Molecules in bipolar proto-planetary nebulae

Abstract

Abstract. Two bipolar proto-planetary nebulae, IRAS 16594-4656 and IRAS 17150-3224, have been detected in various molecular lines, namely CO, 13 CO, 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.

Carbonaceous molecules in the oxygen-rich circumstellar environment of binary post-AGB stars

“…CO molecules can be destroyed due to photodissociation, but the UV radiation of these sources might not be strong enough to dissociate large quantities of CO. Alternatively, X-ray radiation, caused by shocks, is able to break up CO (Woods et al 2005). Another possibility is the Fischer-Tropsch catalysis mechanism, a chemical reaction process that can form hydrocarbons and H 2 O out of CO and H 2 .…”

The UMIST database for astrochemistry 2006

“…The use of astrochemical models to interpret observations of interstellar and circumstellar molecular line observations is widespread; recent years have seen the application of such models to protoplanetary disks (Aikawa & Herbst 1999;Aikawa et al 2002;Markwick et al 2002;Ilgner et al 2004;Semenov et al 2005), protoplanetary nebulae (Cernicharo 2004;Woods et al 2005), hot molecular cores (Viti et al 2004;Nomura & Millar 2004;Wakelam et al 2004), the early universe Lepp 2003) as well as the more traditional applications to diffuse and dark interstellar clouds (Stantcheva & Herbst 2004;Lee et al 2004), interstellar shocks (Bergin et al 1999;, YSOs (Rodgers & Charnley 2003;Doty et al 2004;Stäuber et al 2005), and circumstellar envelopes (Brown & Millar 2003;Willacy 2004). Many of these papers consider grain chemistry either implicitly or explicitly and several have been aimed at exploring deuterium fractionation (Roberts et al 2003(Roberts et al , 2004 and, in particular, the possible role that grains may play in the creation of the high fractionation ratios observed in many multi-deuterated species.…”

Galactic Bulge PNe: Carbon molecules in oxygen-rich environments

“…These tori remain molecular for some time after the ionization of the nebula has started, due to the trapping of the ionization front and due to the shielding effects of the dust column density (Woods et al 2005). A mini PDR (photon-dominated region) is expected in these molecular regions (Phillips et al 2010), slowly being overrun by the advancing ionization front.…”