Mass loss from evolved stars. I - Observations of 17 stars in the CO/2-1/ line

Abstract

Emission in the J =2-1 line of CO has been observed from the molecular envelopes produced by mass loss from 17 evolved stars, four of which are detected as CO sources for the first time. The sizes of several molecular envelopes have been measured; the cloud around IRC+10216 is about 6' in diameter. At a distance of 290 pc, this corresponds to an envelope radius of 0.25 pc and a crossing time of 14,000 years. Comparison with published CO(1-0) data shows that the CO emission from most molecular envelopes is optically thick. A notable exception is provided by Mira, whose antenna temperature in the CO(2-1) line is ~ 10 K. The gas in this envelope appears to be hot and optically thin. Approximate mass loss rates for all of the stars detected in the CO lines are derived from simple model calculations, and vary between ~10-7 M0 yr-1 and ~10-4 M0 yr-1. The results show that, at least for stars in the solar neighborhood, stars for which [C]>[0] lose mass at a greater rate than those for which [C] 3.5 MG. The time scale for mass loss by red giant stars appears to be several X104 years. Values of mass loss rates found from these simple models are used to examine the hypothesis that mass loss is driven by radiation pressure from the central star. This mechanism appears to work for the Mira variables (with [0]>[C]), but the mass loss rates from many of the carbon stars appear to be too high to be driven by radiation pressure. Since this group contains the peculiar planetary nebula NGC 7027, as well as the suggested protoplanetaries CRL 2688 and CRL 618, this could be due to evolution of the central star, although the time scale for this evolution may be too long to be significant. Carbon stars may thus be losing mass by some process other than radiation pressure.