5. Line intensity ratios
Since we observed more than one transition per star for a few objects mean line intensity ratios can be deduced. An example of multi-line observations is shown in Fig. 1 (CW Cnc).
The combined results (IRVs and SRVs) for the different J levels of 12CO are 5 for I(2-1)/I(1-0), 3 for I(3-2)/I(2-1) and around 1 for I(CO(4-3)/I(3-2). All these ratios differ a lot from star to star. The first two ratios for IRVs alone are between 3 and 4 and within the limitations of our very small sample identical with the values given above. Uncertainties of the absolute calibrations of the different antennas add propably additional scatter.
Nevertheless, it seems clear that we observe a different behaviour than in the case of optically thick Mira variables where a ratio of 2 is typical for I(2-1)/I(1-0). Groenewegen et al. (1995) found for their sample of short period (P400d) O-rich Miras these large ratios, too. They argued that the excitation properties in such thin shells are maybe very different and that possibly radiative excitation could be more important that one by collisions. The situation will be clearer with more objects observed in more than one transition. These multiline observations will also play a crucial role for the forseen modelling.
© European Southern Observatory (ESO) 1998
Online publication: July 20, 1998