The Chemistry of PPN
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The Chemistry of PPN. T. J. Millar & P. M. Woods, 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

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The Chemistry of PPN

T. J. Millar & P. M. Woods, 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


- Oxygen-bearing molecules


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

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

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

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