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 evolvedstars: HD 44179 (=AFGL 915, the `Red Rectangle'); HD 56126; and, tentatively,the carbon star V Hya. This brings to seven the number of evolved starenvelopes in which CI has been detected. Upper limits were found for severalother stars, including R CrB. CI was not detected in several oxygen rich postasymptotic giant branch (AGB) stars (OH231.8+4.2, for example), although it isdetected in their carbon-rich analogues. Two trends are evident in the data.First, circumstellar envelopes with detectable CI are overwhelmingly carbonrich, suggesting that much of the CI is produced by the dissociation ofmolecules other than CO. Second, the more evolved the envelope away from theAGB, the higher is the CI/CO ratio. The oxygen-rich supergiant star Alpha Oriremains the only oxygen rich star with a wind containing detectable CI. Thesedata suggest an evolutionary sequence for the CI/CO ratio in cool circumstellarenvelopes. This ratio is small (a few %) while the star is on the AGB, and theCI is located in the outer envelope and produced by photodissociation. Theratio increases to about 0.5 as the star evolves away from the AGB because ofthe dissociation of CO and other carbon-bearing molecules by shocks caused bythe fast winds which appear at the end of evolution on the AGB. Finally, theratio becomes >> 1 as the central star becomes hot enough to photodissociateCO.Comment: 31 pages, AASLaTeX, 8 figure