Nova Cygni 1992 (V1974 Cygni): MERLIN observations from 1992 to 1994

Nova Cyg 1992 was discovered on 1992 February 19. MERLIN has been used at 6 cm and 18 cm to follow the radio development of the nova throughout the outburst. These observations have enabled the authors to resolve the nova from only 80 d after outburst and still detect emission after 682 d. This set of data is unique in the coverage of such an event. In addition to four previously published maps, the authors present two further maps at 6 cm, and five more at 18 cm. The latter maps represent the first resolution of the radio emission from a classical nova at 18 cm during the outburst. Previously published data showed high brightness temperatures at 6 cm, in excess of 40,000 K, and also a high degree of north-south extension in the expansion. The new data show the presence of more recent expansion in a roughly orthogonal direction, and with different velocities. Evidence from the JCMT and the VLA indicates that the emission is thermal free-free and for most of the event this is optically thick, implying that the authors are measuring actual electron temperatures. At 18 cm, brightness temperatures are consistent with the predictions for thermal emission from hot gas, although a mixture of optically thick and optically thin emitting material is required to explain the maps and the radio light curves. The observed dominance of thermal emission mechanisms is in contrast to results for some other stars with radio-emitting outflows, where non-thermal emission has been found. These observations show that detailed modelling of the radio development of classical novae requires models that take into account temperature gradients and asymmetric outflow.