ISM & Astrochemistry Lecture 6

Stellar Evolution

Star Death

IRC (CW Leo) Nearby (~130 pc) high mass-loss carbon star (AGB) Brightest object in the sky at 2 microns – optically invisible Carbon dust envelope detected out to 200’’ = 25,000 AU ( ~ 1 lt yr) Molecular shells at ~ AU >60 molecules detected: CO, C 2 H 2, HC 9 N... Newly discovered anions C 8 H -, C 6 H -, C 4 H -, C 3 N -, C 5 N - Recent detections of H 2 O, OH and H 2 CO Figures from Leao et al. (2006) Lucas and Guelin et al. (1999)

IRC HC 3 N J = 5-4 emission in plane of sky shown as contours Molecular shells/arcs shown as thick green lines Contour is optical V-band image in false colour (Trung & Lim 2008, ApJ, 678, 303) Molecular emission is correlated with dust shells/density enhancements

IRC Comparison of HC 3 N J = 5-4 (greyscale) with HC 5 N J = 9-8 (contours) using the VLA (Trung & Lim 2008) Very close spatial correlation between HC 3 N and HC 5 N emission (and presumably abundances)

Chemical Structure of AGB CSE

LTE Chemistry in AGB Stars CS Maser emission is an excellent probe of conditions near the photosphere, particularly if driven by IR pumping. In IRC+10216, HCN detected up to v=4, SiS detected up to v=0, J=15-14 (Exposito et al. 2006) SO Effect of C/O Ratio on Fractional Abundances

Dust Formation in AGB Stars Dust formation in AGB stars requires: (i) a cooling flow (ii) high collisional rates (iii) time Shocks caused by pulsations are critical – they give large density enhancements, strong post-shock cooling, and lift material slowly away from stellar surface Spatial scale is less than 5 stellar radii Probe with vibrationally excited/maser species (line-widths less than terminal velocity/IR pumped) – eg thermal v=1SiS emission (Exposito et al. 2006) SMA J = 5-4 SiO observations (Schoier et al. 2006) show SiO much more abundant than LTE value in IRC at 3-8 stellar radii but lower at larger radii – condensation on to dust grains?

Pulsations and Shock Waves Density and temperature profiles as a function of phase following a 20 km/s shock at a radial distance of 1.2 stellar radii Shock propagates outward with shock velocity decreasing with increasing radial distance Density cm -3 T = K

Shock Chemistry Input – LTE abundances for IRC parameters Each cycle breaks down into three zones: (i)Immediate post-shock – H 2 and other LTE molecules destroyed (ii) Post-shock – H 2 and other molecules form (ii) Dynamical time less than chemical time – freeze-out of abundances (iv) No significant water formed in shock Abundances at the end of the pulsation zone can be very different from LTE SiO enhanced, HCN, CS destroyed in C-stars (Willacy & Cherchneff 1998)

Shock Chemistry Formation of Molecules in O-rich and S-type Stars Form carbon-bearing molecules in O-rich stars – C/O < 1: (i)HCN increased by 6 orders of magnitude above LTE value (ii)CO 2 increased by 3 orders of magnitude (iii)CS increased by 3 orders of magnitude (Duari, Cherchneff & Willacy 1999) For S-type stars (Chi Cygni) – C/O ~1: HCN in inner envelope increased by 4 orders of magnitude (vibrationally excited HCN detected here – IR pumping within 33 stellar radii) (Duari & Hatchell 2000)

Photochemistry in CSEs Destruction of Parents by IS UV Radiation Field Self-Shielding H 2 – very reactive, daughter (H atoms) unreactive CO – very unreactive, daughters (C, C + ) very reactive Cosmic ray ionisation f(H 3 + ) varies as r 2 N(H 3 + ) ~ cm -2 for CRI rate of s -1, an order of magnitude less than that detected in the interstellar medium Photodissociation and photoionisation Acetylene is the species which determines the complexity of the hydrocarbon chemistry

Photochemistry in CSEs Shell distributions – the photodestruction of acetylene IRC Acetylene has a relatively large photoionisation cross-section Ions and radicals form in outer CSE where both density and UV field are relatively large (Millar, Herbst & Bettens 2000)

Hydrocarbon formation Shell distributions – rapid formation of hydrocarbons IRC C 2 H and C 2 H 2 + are both very reactive with acetylene and derivative species Peak abundances occur at slightly larger radii as size increases Degradation of grains may give inverse behaviour (Millar, Herbst & Bettens 2000)

Cyanopolyyne formation Shell distributions – rapid formation of hydrocarbons IRC Neutral chemistry important in forming cyanopolyynes and other molecules (Millar, Herbst & Bettens 2000) (Guelin et al. 2000)

Anion formation Anion column densities ~ neutral column densities IRC High electron fraction and high density allow formation of abundant anions C 4 H -, C 6 H -, C 8 H -, C 3 N - recently detected in IRC (Millar, Herbst & Bettens 2000)

Modelling the circumstellar shells Leao et al (2006): Concentric rings

H 2 radial density profile time-evolution Density-enhanced packets of gas and dust move outwards through the CSE, their chemistry evolves with time Cordiner & Millar, ApJ, 697, 68 (2009) 2 arcsec thick shells (90 yr enhanced mass-loss) 12 arcsec apart (540 yr between events) Density enhancement = 5

IRC – Standard vs Rate06 + Rings Rings provide additional extinction – so parents are destroyed further out HC 3 N no longer most abundant cyanopolyyne – effect of Rate06

Anion results

IRC results Hydrocarbon radicals No shells – number density Shells – 3mm emission

No shells Shells 3mm emission Hydrocarbon anionsHC 3 N and HC 5 N

Comparison with 3mm emission observations

The effect of anions in the CSE Free electron abundance reduced  increases number of cations Anion abundances > electron abundance within part of envelope

Formation of Water Willacy, ApJL, 600, L87 (2004) In C-rich source (IRC+10216), f(H 2 O) ~ and f(OH) ~ Possibility – evaporation of comets (formed in O-rich material in the neighbourhood – presence of HDO?) Shock chemistry ? No Formation of water by Fischer- Tropsch catalysis CO + 3H 2  CH 4 + H 2 O on iron grains Need few percent of Fe in iron or F-Ni alloy.

IRC summary Successfully model C 6 H -, C 8 H -, C 3 N - abundances C 6 H - and C 8 H - formed by radiative electron attachment, C 3 N - formed by C n H - + N C 4 H - overproduced by ~ 100 times CN - and C 2 H - predicted to be observable in the inner CSE even though the electron attachment rate is very low, due to reaction of H - with HCN and C 2 H 2 Density-enhancements in the CSE may result in narrow emission rings / arcs of carbon-chain species and their anions

Photochemistry in O-rich CSEs Destruction of Parents by IS UV Radiation Field Self-Shielding H2 – very reactive, daughter (H atoms) unreactive CO – very unreactive, daughters (C, C+) very reactive H2O – can self-shield but only weakly, daughter (OH) very reactive Photodissociation and photoionisation Water drives chemistry through production of OH Water photoionisation small so neutral-neutral chemistry dominates

Photochemistry in O-rich CSEs Radical reactions produce oxides OH reacts rapidly with other daughters O to produce O2 N to produce NO S to produce SO SO to produce SO2 Si to produce SiO P to produce PO, etc

Problems C-rich AGB CSEs Detection of water, formaldehyde and OH in outer envelope of IRC Pulsational shocks – no, too little formed – and in inner region Photochemistry – no, not enough O freed from parents Fisher-Tropsch-Type chemistry (grains) – possibly Evaporation of comets - possibly O-rich AGB CSEs Formation of carbon-bearing molecules HCN, HNC, CN, CS, H2CO, OCS Not enough carbon released by photodissociation of CO Pulsational shocks – can help, but in inner region Grain chemistry – not yet studied