AN EXTENSIVE, SENSITIVE SEARCH FOR SiO MASERS IN HIGH- AND INTERMEDIATE-MASS STAR-FORMING REGIONS

We present sensitive Very Large Array observations with an angular resolution of a few arcseconds of the J = 1–0 line of SiO in the v = 1 and 2 vibrationally excited states toward a sample of 60 Galactic regions in which stars of high or intermediate mass are currently forming and/or have recently formed. We report the detection of SiO maser emission in both vibrationally excited transitions toward only three very luminous regions: Orion-KL, W51N, and Sgr B2(M). Toward all three, SiO maser emission had previously been reported, in Orion-KL in both lines, in W51N only in the v = 2 line and in Sgr B2(M) only in the v = 1 line. Our work confirms that SiO maser emission in star-forming regions is a rare phenomenon, indeed, that requires special, probably extreme, physical and chemical conditions not commonly found. In addition to this SiO maser survey, we also present images of the simultaneously observed 7 mm continuum emission from a subset of our sample of star-forming regions where such emission was detected. This is, in most cases, likely to be free–free emission from compact and ultracompact H ii regions.