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28SiO, 29SiO and 30SiO excitation: effects of infrared line overlaps
F. Herpin and
Received 15 December 1999 / Accepted 17 May 2000
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 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 maser, and on the anomalously weak emission. We suggest that 28SiO maser emission in the 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 and lines. 30SiO, , and maser emissions could also result from line overlaps, and we find that several overlaps could explain the newly discovered maser line 30SiO . 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 (firstname.lastname@example.org)
Online publication: July 13, 2000