Atomic Carbon in the Circumstellar Envelopes of Evolved Stars

Abstract

We report James Clerk Maxwell Telescope observations of the 609 μm ground-state fine-structure line of neutral atomic carbon in the circumstellar envelopes of a sample of evolved late-type stars. C I emission is detected in the envelopes of α 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 detections are in good agreement with previous C I observations, indicating that C I is not present in the inner envelope of IRC +10216 (with a C I/CO abundance ratio of ≲0.01) but is present in a shell structure of radius 14'', whereas in α Ori, C I is the main carrier of carbon in the inner envelope, with C I/CO ≈ 5. The absence of C I emission from the other stars places upper limits on C I/CO in the bulk of their envelopes of ≲1 in most cases, implying that CO is a good indicator of their mass-loss rates. The extreme case of α Ori can be ascribed to its supergiant status and the presence of a chromosphere.