Atomic Carbon in the Circumstellar Envelopes of Evolved Stars

Abstract

We report James Clerk Maxwell Telescope observations of the 609 {μ}mground-state fine-structure line of neutral atomic carbon in thecircumstellar envelopes of a sample of evolved late-type stars. C iemission is detected in the envelopes of alpha Orionis and IRC +10216with line intensities of 10.9 and 3.9 K km s^-1, respectively, but isnot detected in other envelopes (including Mira) down to the much lowerlevels of 0.4-1.9 K km s^-1. The detections are in good agreement withprevious C i observations, indicating that C i is not present in theinner envelope of IRC +10216 (with a C i/CO abundance ratio of{\lt}~0.01) but is present in a shell structure of radius 14'', whereas inalpha Ori, C i is the main carrier of carbon in the inner envelope, withC i/CO ~ 5. The absence of C i emission from the other stars placesupper limits on C i/CO in the bulk of their envelopes of {\lt}~1 in mostcases, implying that CO is a good indicator of their mass-loss rates.The extreme case of alpha Ori can be ascribed to its supergiant statusand the presence of a chromosphere.