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6. SiO masers in an IRAS two-color diagram

An IRAS two-color diagram can be used to roughly identify stars in different stages of their evolution (cf. Olnon et al. 1984; Bedijn 1987; van der Veen & Habing 1988). Due to the different properties of their dust shells, OH/IR stars, PPNe, and PNe occupy slightly different regions in the diagram (although there is also an overlap), and the diagram has been found to be especially useful in the selection of PPNe (e.g., Hu et al. 1993a). Figure 3 (click here) shows a plot of [F60/F25] versus [F25/F12] for the sources in this sample and also from previous observations by Gomez et al. (1990), Nyman et al. (1993a), and the OH/IR objects detected by Jewell et al. (1991). The colors are defined from the IRAS fluxes in the following way: tex2html_wrap_inline3080.

  figure632
Figure 3: IRAS two-color diagram of detected and non-detected sources in our sample and from previous observations by Gomez et al. (1990), Nyman et al. (1993a), and Jewell et al. (1991)

As was also found by Gomez et al. (1990) very few sources are detected beyond [F25/F12]=1.2. Only two sources with redder colors have been found to have SiO masers: OH 231.8+4.2 and OH 15.7+0.8 which are discussed in the previous section. Bedijn (1987) suggests that sources with [F25/F12]>1.2 have ceased their mass loss and are surrounded by a detached envelope.

To study in more detail how the detection rate of SiO masers changes with IRAS colors, we have taken the data from our observations as well as from the papers listed above and calculated the detection rates as function of [F25/F12]. The data were separated into bins of 0.4 units in IRAS color, except for the last bin which is three times larger to contain more objects. The results are shown in Table 3 (click here). The statistical error in the detection rate is given within parenthesis in Table 3 (click here), and was calculated assuming that the detected sources have a Poisson distribution. As found by Gomez et al. (1990) there is a gradual decrease in detection rate with increasing IRAS color. The gradual decrease is partly due to a small decrease in the detection rate of the OH/IR stars, but mainly due to an increase in the number of PPNe and unclassified sources with increasing [F25/F12].

 

Our sample and data fromOur sample,Our sample,
literature, all objectsall objectsOH/IR objects only

[F25/F12]

No. of No. of Det rate No. of No. of Det rate No. of No. of Det rate
obj. det. (%) obj. det. (%) obj. det. (%)
-0.4 - 0.0 22 15 68 (18) 6 4 67 (33) 3 3 100 (56)
tex2html_wrap_inline3094 46 21 45 (10) 21 12 55 (16) 18 11 61 (18)
tex2html_wrap_inline3096 43 18 42 (10) 21 9 43 (14) 10 8 80 (28)
tex2html_wrap_inline3098 46 11 24 (17) 27 6 22 (9) 12 5 42 (19)
tex2html_wrap_inline3100 13 1 8 (8) 9 0 0 0
tex2html_wrap_inline3102 19 0 0 16 0 0 0
tex2html_wrap_inline3104 7 0 0 3 0 0 0
tex2html_wrap_inline3106 8 1 12 (12) 5 1 20 (20) 0
tex2html_wrap_inline3108 6 0 0 4 0 0 0
Table 3: Detection rates

 


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