A STUDY OF CIRCUMSTELLAR ENVELOPES AROUND BRIGHT CARBON STARS .1. STRUCTURE, KINEMATICS, AND MASS-LOSS RATE

We have performed a survey of circumstellar CO emission on a sample of bright carbon stars, which is relatively complete out to about 900 pc from the Sun. In total, 68 detections were made. All objects within 600 pc of the Sun were detected. The result suggests that the large majority of all carbon stars have circumstellar envelopes. The CO-emitting parts of these envelopes appear to have angular sizes less than about 15'. The median gas expansion velocity is 12.5 km s-1, and the expansion velocities for the majority of the objects fall in the range 9-15 km s-1. We find no significant differences between the expansion velocities estimated from the CO (1-0) and CO (2-1) lines. The expansion velocities show a tendency to be higher for stars that lie close to the Galactic plane. Relatively accurate stellar radial velocities have also been obtained. The mass-loss rate estimated from the CO data correlates well with an infrared excess estimate. This has been used to obtain mass-loss rates for the entire sample. The median mass-loss rate is 1.5 x 10(-7) M. yr-1, and the mass-loss rate for the majority of the stars lies within the narrow range (0.8-2.5 ) X 10(-7) M. yr-1. The mass-loss rates of the Mira variables are on the average a factor of 10 higher than those of the Lb, SRa, and SRb variables, which are, on the other hand, very similar. A corollary to this is that the mass-loss rate increases with period and variability amplitude. We find a dependence of the mass-loss rate on the stellar effective temperature, but no apparent dependence on the photospheric carbon excess (with respect to oxygen) and C-12/C-13 ratio in spite of the fact that the stars span quite a range in these properties. The gas expansion velocity shows a dependence on the stellar effective temperature, and possibly also on the carbon excess. The mass-loss rate and the gas expansion velocity are weakly correlated with each other. This trend could be a luminosity effect due to radiation pressure on grains. The substantial scatter in the data suggests a mass-loss mechanism that can produce widely different mass-loss rates with only marginal effects on the expansion velocity. We present results that imply that the simple estimate of the mass-loss rate from the strength of the CO emission used in this paper systematically underestimates the mass-loss rate. The underestimate becomes larger the lower the mass-loss rate, and it amounts to about a factor of 2-4 for the objects with the lowest mass-loss rates (< 10(-7) M. yr-1). Accurate estimates of the mass-loss rate using the circumstellar CO emission may in fact be quite difficult to obtain before a significantly better physical understanding of the envelopes is at hand. The sample contains three stars-U Ant, S Sct, and TT Cyg-where there is convincing evidence for highly episodic mass loss, possibly caused by a thermal pulse. The scarcity of such objects in our sample, and the assumption that all carbon stars go through this phase at least once, suggest carbon star lifetimes of about 2 X 10(5) yr or more and a carbon star birthrate consistent with other data.