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Astron. Astrophys. 359, 1117-1123 (2000)

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28SiO, 29SiO and 30SiO excitation: effects of infrared line overlaps

F. Herpin and A. Baudry

Observatoire de Bordeaux, B.P. 89, 33270 Floirac, France

Received 15 December 1999 / Accepted 17 May 2000

Abstract

We have investigated the influence of several line overlaps between 28SiO, 29SiO and 30SiO on maser emission from the envelopes of late-type stars using the Large Velocity Gradient radiative transfer formalism modified to account for overlaps. Our model includes: (i) 40 rotational levels for each of the [FORMULA] to 4 vibrational states and each isotopic species; (ii) collisions with molecular and atomic hydrogen; and (iii) radiative pumping by a shell of dust grains surrounding the SiO masing layers.

We have shown that several line overlaps may play a role on the excitation of the 28SiO [FORMULA] maser, and on the anomalously weak [FORMULA] emission. We suggest that 28SiO maser emission in the [FORMULA] line, the only one observed in this vibrational level, may also result from line overlaps. For 29SiO, infrared line overlaps successfully explain observed maser emission in the [FORMULA] and [FORMULA] lines. 30SiO, [FORMULA], and [FORMULA] maser emissions could also result from line overlaps, and we find that several overlaps could explain the newly discovered maser line 30SiO [FORMULA]. Finally, we make some predictions for new 28SiO, 29SiO and 30SiO maser lines.

Key words: line: formation – masers – stars: circumstellar matter – stars: late-type

Present address: Dept Física Molecular, I.E.M., C.S.I.C, Serrano 121, 28006 Madrid, Spain

Send offprint requests to: F. Herpin (herpin@isis.iem.csic.es)

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© European Southern Observatory (ESO) 2000

Online publication: July 13, 2000
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