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Astron. Astrophys. 361, 601-613 (2000)


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Excitation of HCN in the circumstellar envelope of carbon stars. Maser emission and hyperfine line interaction

Dinh-V-Trung 1,2,3 and Nguyen-Q-Rieu 3

1 Institute of Astronomy and Astrophysics, Academia Sinica, P.O. Box 1-87, Nankang, Taipei 11529, Taiwan, ROC
2 Institute of Physics, P.O. Box 429 BoHo 10000, Hanoi, Vietnam
3 Observatoire de Paris, DEMIRM, 61 Avenue de l'Observatoire, 75014 Paris, France

Received 8 September 1998 / Accepted 9 June 2000

Abstract

We have investigated the excitation of the molecule HCN in the circumstellar envelopes of carbon stars, particularly the HCN(J=1-0) maser emission using a model which takes into account the collisional and radiative excitation, as well as the effect of the overlap of hyperfine lines.

We calculate consistently both the gas temperature and the mass loss rate based on the energy balance equation and the fitting of CO observations. In this way we can obtain more realistic data of the envelope.

In optically thin envelopes such as that around Y CVn, our results suggest that the HCN(J=1-0) transition is inverted by the absorption of 3 µm stellar photons which transfers the population to the (001) vibrational state and by subsequent cascades down to the fundamental state. Mutual amplification of hyperfine components results in HCN line profiles containing several strong maser spikes.

In thicker envelopes, such as IRC+10216, we find that HCN and H13CN are excited mainly by the absorption of infrared photons emitted by the warm dust at 14 µm. The HCN(J=1-0) and H13CN(J=1-0) line profiles calculated by our model contain several features as observed. We also discuss the implication of our results on the mass loss history of IRC+10216.

Key words: masers – radiative transfer – stars: circumstellar matter – stars: individual: IRC+10216, Y CVn

Send offprint requests to: Dinh-V-Trung (trung@asiaa.sinica.edu.tw)

© European Southern Observatory (ESO) 2000

Online publication: October 2, 2000

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