Astron. Astrophys. 337, 832-846 (1998)

6. Conclusions

In this paper we showed that the models of photoevaporating disks around intermediate mass stars cannot explain the large number of "unresolved" UCHII 's observed by Wood & Churchwell (1989) and Kurtz et al. (1994), because the inferred dust temperatures of these objects are in most cases an order of magnitude lower than those obtained in the numerical models. But the question remains whether the disks of more massive stars than considered here could be responsible for the high abundance of the "unresolved" UCHII 's. Disks around close companions of massive stars should be treated in greater detail. If we assume that circumstellar disks are the rule in the process of star formation, the simplicity and straightforwardness of the model make it favorable compared to alternative suggestions. The extremely high radiation pressure in the vicinity of massive stars could lead to a larger distance between star and disk and thus to smaller dust temperatures.

Another important result of this work is that the profiles of forbidden lines in the optical for the models G2, G3 and G4 with wind velocities of 400-1000 km s^-1 are almost independent of . This is due to the fact that the mass loss rate and velocity of the evaporated disk material is not affected by its interaction with the stellar wind, but by the rate of ionizing photons and the peculiar shock structure which is very similar in the numerical models (see Fig. 3). Since the total mass loss rate is dominated by the evaporated component with low velocity, and the emission is proportional to , the intensity in the lines does not depend on details of the stellar wind and their profiles show the same low-velocity components.

Treatment of optically thick line emission and scattering effects is not possible with the method presented above. In order to compare non-LTE-effects like masing lines, which are observed in various objects related with the formation of massive stars, one has to refer to different methods, e.g. the Monte-Carlo method presented by Juvela (1996). This would immensely help us in our understanding of the process of formation and evolution of massive stars.

© European Southern Observatory (ESO) 1998

Online publication: August 27, 1998
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