Molecules in bipolar proto-planetary nebulae

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.