonuclear runaway.