Dust formation in O-rich Miras and IK Tau

Transcription

1 Dust formation in O-rich Miras and IK Tau David Gobrecht & Isabelle Cherchneff Basel University Colls.: Arkaprabha Sarangi & John Plane Why Galaxies Care About AGB Stars III Vienna 30 July 2014

2 Overview The inner wind of AGBs Gas-phase chemistry & dust nucleation routes A model for the O-rich Mira IK Tau Results on molecules & dust clusters Dust condensation & grain size distributions Conclusions

3 The inner wind of AGBs No dust With dust Nowotny et al The outer atmosphere is periodically shocked layers of dense, warm gas bound to the star conducive to the formation of molecules, dust clusters & dust grains Cherchneff 1996/2006/11/12, Willacy & Cherchneff 1998, Duari 1999,

4 The inner wind of AGBs Gobrecht et al. 2014

5 Gas-phase chemistry In O-rich AGB inner winds, detection of : H2O, OH, SiO, SiS, CO, CO2, HCN, CS, SO, SO2, PN, PO Justtanont 1998, Decin 2010, Justtanont 2012, De Beck 2013 Chemical-kinetic approach - all relevant processes for hot-warm, dense post-shock gas: termolecular & bimolecular (neutral-neutral, fragmentation, radiative association) processes no ions For IK Tau, reaction network includes 105 molecular species and 426 chemical reactions

6 Nucleation routes Network contains gas-phase pathways to the formation of dimers of alumina (Al2O3) & forsterite (Mg2SiO4), enstatite (MgSiO3) of metal oxides (MgO, FeO, TiO), and pure metal clusters (Fe, Al, Si) Alumina dimers Al4O6: Structures from Monte-Carlo-based candidate search & subsequent Density Functional Theory quantum calculations Al4O6 Al4O6 Dimerisation of AlO in (AlO)2 termolecular Oxygen addition via H2O, OH or O2 to form Al2O3 & dimerisation (Biscaro & Cherchneff 2014)

7 Nucleation routes Silicates: forsterite dimers Mg4Si2O8 Goumans & Bromley 2012 Enstatite formation pathway Zachariah & Tsang 1995 Formation of silica in silane-rich flame SiO dimerisation too slow to start silicate nucleation Nucleation goes via HSiO, H2Si2O2 & H2Si2O3 formation Growth via successive oxidation & Mg inclusion steps Efficient mechanism to synthesise silicate dimers (enstatite and forsterite) between ~ 4 R* and 6 R*

8 IK Tau periodic pulsator model Galactic (250 pc), oxygen-rich (C/O=0.75) Regular pulsator at the tip of the AGB Stellar Parameters:

9 IK Tau periodic pulsator model Fox & Wood 1985 Betschinger & Chevalier 1985 Cherchneff 1996 Willacy & Cherchneff 1998 Duari et al Cherchneff 2006, 2011 & 2012 Gobrecht et al. 2014

10 Results on molecules 1 R* CO, H2O, SiO, SiS, HCl, AlOH & PN form close to the star as soon as gas cools down. 3 R* Some molecules are more shock chemistrydependent. C-bearing species form from CO breaking by shocks SO, HCN, CS, CO2

11 Results on molecules Our parent species include CO, H2O, SiO, SiS, PN, SO, HCN, CS, CO2, AlOH, TiO, HCl & NaCl Modelled abundances for 12 molecules at 8 R* agree well with observations Validate shock chemistry scenario strong impact of shocks on the gas and solid phases of the inner wind Discrepancy for SO2

12 Results on dust clusters Alumina dimers Al4O6: 1 R* Form at 1 R* when the Tgas< 2000 K Abundance of dimers on the low side - other nucleation routes may be involved - via AlOH?

13 Results on dust clusters Silicates: forsterite dimers Mg4Si2O8 Start forming at 3.5 R* from HSiO dimerisation

14 Dust condensation Formalism based on Brownian diffusion, which accounts for the scattering, collision, and coalescence of the grains through Brownian motion Plane 2013, Sarangi & Cherchneff 2014 Grains size distributions are derived for silicates of forsterite and enstatite stoichiometry, and alumina. At each pulsation and shock, molecules are destroyed and reform over the next pulsation while dust is not destroyed by the shock and keeps growing Consider from hydro models Bowen 1988, Nowotny 2010 small drift velocities close to the star for alumina kms-1 and 6 pulsations to cover 0.5 R* larger drift velocities at r > 3 R* for silicates 1.5 kms-1 and 2 pulsations to cover 0.5 R*

15 Grain size distributions: alumina Dust-to-gas mass ratio 3.3 E E E E E E-05 Large grains > 0.1 μm are already formed after a few pulsations because gas densities are high between 1 R* and 2 R*

16 Grain size distribution: forsterite Dust/gas Mass ratios 3.5R* 1.3 E R* 4.6 E R* 6.0 E R* 7.3 E R* 1.0 E R* 1.3 E R* 1.6 E R* 2.0 E R* 2.3 E-03 Forsterite grains grow to larger sizes with increasing number of pulsations and radius Dust/gas mass ratio after 8 R* agrees with observations Grain size peaks at 0.02 µm, which is a bit low (from obs. a = 0.1 µm)

17 Enhanced density: forsterite Dust/gas Mass ratios 3.5R* 4.7 E R* 1.1 E R* 1.9 E R* 3.0 E R* 5.3 E R* 6.1 E R* 6.7 E R* 7.5 E R* 8.0 E-03 A factor x10 in gas density results in grain size distributions peaking at ~ 0.1 μm inhomogeneous wind will help!

18 Conclusions Shock-induced chemistry in the inner wind well explains observed molecular abundances Formation of large alumina grains (> 0.1 µm) close to the star at r 2 R* & formation of silicate grains between 4 R* and 6 R* from a new nucleation route involving HSiO Consistent with recent MIDI/VLT observations Karovicova 2013 Dust synthesis results from shock-induced nucleation chemistry in the gas-phase & the continuous dust growth over several stellar pulsations in the shocked inner wind at r < 10 R * Other dust formation models in O-rich AGBs lack pulsations Tielens 1998, Ferrarotti & Gail 2001/2/6, Dell Agli 2013 Large silicate grains form with enhanced gas densities Inhomogeneous wind? growth from SiO-based cluster deposition at lower T in the intermediate envelope? Derived dust-to-gas mass ratio agrees with observations Next: investigate spinel, inhomogeneous wind, chemistry along the AGB and at various metallicities...