1996A&A...314..883B

We present high-sensitivity observations of the SiO transition v=2 J=2-1, which is known to present anomalous maser emission, together with simultaneous observations of the v=1 J=2-1 intense maser. Only 3 stars were previously detected in the v=2 J=2-1 transition. We present 8 new detections (out of a total of 10 detected stars) and significant upper limits for 37 other stars, which allows for the first time a statistical study of this maser transition. The unexpectedly weak emission in this line is confirmed. There is also a clear trend favoring the emission of the v=2 J=2-1 line in S-type stars, with respect to O-rich sources (that are the most common emitters in the other SiO lines): the v=2/v=1 J=2-1 line ratio is at least 10 times larger in S-type stars than in M-type sources. We also find a trend of the v=2 J=2-1 emission to be relatively red-shifted, confirming a previous result for one source. The possible origin of these properties is discussed. We conclude that the mechanism previously proposed by Olofsson et al., introducing the effects of line overlap between one ro-vibrational transition of SiO and another one of H₂O, can explain the observed peculiarities of the SiO v=2 J=2-1 line.