Atomic Carbon in the Circumstellar Envelopes of Evolved Stars

Astronomy and Astrophysics – Astronomy

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Stars: Circumstellar Matter, Ism: Atoms, Radio Lines: Stars, Stars: Late-Type, Stars: Mass Loss

Scientific paper

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 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 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 alpha 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 alpha Ori can be ascribed to its supergiant status and the presence of a chromosphere.

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