1. The Chemistry of PPN T. J. Millar, 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, ~ 10 5 – 10 7 F(ISM) Previous high mass loss rate but current mass loss ceased Dense gas, n(H 2 ) ~ 10 7 – 10 9 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 AGBPPNPN HNC/HCN0.00510.5 CN/HCN0.5110 HCO + /HCN0.00050.10.5

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, T eff ~ 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 CH 4, C 2 H 2, C 4 H 2, C 6 H 2, CH 3 CCH, CH 3 C 4 H, C 6 H 6 - Cyanopolyynes HC 3 N, HC 5 N - Oxygen-bearing molecules OH, H 2 O, H 2 CO

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 H 2 O AGB Envelope The remnant of the AGB CSE, dilution due to expansion, photochemistry by internal and external UV photons

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

7. The Chemistry of PPN Redman et al. MNRAS, 345, 1291 (2003) – clumps in expanding AGB winds – follow evolution to PN phase Clumps: n(t) ~ t -3/2, r(t) ~ t 1/2, A V ~ t -1, d(t) ~ t, Tt) ~ t -1/4, G ~ t -2 Initially: 10 7 cm -3, 10 14 cm, 100 mag,, 10 16 cm, 300 K, 100 Molecules survive better in clump than in interclump gas CN/CO ratio increases from AGB – PPN – PN phase In PPN phase, column densities are determined by interclump chemistry

8. The Chemistry of PPN Cernicharo, ApJ, 608, L41 (2004) models the PDR precursor (PDRP) Zone I – G 0 = 10 4, A V = 1 mag Zone II – A V = 2 mag, H 2 self-shielded, CO photodissociated Zone III – A V = 3 mag, CO not photodissociated In all zones, T = 300K, n(H 2 ) = 10 7 cm -3, zone thickness = 10 14 cm, initial molecules H 2, CO, C 2 H 2, CH 4, C 2 H 4 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, H 2 O, CO 2, H 2 CO in Zones I, II, III

9. 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, A UV ~ 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 10 15 cm Initial H 2 abundance, 1.6 10 9 cm -3 Initial extinction, A V = 160 mags Initial UV flux enhancement, 3.2 10 6 Initial CR rate enhancement, 500 Initial temperature, 250 K C/O = 1.2 Initial abundances from AGB observations and calculations

10. The Chemistry of PPN ‘No’ chemistry when A V 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

11. The Chemistry of PPN CRL 618: Observed (heavy) and model (light) abundances, calculated at 9 10 15 cm

12. 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

13. The Chemistry of PPN

14. 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