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
AGB PPN PN
HNC/HCN 0.005 1 0.5
CN/HCN 0.5 1 10
HCO+/HCN 0.0005 0.1 0.5
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 ChemistryUV photons dissociate molecules formed in AGB envelope,
produce radicals which then form new species, primarily carbon chainsUV radiation dissociates CO which injects O atoms into chemistry
Shock ChemistryInteraction of HV outflow with remnant AGB envelope. High
temperature chemistry converts O into OH and H2O
AGB EnvelopeThe remnant of the AGB CSE, dilution due to expansion,
photochemistry by internal and external UV photons
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
The Chemistry of PPNRedman 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 PPNCernicharo, 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 PPNWoods 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 PPNWoods 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 evolves0.79
0 0 0( / ) where
increases from 100 to 1000K in 1000 yr
T T r r T
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