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

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

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 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, [FORMULA] , 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.