1996A&A...313L..13G

We have performed detailed radiative transfer calculations which show that, in circumstellar envelopes of late-type stars, some submillimeter rotational transitions of ²⁹SiO and ³⁰SiO can be inverted with amplification high enough to be detected. The pumping mechanism is due to overlaps between ro-vibrational lines of the three stable SiO isotopomers. We present the detection of one of these lines, the v=3 J=8-7 transition of ²⁹SiO, in four circumstellar sources. This is the highest frequency SiO maser detected up to date, and provides a strong confirmation of infrared SiO overlaps as efficient pumping agents of SiO masers.