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M. Jura (University of California, Los Angeles), C. Kahane (Observatoire de Grenoble)
We report the detection of a narrow (FWHM ~ 5 km s-1) CO (2-1) emission line from the carbon-rich red giant with an oxygen-rich envelope EU And, as well as a highly probable detection of an even narrower CO (2-1) line from the carbon star HD 100764. In addition, we report narrow 13CO emission from the carbon-rich red giant BM Gem as well as upper limits to the emission of HCN, SiO, SO, HCO+ and CS from this star. We interpret the narrow CO emission lines as signatures of long-lived reservoirs of orbiting gas.
By including the Red Rectangle and AC Her, narrow CO emission has been detected from five systems, and we can begin to characterize these apparently similar environments where substantial chemical and dynamical evolution probably has occurred. Some common characteristics are: (1) Their diameters are typically ~1000 AU. (2) The masses of CO are within a factor of 100 of 1027 g. (3) Unlike the envelopes around mass-losing carbon stars where MCO/Mdust ~ 2, the circumstellar orbiting reservoirs 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 0.002 cm to 0.2 cm in radius may be common in these systems. (6) The reservoirs are sometimes asymmetric and possess large clumps.