Free access article

A&A 429, 977-992 (2005)
DOI: 10.1051/0004-6361:20034388

Molecules in bipolar proto-planetary nebulae

P. M. Woods^{1, 2}, L.-Å. Nyman^{2, 3}, F. L. Schöier⁴, A. A. Zijlstra¹, T. J. Millar¹ and H. Olofsson<sup>4

1</sup>  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)

Abstract
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( J=2-1) and CO( J=3-2) line profiles are compared to new spectra of similar PPN candidates, previously undetected in CO( J=2-1): CPD-53°5736 , IRAS 17106-3046 , IRAS 17245-3951 and IRAS 17441-2411 . CO( J=2-1) 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( J=2-1) 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 ( 200-400 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

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© ESO 2005

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