THE ANOMALOUS SIO MASER TRANSITION V=2 J=2-1

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 ne w detections (out of a total of 10 detected stars) and significant upp er limits for 37 other stars, which allows for the first time a statis tical study of this maser transition. The unexpectedly weak emission i n this line is confirmed. There is also a clear trend favoring the emi ssion of the v=2 J=2-1 line in S-type stars, with respect to O-rich so urces (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 t han 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 sou rce. The possible origin of these properties is discussed. We conclude that the mechanism previously proposed by Olofsson et al., introducin g the effects of line overlap between one re-vibrational transition of SiO and another one of H2O, can explain the observed peculiarities of the SiO v=2 J=2-1 line.