Astron. Astrophys. 355, 979-993 (2000)

6. Conclusions

We have performed observations of the H₂O maser emission in the circumstellar envelopes of 11 late-type stars with the VLA. In order to study the maser spatial and velocity morphology, we have constructed a computer program ("FG3", described in the appendix). Using this program, we have been able to place the H₂O masers on an expanding shell around the central star in five cases (IRC +60169, R Crt, RX Boo, U Her, and R Cas, see Table 8), despite of the fact that the emission is rather clumpy and the shells appear incomplete. A characteristic radius of  AU emerges for H₂O maser emission in AGB stars. At this distance from the star, dust is still forming (or dust grains are still getting larger), so H₂O masers are in the region of acceleration, where the expanding envelope has not reached its terminal velocity. Moreover, our maps confirm that there is a correlation between the overall sizes of the H₂O maser regions and the mass-loss rate from the star, in agreement with the theoretical predictions. Note that the VLA lacks the resolution needed to resolve the individual compact maser spots.

An important advantage in the use of "FG3", over earlier studies, is the much better spectral linewidth estimate, compared to the usual procedure of fitting positions in each spectral channel map and later using these results to infer linewidths. The maser linewidth contains information about the physical conditions in the region where the maser phenomena occur. In particular, it provides a powerful tool to discriminate whether or not these masers are saturated. For completely unsaturated masers, maser theory predicts that the strongest features must have also the narrowest linewidths. We clearly see evidence for line narrowing toward U Her, RX Boo, and R Cas, which indicates that these masers are unsaturated.

© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000
helpdesk.link@springer.de