| Issue |
A&A
Volume 429, Number 3, January III 2005
|
|
|---|---|---|
| Page(s) | 977 - 992 | |
| Section | Stellar structure and evolution | |
| DOI | https://doi.org/10.1051/0004-6361:20034388 | |
| Published online | 05 January 2005 | |
Molecules in bipolar proto-planetary nebulae *
1
Department of Physics, UMIST, PO Box 88, Manchester M60 1QD, UK e-mail: Tom.Millar@umist.ac.uk
2
European Southern Observatory, Alonso de Cordova 3107, Casilla 19001, Santiago 19, Chile
3
Onsala Space Observatory, 439 92 Onsala, Sweden
4
Stockholm Observatory, AlbaNova, 106 91 Stockholm, Sweden
Received:
9
September
2003
Accepted:
12
July
2004
Two bipolar proto-planetary nebulae, IRAS 16594-4656
and IRAS 17150-3224, have been detected in various molecular
lines, namely CO, 13CO, HCN and CN, and remain undetected in
several other species. CO(
–1) and CO(
–2) line profiles are
compared to new spectra of similar PPN candidates, previously
undetected in CO(–1): CPD-53°5736,
IRAS 17106-3046, IRAS 17245-3951 and
IRAS 17441-2411. CO(–1) maps of IRAS 16594-4656
and IRAS 17150-3224 show that both PPNe have little separation
between blue, centre and red-shifted emission, and also that the
CO(–1) emission is of a similar size to the telescope beam.
Fractional abundances of all detected molecules (except CO) are
calculated using the results of CO line modelling and a simple
photodissociation model. For those species not detected, upper limits
are derived. Comparisons between these fractional abundances and those
of other PPNe show that IRAS 16594-4656 and
IRAS 17150-3224 are quite under-abundant when compared to
molecule-rich sources like CRL618, CRL2688 and
OH231.8+4.2. As a reason for this deficit, the difference in
circumstellar envelope/torus density between the molecule-rich sources
and the molecule-poor sources is proposed, and supported by a chemical
model which follows the transition of a circumstellar envelope through
the AGB phase and into the PPN phase of evolution. The model includes
the effects of UV radiation, cosmic rays and also X-rays. Finally, the
post-AGB ages of these two objects (
yr) are estimated
using CN/HCN and HCN/CO ratios and both ages are found to be in
agreement with previous figures cited in the literature,
IRAS 17150-3224 being the younger of the two PPNe.
Key words: astrochemistry / stars: AGB and post-AGB / stars: carbon / stars: circumstellar matter / stars: individual: IRAS 16594-4656 / stars: individual: IRAS 17150-3224
© ESO, 2005
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