Atomic Carbon in the Envelopes of Carbon‐rich Post–Asymptotic Giant Branch Stars

Abstract

Atomic carbon has been detected in the envelopes of three carbon-richevolved stars: HD 44179 (=AFGL 915, the ``Red Rectangle''), HD 56126,and, tentatively, the carbon star V Hya. This brings to seven the numberof evolved star envelopes in which C I has been detected. Upper limitswere found for several other stars, including R CrB. C I was notdetected in several oxygen-rich post-asymptotic giant branch (AGB) stars(OH 231.8+4.2, for example), although it is detected in theircarbon-rich analogs. Two trends are evident in the data. First,circumstellar envelopes with detectable C I are overwhelminglycarbon-rich, suggesting that much of the C I is produced by thedissociation of molecules other than CO. Second, the more evolved theenvelope away from the AGB, the higher the C I/CO ratio. The oxygen-richsupergiant star {α} Ori remains the only oxygen-rich star with awind containing detectable C I. These data suggest an evolutionarysequence for the C I/CO ratio in cool circumstellar envelopes. Thisratio is small (a few percent) while the star is on the AGB, and the C Iis located in the outer envelope and produced by photodissociation. Theratio increases to about 0.5 as the star evolves away from the AGBbecause of the dissociation of CO and other carbon-bearing molecules byshocks caused by the fast winds which appear at the end of evolution onthe AGB. Finally, the ratio becomes {\gt}{\gt}1 as the central starbecomes hot enough to photodissociate CO.