Volume 429, Number 3, January III 2005
|Page(s)||977 - 992|
|Section||Stellar structure and evolution|
|Published online||05 January 2005|
Molecules in bipolar proto-planetary nebulae *
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
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
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.