Molecules in the Circumstellar Disk Orbiting BP Piscium

BP Psc is a puzzling late-type, emission-line field star with large infrared excess. The star is encircled and enshrouded by a nearly edge-on, dust circumstellar disk, and displays an extensive jet system similar to those associated with pre-main sequence (pre-MS) stars. We conducted a mm-wave molecular line survey of BP Psc with the 30 m telescope of the Institut de Radio Astronomie Millimetrique (IRAM). We detected lines of 12CO and 13CO and, possibly, very weak emission from HCO+ and CN; HCN, H2CO, and SiO are not detected. The CO line profiles of BP Psc are well fit by a model invoking a disk in Keplerian rotation. The mimumum disk gas mass, inferred from the 12CO line intensity and 13CO/12CO line ratio, is ~0.1 Jupiter masses. The weakness of HCO+ and CN (relative to 13CO) stands in sharp contrast to the strong HCO+ and CN emission that characterizes most low-mass, pre-main sequence stars that have been the subjects of molecular emission-line surveys, and is suggestive of a very low level of X-ray-induced molecular ionization within the BP Psc disk. These results lend some support to the notion that BP Psc is an evolved star whose circumstellar disk has its origins in a catastrophic interaction with a close companion.