Orbiting molecular reservoirs around evolved red giant stars

We report molecular emission from the circumstellar envelopes of two carbon -rich stars with oxygen-rich envelopes, EU And and BM Gem. We find a narrow (FWHM similar to 5 km s(-1)) CO (2-1) emission line from EU And and an eve n narrower (FWHM similar to 1 km s(-1)) (CO)-C-13 emission line from BM Gem . We also place upper Limits to the emission of HCN, SiO, SO, HCO+, and CS from BM Gem. We argue that the narrow CO emission lines are signatures of l ong-lived reservoirs of orbiting gas and that standard models for CO emissi on from red giant winds are not appropriate for these two stars. By includi ng the Red Rectangle and AC Her, narrow CO emission characteristic of gravi tationally bound gas has been detected from four post-main-sequence systems , and we can begin to characterize these apparently similar environments. S ome common characteristics are the following: (1) Their diameters are typic ally between similar to 100 and similar to 1000 AU. (2) The masses of CO ar e near 10(27) g. (3) Unlike the envelopes around mass-losing carbon stars w here M-CO/M-dust similar to 2, the circumstellar orbiting reservoirs often appear to have M-CO < M-dust. (4) Molecules in addition to CO seem to be ra re; we have yet to detect any other abundant gas-phase molecule besides CO. (5) Grains from 20 mu m to 0.2 cm in radius may be common in these systems . (6) The reservoirs can possess large clumps. These properties can be unde rstood if substantial chemical and dynamical evolution has occurred during the long lifetime of the orbiting reservoirs which are probably produced du ring mass loss in a binary system.