 |
 |
Astron. Astrophys. 317, 859-870 (1997)
6. Conclusions
One of the main aims of this study was to investigate what kind of
dust is necessary to explain the unusual infrared spectrum observed
from IRAS 22272 ![[FORMULA]](img1.gif)
5435. We have found that PAHs
(or molecules with very similar properties, e.g. hydrogenated
solid amorphous carbon - see Buss et al. 1990) are essential for
an interpretation of the spectral energy distribution. We also find
that quantum treatment of their heating and cooling processes to be
very important even for such a low temperature as 5300 K for the
central star. It has been shown also that if the temperature of pure
MgS-FeS grains is high enough we are able, in principle, to explain
the observed 30 µm band by their IR emission, especially
if we have a variety of shapes of grains inside the envelope. Until
measurements of the absorption properties of MgS in the UV wavelengths
become available, it will be rather difficult to rule out MgS as a
possible carrier of the feature. On the other hand, sulfide mantles
with amorphous carbon cores are found to be a less probable carrier of
the feature as they possess two separate maxima with minimum around 30
µm in their absorption properties. The minimum disappears
for thick coats, but this needs more sulphur than is available if
coats are distributed proportionally to the surfaces of all AC grains
present. Because the emitter(s) responsible for the 21 and 30
µm features are presently unknown, we have constructed an
empirical opacity function which gave us a reasonable fit to the
energy distribution between 18 and 48 µm.
The best fit constructed for IRAS 22272 allowed us to draw some
conclusions concerning the evolutionary status of the source. We have
shown that the estimated distance to this source is about 1.45 kpc if
the mass of the central star is close to 0.60 ![[FORMULA]](img2.gif)
.
The derived dynamical age of the nebula is then in agreement with the
expected post-AGB evolution of such a hydrogen-burning star.
© European Southern Observatory (ESO) 1997
Online publication: July 8, 1998
helpdesk.link@springer.de