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 mainly concerned in this paper. We observed in total 75 SiO maser sources with the Nobeyama 45m 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 SiO J = 1−0 v = 0 and J = 2−1 v = 0, and SiO J = 1−0 v = 0 lines, and the H2O 61,6−52,3 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&2 lines. Two overlap lines of H2O (i.e., 116,6 ν2 = 1→127,5 ν2 = 0 and 50,5 ν2 = 2→63,4 ν2 = 1) might explain these correlation if these overlap lines become stronger with increase of infrared colors, although the phenomena also might be explained by more fundamental ways if we take into account the variation of opacity from object to object. Subject headings: masers — stars: AGB and post-AGB — stars: late-type — stars: mass loss — stars: statistics Academia Sinica Institute of Astronomy and Astrophysics, P.O. Box 23-141, Taipei 10617, Taiwan; email(JN): [email protected] Department of Astronomy, University of Illinois at Urbana-Champaign, 1002 W. Green St., Urbana, IL 61801 Nobeyama Radio Observatory, National Astronomical Observatory, Minamimaki, Minamisaku, Nagano 384-1305, JAPAN; [email protected]