Orbiting Molecular Reservoirs around Evolved Red Giant Stars

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Abstract
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 ~ 5 km s-1) CO (2-1) emission line from EU And and an even narrower (FWHM ~ 1 km s-1) 13CO 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 long-lived reservoirs of orbiting gas and that standard models for CO emission from red giant winds are not appropriate for these two stars. By including the Red Rectangle and AC Her, narrow CO emission characteristic of gravitationally bound gas has been detected from four post-main-sequence systems, and we can begin to characterize these apparently similar environments. Some common characteristics are the following: (1) Their diameters are typically between ~100 and ~1000 AU. (2) The masses of CO are near 1027 g. (3) Unlike the envelopes around mass-losing carbon stars where MCO/Mdust ~ 2, the circumstellar orbiting reservoirs often appear to have MCO < Mdust. (4) Molecules in addition to CO seem to be rare; we have yet to detect any other abundant gas-phase molecule besides CO. (5) Grains from 20 μm to 0.2 cm in radius may be common in these systems. (6) The reservoirs can possess large clumps. These properties can be understood if substantial chemical and dynamical evolution has occurred during the long lifetime of the orbiting reservoirs which are probably produced during mass loss in a binary system.

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