Correlation between Infrared Colors and Intensity Ratios of SiO Maser Lines

Abstract

We present the results of SiO millimeter line observations of a sampleof known SiO maser sources covering a wide dust temperature range. Acold part of the sample was selected from the SiO maser sources found inour recent SiO maser survey of cold dusty objects. The aim of thepresent research is to investigate the causes of the correlation betweeninfrared colors and SiO maser intensity ratios among differenttransition lines. In particular, the correlation between infrared colorsand SiO maser intensity ratio among the J=1-0 v=1, 2, and 3 lines areour main concern in this paper. We observed in total 75 SiO masersources with the Nobeyama 45 m telescope quasi-simultaneously in the SiOJ=1-0 v=0, 1, 2, 3, 4 and J=2-1 v=1, 2 lines. We also observed thesample 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}O6_{1,6}-5_{2,3} line. As reported in previous papers, weconfirmed that the intensity ratios of the SiO J=1-0 v=2 to v=1 linesclearly correlate with infrared colors. In addition, we found possiblecorrelation between infrared colors and the intensity ratios of the SiOJ=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--{\gt}12_{7,5}{ν}_{2}=0 and 5_{0,5}{ν}_{2}=2--{\gt}6_{3,4} {ν}_{2}=1) mightexplain this correlation if these overlap lines become stronger with anincrease of infrared colors, although the phenomena also might beexplained in more fundamental ways if we take into account the variationof opacity from object to object.