The peculiar, fast Nova Herculis 1991

Unlike previously observed fast novae, the temporal evolution of Nova Her 1991 is similar to slower, dusty novae in which the appearance of thermal infrared emission from condensing dust was coincident with a transition in the visible light curve. However, the total luminosity radiated by Nova Her 1991 in the infrared was only ∼5% of the outburst luminosity. We suggest that the dust formed in the shell of Nova Her 1991 consisted of dense clumps with a covering factor of less than unity, and that one of these clumps formed along the line of sight to the central star. In a more uniform shell, the density might not have been high enough for dust to condense at the time the ejecta reached the condensation zone. The decline in dust temperature and total near-infrared flux with time subsequent to outburst is consistent with optically thick clumps moving radially outward from the star at the observed expansion velocity of the ejecta.