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The Chemistry of PPN. T. J. Millar, School of Physics and Astronomy, University of Manchester. 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
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The Chemistry of PPN T. J. Millar, School of Physics and Astronomy, University of Manchester
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
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
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
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
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
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) ~ t1/2 , AV ~ t-1 , d(t) ~ t, Tt) ~ t-1/4 , G ~ t-2 Initially: 107 cm-3, 1014 cm, 100 mag,, 1016 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
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
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
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
The Chemistry of PPN CRL 618: Observed (heavy) and model (light) abundances, calculated at 9 1015 cm
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
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