The Chemistry of PPN

1. The Chemistry of PPN T. J. Millar & P. M. Woods, School of Physics and Astronomy, University of Manchester

2. The Chemistry of PPN • Short time scales, ~ 1000 yr • Fast bipolar outflows, up to 200 km s-1 in CRL 618 • Interacting stellar winds model • Hot central object, 10,000 – 30,000 K • Strong increasing central UV field, ~ 105 – 107 F(ISM) • Previous high mass loss rate but current mass loss ceased • Dense gas, n(H2) ~ 107 – 109 cm-3 • Evolution of AGB molecular envelope • Over 20 molecules detected

3. Molecular Line Observations of PPN Decrease in complexity from AGB → PPN → PN 50 → 20 → 8 molecules Large increase in HCO+ abundance in PPN CN and HNC abundances increase in the post-AGB phase Importance of UV increases, of shocks decrease as PPN evolve

4. Molecular Line Observations of PPN CRL 618 (Cernicharo et al. 2001a,b; Herpin & Cernicharo 2000) intermediate age PPN, 200-1000 yr old, B0 star, Teff ~ 30,000 K, compact HII region, confined by a dense torus, bipolar outflow at ~ 200 km s-1, CSE expansion at ~ 20 km s-1 - Large hydrocarbon species CH4, C2H2, C4H2, C6H2, CH3CCH, CH3C4H, C6H6 - Cyanopolyynes HC3N, HC5N - Oxygen-bearing molecules OH, H2O, H2CO

5. Modelling the Chemistry of PPN Photon-dominated Chemistry UV photons dissociate molecules formed in AGB envelope, produce radicals which then form new species, primarily carbon chains UV radiation dissociates CO which injects O atoms into chemistry Shock Chemistry Interaction of HV outflow with remnant AGB envelope. High temperature chemistry converts O into OH and H2O AGB Envelope The remnant of the AGB CSE, dilution due to expansion, photochemistry by internal and external UV photons

6. The Chemistry of PPN CRL 618 Herpin & Cernicharo, ApJ, 530, L129 (2000) identified three main molecular components – a torus (with PDR), a HV outflow and the AGB CSE

7. The Chemistry of PPN Cernicharo, ApJ, 608, L41 (2004) models the PDR precursor (PDRP) Zone I – G0 = 104, AV = 1 mag Zone II – AV = 2 mag, H2 self-shielded, CO photodissociated Zone III – AV = 3 mag, CO not photodissociated In all zones, T = 300K, n(H2) = 107 cm-3, zone thickness = 1014 cm, initial molecules H2, CO, C2H2, CH4, C2H4 and HCN Abundance peaks ~ 0.2 yr Steady state ~ few yr Faster than expansion of HII region High fractional abundances of carbon chains, etc in Zones II and III O freed from CO forms OH, H2O, CO2, H2CO in Zones I, II, III

8. The Chemistry of PPN Woods et al. ApJL, 574, L167 (2002) & A&A, 402, 189, (2003) Modelled a thin slab of high-density gas as it moved away from central object – the expanding inner edge of the remnant AGB circumstellar envelope Constant thickness, Δr, density n(r) ~ r-2, AUV ~ r-2 Expansion velocity 5 km s-1 (if v = 20 km s-1, dilution is rapid and photodissociation dominates; no complex molecules formed) Equivalent mass-loss rate, 3 10-3 solar masses per yr Initial radius, 2.5 1015 cm Initial H2 abundance, 1.6 109 cm-3 Initial extinction, AV = 160 mags Initial UV flux enhancement, 3.2 106 Initial CR rate enhancement, 500 Initial temperature, 250 K C/O = 1.2 Initial abundances from AGB observations and calculations

9. The Chemistry of PPN ‘No’ chemistry when AV is less than about 10 mags – photodestruction dominates – ‘radiation catastrophe’ Collision times very short ~ 0.1 s, so complex species are formed rapidly once parent species start to break down

10. The Chemistry of PPN CRL 618: Observed (heavy) and model (light) abundances, calculated at 9 1015 cm

11. The Chemistry of PPN Woods et al. Molecules in Bipolar Proto-Planetary Nebulae, A&A, in press SEST observations of IRAS16594-4656 (~ 400 yr old) and 17150-3224(~ 200 yr old) Other than CO, only HCN and CN detected; many upper limits conclude that these 2 PPN are molecule-poor Chemical model: Calculate radial distributions in a C-rich CSE Expansion velocity = 14 kms-1 Mass-loss rate = 10-5 solar masses per yr X-ray and CRP ionisation included Envelope heating as central star evolves

12. The Chemistry of PPN

13. The Chemistry of PPN • Summary: • Importance of photons • CO dissociation leads to OH and H2O formation • High-densities, short time-scales, seconds to years • Rich organic chemistry driven by acetylene parent • Shock chemistry may be important in some PPN • Fine balance between UV as a promoter of molecular complexity and as a destructive force – radiation catastrophe • UV eventually destroys molecules – PN stage is molecule poor