Combined BIMA and OVRO Observations of Comet C/1999 S4 (LINEAR)

We present results from an observing campaign of the molecular content of the coma of comet C/1999 S4 (LINEAR) carried out jointly with the millimeter arrays of the Berkeley-Illinois-Maryland Association (BIMA) and the Owens Valley Radio Observatory (OVRO). Using the BIMA array in autocorrelation ("single-dish") mode, we detected weak HCN J = 1–0 emission from comet C/1999 S4 (LINEAR) at 14 ± 4 mK km s-1 averaged over the 143″ beam. The 3 days over which emission was detected, 2000 July 21.9–24.2, immediately precede the reported full breakup of the nucleus of this comet. During this same period, we find an upper limit for HCN J = 1–0 of 144 mJy beam-1 km s-1 (203 mK km s-1) in the 9″ × 12″ synthesized beam of combined observations of BIMA and OVRO in cross-correlation ("imaging") mode. Together with reported values of HCN J = 1–0 emission in the 28″ IRAM 30 m beam, our data probe the spatial distribution of the HCN emission from radii of 1300 to 19,000 km. Using literature results of HCN excitation in cometary comae, we find that the relative line fluxes in the 12″ × 9″, 28″, and 143″ beams are consistent with expectations for a nuclear source of HCN and expansion of the volatile gases and evaporating icy grains following a Haser model.