Correlation between Infrared Colors and Intensity Ratios of SiO Maser Lines

We present the results of SiO millimeter line observations of a sample of known SiO maser sources covering a wide dust temperature range. A cold part of the sample was selected from the SiO maser sources found in our recent SiO maser survey of cold dusty objects. The aim of the present research is to investigate the causes of the correlation between infrared colors and SiO maser intensity ratios among different transition lines. In particular, the correlation between infrared colors and SiO maser intensity ratio among the J = 1-0 v = 1, 2, and 3 lines are our main concern in this paper. We observed in total 75 SiO maser sources with the Nobeyama 45 m telescope quasi-simultaneously in the SiO J = 1-0 v = 0, 1, 2, 3, 4 and J = 2-1 v = 1, 2 lines. We also observed the sample in the 29 SiO J = 1-0 v = 0 and J = 2-1 v = 0 lines, 30 SiO J = 1-0 v = 0 line, and the H 2 O 6 1,6 -5 2,3 line. As reported in previous papers, we confirmed that the intensity ratios of the SiO J = 1-0 v = 2 to v = 1 lines clearly correlate with infrared colors. In addition, we found possible correlation between infrared colors and the intensity ratios of the SiO J = 1-0 v = 3 to v = 1 and 2 lines. Two overlap lines of H 2 O (i.e., 11 6,6 ν 2 = 1 → 12 7,5 ν 2 = 0 and 5 0,5 ν 2 = 2 → 6 3,4 ν 2 = 1) might explain this correlation if these overlap lines become stronger with an increase of infrared colors, although the phenomena also might be explained in more fundamental ways if we take into account the variation of opacity from object to object.