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
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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

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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
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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
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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
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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
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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
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The Chemistry of PPN
Woods et al. ApJL, 574, L167 (2002) AA, 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, ?r,
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
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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
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The Chemistry of PPN
CRL 618 Observed (heavy) and model (light)
abundances, calculated at 9 1015 cm
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The Chemistry of PPN
Woods et al. Molecules in Bipolar Proto-Planetary
Nebulae, AA, 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
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The Chemistry of PPN
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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