Observations of sio masers in the circumstellar envelopes of late type stars

The properties of silicon monoxide masers toward late type stars have been investigated by means of time monitoring studies and very long baseline interferometry. Spectra were obtained in the J = 1 - 0, v = 1, 2, and 3 (43 Ghz) and J = 2 - 1, v = 1 (86 Ghz) transitions of SiO toward twelve long-period variables and Orion/IRc2 at twenty-two epochs over a 2.5 year period. The integrated SiO line flux for most sources exhibits both periodic variations which correlate with the stellar infrared light cycle and secular variations. Detailed analysis of the correspondence of velocity structure in different transitions indicates a common location for the J = 1 - 0 masers in different vibrational states but a different origin for the v = 1, J = 2 - 1 masers. Line intensity ratios of different transitions are examined in order to further constrain maser pump models. The v = 1, J = 1 - 0 and v = 2, J = 1 - 0 profiles are systematically skewed to the red in nine out of ten sources for which reliable determination of the stellar velocity is available; the average velocity shift of the emission centroid is 2.7 km/s. At most epochs, 65 - 100% of the flux in these transitions is from redshifted gas. Over the same period, a similar predominance of redshifted emission is not present in the v = 1, J = 2 - 1 line. No evidence was found for any phase dependent variations in the fraction of the total flux which is redshifted or in any other velocity structure characteristics of the lines. Results from two VLBI experiments using the 75 km baseline between FCRAO and Haystack are discussed. The apparent sizes of the smallest features in the J - 1 - 0 lines toward Mira variables are 2 - 6 x 1013cm, while for the supergiant VX Sgr, the sizes are 2 - 4 x 1014cm