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

Atomic carbon has been detected in the envelopes of three carbon-rich evolved stars: HD 44179 (=AFGL 915, the "Red Rectangle"), HD 56126, and, tentatively, the carbon star V Hya. This brings to seven the number of evolved star envelopes in which C I has been detected. Upper limits were found for several other stars, including R CrB. C I was not detected in several oxygen-rich post-asymptotic giant branch (AGB) stars (OH 231.8+4.2, for example), although it is detected in their carbon-rich analogs. Two trends are evident in the data. First, circumstellar envelopes with detectable C I are overwhelmingly carbon-rich, suggesting that much of the C I is produced by the dissociation of molecules other than CO. Second, the more evolved the envelope away from the AGB, the higher the C I/CO ratio. The oxygen-rich supergiant star α Ori remains the only oxygen-rich star with a wind containing detectable C I. These data suggest an evolutionary sequence for the C I/CO ratio in cool circumstellar envelopes. This ratio is small (a few percent) while the star is on the AGB, and the C I is located in the outer envelope and produced by photodissociation. The ratio increases to about 0.5 as the star evolves away from the AGB because of the dissociation of CO and other carbon-bearing molecules by shocks caused by the fast winds which appear at the end of evolution on the AGB. Finally, the ratio becomes ≫1 as the central star becomes hot enough to photodissociate CO.