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