Atomic carbon in the circumstellar envelopes of evolved stars

We report James Clerk Maxwell Telescope observations of the 609 mu m ground -state fine-structure line of neutral atomic carbon in the circumstellar en velopes of a sample of evolved late-type stars. C I emission is detected in the envelopes of alpha Orionis and IRC +10216 with line intensities of 10. 9 and 3.9 K km s(-1), respectively, but is not detected in other envelopes (including Mira) down to the much lower levels of 0.4-1.9 K km s(-1). The d etections are in good agreement with previous C I observations, indicating that C I is not present in the inner envelope of IRC +1.0216 (with a C I/CO abundance ratio of less than or similar to 0.01) but is present in a shell structure of radius 14 ", whereas in alpha Ori, C I is the main carrier of carbon in the inner envelope, with C I/CO approximate to 5. The absence of C I emission from the other stars places upper limits on C I/CO in the bul k of their envelopes of less than or similar to 1 in most cases, implying t hat CO is a good indicator of their mass-loss rates. The extreme case of al pha Ori can be ascribed to its supergiant status and the presence of a chro mosphere.