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Astron. Astrophys. 346, 243-259 (1999)

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5. Conclusions

We have used the results of our multi-wavelength observational campaign in the near- and mid-infrared as well as at 1.3 mm to argue on the nature of the ultracompact HII region G5.89. The main aspects of our reasoning are:

  • G5.89 is a spherical shell of dust around an O6 ZAMS star. The interior of this shell is free of dust and non-uniformly filled with ionized gas.

  • The spherical shell has a channel opening which manifests itself in asymmetric appearances at shorter (near-) infrared wavelengths and is clearly visible on the VLA maps.

  • At least one outflow escapes through these openings. It is visible by two [FORMULA] emission features which probably mark its ends north and south of the source.

  • G5.89 shows no signs of a disk being present around its central star. However, the structure of the radio shell and the presence of the outflow point to the former presence of such a disk, the remains of which might still be undergoing the final stage of their destruction.

  • G5.89 is surrounded by a large, dense cloud of cold dust which is visible at 1.3 mm and obscures the view to half of G5.89's shell as well as to background sources south and southwest of G5.89.

  • The rim of this cloud is reflecting light and probably is also shock excited and/or ionized by neighbouring sources, some of which are identified as massive stars themselves.

  • As such, G5.89 seems to be a very young member of a larger cluster, where star formation seems to have happened in the past and is still going on.

Together with our findings from Paper I and from Stecklum et al. (1998), we can now state that UCHII s do form around single stars, either as externally ionized disks around low-mass stars or as spherical shells around high-mass stars as predicted by classical theories of UCHII s. However, it seems common that massive stars form in clusters, and that ionized regions of individual members will eventually merge and create larger ionized structures. This may help to explain the variety of shapes and the longevity of the ultracompact phase.

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© European Southern Observatory (ESO) 1999

Online publication: May 6, 1999