Issue |
A&A
Volume 431, Number 2, February IV 2005
|
|
---|---|---|
Page(s) | 523 - 538 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20035746 | |
Published online | 04 February 2005 |
Physical conditions in Photo-Dissociation Regions around Planetary Nebulae *,**
1
SRON National Institute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands e-mail: jbs@isc.astro.cornell.edu
2
Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
3
Department of Astronomy, Cornell University, 219 Space Sciences Building, Ithaca, NY 14853, USA
Received:
25
November
2003
Accepted:
1
October
2004
We present observations of the infrared fine-structure lines
of ] (34.8 μm),
] (63.2 and 145.5 μm)
and
] (157.7 μm) obtained with the ISO SWS and LWS
spectrographs of nine Planetary Nebulae (PNe). These lines originate in
the Photo-Dissociation Regions (PDRs) associated with the nebulae
and provide useful information on the evolution and excitation
conditions of the ejected material in these regions. In order to
interpret the observations, the measured line intensities have been
compared with those predicted by photo-dissociation models. This
comparison has been done taking into account the C/O content in the
nebulae. The densities derived with this comparison show a large
scatter for some nebulae, probably because the density is higher
than the critical density. Therefore, they are no longer sensitive
to this parameter implying that transitions from other species with
higher critical density should be used. The possible contribution of
shocks to the observed emission characteristics of these PNe is
briefly discussed and it is shown that the radiation field is the
main driving force responsible for the atomic lines in the PNe that
have been studied. In addition, data on the pure rotational lines
of H2 in three nebulae (NGC 7027, NGC 6302 and Hb 5) are also
presented. Assuming local thermal equilibrium the rotational
temperature and densities have been derived. We have derived the
mass of atomic gas in the PDR associated with these PNe and compared
those to ionic masses derived from Hβ and molecular masses
derived from low J CO observations. This comparison shows that for
these nebulae, the PDR is the main reservoir of gas surrounding
these objects. A comparison of the results of these evolved
PNe with very young PNe from the literature suggests that as the
nebula ages the relative amount of ionic gas increases at the
expense of the atomic and molecular mass.
Key words: ISM: lines and bands / ISM: atoms / ISM: molecules / ISM: planetary nebulae: general
© ESO, 2005
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