Astron. Astrophys. 346, 243-259 (1999)
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
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 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
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
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.
© European Southern Observatory (ESO) 1999
Online publication: May 6, 1999