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 ^ { 29 } SiO J = 1 - 0Â v = 0 and J = 2 - 1 v = 0 , and ^ { 30 } SiO J = 1 - 0 v = 0 lines , 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 \& 2 lines .
Two overlap lines of H _ { 2 } O ( i.e. , 11 _ { 6 , 6 } Â \nu _ { 2 } = 1 \rightarrow 12 _ { 7 , 5 } Â \nu _ { 2 } = 0 and 5 _ { 0 , 5 } Â \nu _ { 2 } = 2 \rightarrow 6 _ { 3 , 4 } Â \nu _ { 2 } = 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 .