Spectroscopic monitoring of comet C/1996 B2 (Hyakutake) with the JCMT and IRAM radio telescopes

Molecular radio lines were monitored in comet C/1996 B2 (Hyakutake) from 19 96 February 10 to June 23, using the James Clerk Maxwell Telescope and the 30 m telescope and the Plateau de Pure interferometer of the Institut de Ra dioastronomie Millimetrique. We report on observations of HCN, CH3OH, CO, H 2CO, CS, and H2S and on the evolution of their production rates with helioc entric distance (r(h)), from 1.86 down to 0.24 AU at perihelion. Most produ ction rates increased roughly as r(h)(-2.2) down to 0.6 AU preperihelion. C loser to the Sun, they stalled before decreasing beyond 0.6 AU postperiheli on, when observations resumed. The CS/HCN ratio varied as r(h)(-0.8) from 1 .2 to 0.24 AU. A rapid increase of the mean gas temperature in the coma is measured, and the gas expansion velocity increased from 0.55 to 1.6 km s(-1 ), as the comet approached the Sun from 1.6 to 0.3 AU. Molecular abundances of the minor species around 1 AU are similar to those observed in other co mets, while the CO abundance relative to water is high (approximate to 22%) . Coarse mapping was used to check the comet's position and to investigate the density distribution of the molecules within the coma. It provides cons traints on the size of the extended source of formaldehyde, found to be bet ween 1.2 and 2 times the scale length of H2CO itself. The density distribut ion of CS is compatible with its production from the photodissociation of a short-lived molecule such as CS2. The density distribution observed for CO can be mostly explained by a nuclear source.