Perugia David Gobrecht & Isabelle Cherchneff Departement Physik Universität Basel Switzerland Dust in AGB stars

Overview  Red giant stars and galactic ecology  Observations of dust in evolved stars  Chemical routes to carbon and silicate dust  Physics and chemistry of AGB winds  Shock-induced chemistry of dust: IRC  Shock-induced chemistry of dust: IK Tau  Conclusions

Red giant stars and galactic ecology AGB star = late stage of evolution of a low-to- intermediate mass star (1-8 Msun)  Lose ~ 50 % of its mass on the AGB  Ascending the AGB, switch from an oxygen-rich to a carbon-rich photosphere due to thermal pulses and 3rd dredge-up  Cool photosphere (~2500 K) and dust formation close to the star  Develop strong mass loss & winds due to dust: from (O- rich) to Msun/yr (C-rich) C-rich O-rich Prevalent molecule & dust providers to galaxies locally & in the early universe (?) Herschel – ALMA - JWST

Kemper et al. (2002) Si-O stretching (~ 9 μm) O-Si-O stretching (18 μm) Observations of dust in evolved stars O-rich AGB star: OH Amorphous silicates & alumina (Al 2 O 3 )

Observations of dust in evolved stars O-rich Semi-Regular Variables (early AGBs): Posch et al Spinel MgAl 2 O 4 TiC? 21  m MgS ~ 27  m

Observations of dust in evolved stars C-rich AGB star: IRAS Speck et al. (2009) Amorphous carbon, SiC & MgS SiC 11.3  m MgS ~ 27  m

Observations of dust in evolved stars C-rich Post-AGB: HD Hony et al Amorphous carbon, HAC, MgS & TiC TiC? 21 μm MgS ~ 27 μm

Observations of dust in evolved stars  No satisfactory theory (Thermodynamic Equilibrium – Classical Nucleation Theory) to model the formation of solids in AGBs  A satisfactory theory of dust formation needs to reproduce the very well characterised gas phase (IRAM-Herschel- ALMA) chemical kinetics approach

First-ring formation: Dominant pathway in acetylenic flames: propargyl C 3 H 3 recombination Westmoreland et al. (1989), Miller & Melius (1992), Miller & Klippenstein (2003) + + H Intermediates: dimethylenecyclobutene, fulvene ~ 90 % ~ 10 % Carbon with hydrogen: hydrocarbon & aromatic route (combustion chemistry) Chemical routes to carbon and silicate dust C6H5C6H5 C6H6C6H6

H-rich ( A) Aromatic ( B) Polyynes Growth via H abstraction/C 2 H 2 addition Growth via PAH coalescence Polymerization of polyynes (C 6 H 2 ) on surface of radical sites (Frenklach et al. 1986) (Mukherjee et al. 1994) (Krestinin et al. 2000, Naydenova et al. 2005) (Tielens 2008) Combustion chemistry: neutral-neutral processes with activation energy barrier Chemical routes to carbon and silicate dust

Silicate: formation of forsterite (Mg 2 SiO 4 ) dimers Enstatite dimer Si 2 O 2 Si 2 O 3 Goumans & Bromley (2012) Oxidation by H 2 O, O 2 & SO, and Mg addition are down-hill processes: no activation barrier O Addition via H 2 O, O 2, SO Mg addition Chemical routes to carbon and silicate dust SiO

1 R* 10 R*100 R* 1000 R* 2500 – – – – 10 4 T (K) - ngas (cm -3 ) Inner envelope Intermediate envelope Outer envelope TE chemistry Kinetics – dust formation zone Gas-solid chemistry (?)Photo-chemistry: ionisation & dissociation Wind terminal velocity km/s Formation of NH3, CH4, SiH4, H2S on grains? ISM UV field Cosmic Rays Formation of complex species, e.g., carbon chains Physics and chemistry of AGB winds Parent species injected Daughter species

Physics and chemistry of AGB winds 1 10Radius(R*) Thermal Equilibrium Non-Equilibrium Chemistry Gas-solid surface chemistry Chemistry 2500; ; ; 10 4 T(K) - n(cm -3 ) km/s Convection CO HCN C2H2C2H2 Pulsating Photosphere Shocked layers Dust formation zone CS HCN CO SiO CS NH 3, CH 4, SiH 4, H 2 S ? Fully accelerated wind AC, SiC MgS, FeS ? H2H2 H2H2 H2OH2O Shocks km/s HF HCl c-SiC2 Molecules as parents species HERSCHEL ALMA

Physics and chemistry of AGB winds Based on Thermodynamic Equilibrium (Tsuji 1973) : If O-rich: oxygen chemistry (CO, H 2 O, OH, SiO…) If C-rich: carbon chemistry (CO, HCN, CS, C 2 H 2 …) Ziurys (2006) AGB winds are great laboratories for studying molecule and dust formation in space IRC+10216: extreme carbon star C/O = 1.4 d ~ 120 pc r = 5 · cm M loss = 2 · M sun /yr

Physics and chemistry of AGB winds Dichotomy broken: the following species are observed & not predicted by TE  C-bearing molecules in O-rich AGBs: HCN, CS, OCS, HNC, CN & CO 2 in M stars (Deguchi & Goldsmith 1985, Lindqvist et al. 1988, Omont et al. 1993, Bujarrabal et al Justtanont et al. 1998, Decin et al. 2008)  O-bearing species in carbon stars: H 2 O, OH, SiO (Melnick et al. 2001, Hasegawa et al. 2006, Ford et al. 2004, Schöier et al. 2006) Observational surveys of HCN & SiO in M, S, & carbon stars (Olofsson et al. 1998, Bieging et al. 2000, Schoier et al. 2006)

Physics and chemistry of AGB winds Proposed non-TE formation mechanisms HCN, CS etc.:  Photo-dissociation in the outer envelope Wilacy & Millar (1997), Agũndez & Cernicharo (2006)  Shock-induced chemistry in the inner wind Willacy & Cherchneff (1998), Duari et al. (1999), Cherchneff (2006) H2O:  Comets in the intermediate envelope Melnik et a. (2001), Ford et al. (2004)  Surface chemistry in the intermediate envelope Willacy (2004)  Photo-dissociation of 13 CO & SiO in a clumpy wind Agũndez et al. (2010)  Shock-induced chemistry in the inner wind Cherchneff (2011, 2012)

Shock-induced chemistry of dust: IRC Observation of H 2 O with Herschel SPIRE/PACS & HIFI in carbon stars including IRC (Decin et al. 2010, Neufeld et al 2010, 2011)  From HIFI, H 2 O forms at r < 6 R* x(H 2 O) ~ 1 x cm-3  Rule out comets and surface chemistry in the intermediate wind  Similar shape of the H 2 O & SiO lines pointing to a common formation locus

Shock-induced chemistry of dust: IRC Cherchneff (2011, 2012) At r s = 1.2 R* Molecules are destroyed and reform due to non-equilibrium chemistry in the post-shock gas CO dissociation by collisions in the post-shock gas releases atomic O formation of SiO and H 2 O Shocked inner-wind

Shock-induced chemistry of dust: IRC Cherchneff (2012) No need of UV field in clumpy wind to form water! H2O and SiO are competitors for OH consumption at r < 4R* H 2 O & SiO exhibit similar line shapes ? (Neufeld et al. 2010) All chemical families are linked, including carbon dust precursors

Shock-induced chemistry of dust: IRC Good agreement between gas-phase model & data, except for PN Gas-Phase

Shock-induced chemistry of dust: IRC Hydrocarbons & Aromatics C 6 H 6 Benzene formation via propargyl (C 3 H 3 ) recombination PAHs growth via H- abstraction-C 2 H 2 - addition as in flames C 16 H 10 At small radii: Oxidation by OH No PAHs At larger radii: Low enough temperatures at late phases PAHs form

Shock-induced chemistry of dust: IRC Hydrocarbons & Aromatics Go from 2 R* to 5 R* after ~ 15 pulsations Dust mass/gas mass ~ 1.3 x From observations: 1 x – 4 x Indication that carbon dust forms in a very specific zone in the inner wind Bowen (1988)

 Rapid conversion of SiC into c-SiC2 and (SiC)2 small clusters at ~ 1.5 R* & at high temperatures as in the laboratory  SiC lags behind carbon dust (low absorption coefficient at T~2500K & radiation pressure force)  Indicate a separate SiC cluster population from AC grains (~ 3 R*) as observed in meteorites (Hynes et al. 2007) ? SiC Shock-induced chemistry of dust: IRC+0216

Oxygen-rich AGB: IK Tau Gas-phase species observed Shock-induced chemistry of dust: IK Tau IK Tau Parameters: T(1R*) = 2100 K L (1R*) = 2.0 · 10 3 L sun R = 305 R sun M = 1 M sun P = 470 d n (1R*) = 3.7 · cm −3 R s = 1.0 R* C/O = 0.75

Shock-induced chemistry of dust: IK Tau  Modelled abundances agree well with observations so far  SO 2 abundances are too low compared to SO

Shock-induced chemistry of dust: IK Tau  Silicates seem to form at early phases close to the star Pulsation phase 1R* 3R* Pulsation phase

Conclusions ● TE does hold in the photosphere but not in the dust formation region, i.e., inner wind ● Non-equilibrium chemistry induced by shocks explains the observed ‘unexpected species’ in AGB stars, including H 2 O → Collisional dissociation of CO drives their formation  Specific zone where carbon dust grows preceded by SiC formation separate population of SiC grains as observed in presolar grains  For O-rich AGBs, gas-phase well reproduced - problem with SO 2 (formation on grains?)  Forsterite and enstatite clusters form very efficiently Need to include alumina, spinel etc… … still a lot of work to be done…. stay tuned!