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

The Chemistry of PPN

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


The chemistry of ppn

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

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 ppn1

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

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 ppn1

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 ppn2

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 ppn3

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 ppn4

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 ppn5

The Chemistry of PPN

CRL 618: Observed (heavy) and model (light) abundances, calculated at 9 1015 cm


The chemistry of ppn6

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 ppn7

The Chemistry of PPN


The chemistry of ppn8

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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