Dust formation in O-rich AGBs and IK Tau

David Gobrecht & Isabelle CherchneffBasel University

Colls.: Stefan Bromley, Arkaprabha Sarangi & John Plane

SeminarTeramo 15 th of October 2014

Overview

AGBs: types & evolution 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 Semiregular variables Conclusions

Evolution towards the AGB

Low mass: Intermediate mass:

Maercker PhD thesis 2009

Types of AGB stars

M-type: oxygen-rich, C/O <1

S-type: C/O ~ 1

C-type: carbon-rich, C/O > 1

Low mass: 3rd dredge up mixes carbon to the photosphere → C/O increases

Intermediate mass: Hot bottom burning converts C in N and other CNO products

→ C/O decreases

Hoefner 2009

Timescale for AGBs

Main sequence: ~ 107 – 1010 years

Time on the (TP) AGB: ~ 106 years

Time between dredge-up episodes: ~ 104 years

Time between stellar pulsations:

> 100 days

Chemical timescale: 1/(kAB*nA*nB)

large range → stiff system of coupled ODEs

The inner wind of AGBs

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,

No dust With dust

Nowotny et al. 2010

The inner wind of AGBs

Gobrecht et al. 2014 in submission

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

Nucleation routesNetwork 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

• Dimerisation of AlO in (AlO)2 – termolecular • Oxygen addition via H2O, OH or O2 to form Al2O3 &

dimerisation (Biscaro & Cherchneff 2014)

Al4O6

Nucleation routes

Goumans & Bromley 2012Enstatite formation pathway

Zachariah & Tsang 1995Formation 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*

Silicates: forsterite dimers Mg4Si2O8

IK Tau periodic pulsator model

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

IK Tau periodic pulsator model

Fox & Wood 1985Betschinger & Chevalier 1985Cherchneff 1996Willacy & Cherchneff 1998Duari et al. 1999Cherchneff 2006, 2011 & 2012

Gobrecht et al. 2014

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 chemistry-dependent. C-bearing species form from CO

breaking by shocksSO, HCN, CS, CO2

Results on molecules

•Modelled abundances for 12 molecules at 6 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

Our parent species include

CO, H2O, SiO, SiS, PN, SO, HCN,

CS, CO2, AlOH, TiO, HCl & NaCl

Results on dust clusters

Form at 1 R* when Tgas< 2000 K

Abundance of dimers on the low side - other nucleation routes may be involved - via AlOH?

Alumina dimers Al4O6: 1 R*

Results on dust clusters

Silicates: forsterite dimers Mg4Si2O8

Start forming at 3.5 R* from HSiO dimerisation

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 - 0.5 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*

Grain size distributions: alumina

Large grains > 0.1 μm are already formed after a few pulsations because gas densities are high between 1 R* and 2 R*

3.3 E-061.0 E-052.1 E-052.2 E-052.4 E-052.8 E-05

Dust-to-gas mass ratio

Grain size distribution: forsterite

• 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)

3.5R* 1.3 E-044.0R* 4.6 E-044.5R* 6.0 E-045.0R* 7.3 E-046.0R* 1.0 E-037.0R* 1.3 E-038.0R* 1.6 E-03 9.0R* 2.0 E-03 10.0R* 2.3 E-03

Dust/gasMass ratios

Enhanced density: forsterite

3.5R* 4.7 E-044.0R* 1.1 E-034.5R* 1.9 E-035.0R* 3.0 E-036.0R* 5.3 E-037.0R* 6.1 E-038.0R* 6.7 E-03 9.0R* 7.5 E-03 10.0R* 8.0 E-03

Dust/gasMass ratios

A factor x10 in gas density results in grain size distributions peaking at ~ 0.1 μm → inhomogeneous wind will help!

Semiregular variables

Classes Mira, SRa, SRb and L are rather loosely defined in GCVS

SRb's & SRa's pulsate in overtone modes, whereas Miras are fundemental mode pulsators

Trends:

Temperatures and stellar masses higher

Pulsation periods, radii, C/O smaller

Luminosities: rather smaller,

P-L relation

Results: SRa

H/H2 balance depends on the shock speed

Alumina grains form, silicates do not

PRELIMINARY!

Results: S starH2 dominates the chemistry,Alumina and forsterite grains form in quantity and large sizes

PRELIMINARY!

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...