How to kill a giant Dust-driven stellar winds on the AGB Susanne - - PowerPoint PPT Presentation

How to kill a giant

Dust-driven stellar winds

• n the AGB

Susanne Höfner Department of Physics and Astronomy Uppsala University

Large-scale structure of the wind

V-band image of IRC+10216 show- ing shell-like structures in the circumstellar en- velope (90''x 90'') Mauron & Huggins (2010)

A fascinating picture of stellar mass loss and the cosmic matter cycle at work ...

GALEX images of IRC+10216 (left: composite NUV+FUV, right: FUV) showing wind - ISM interaction (field

• f view 62'x 62')

Sahai & Chronopoulos (2010)

Dust-driven winds: micro-physics

Stellar photon hits dust grain dust grain absorbs & emits

• r scatters photon

dust gains momentum dust collides with gas particle gas gains momentum dust loses momentum gas particle collides with

• ther gas particle

momentum redistribution in gas phase

simple estimate for grain temperature:

• radiative equilibrium
• Planckian radiation field, geom. diluted
• dust opacity approximated by power law

condensation distance: Tgrain = Tc

Dust: condensation distance

condensation temperature (material property)

Dust: condensation distance

T* = 3000 K

Grain opacity full-drawn: lab data dashed: power law fit

Dust: condensation distance

Example:

• am. carbon

Rc = 2-3 R*

T* = 3000 K

Dust: condensation distance

How to bridge the gap between the stellar photosphere and the condensation distance ? Early ideas:

• acoustic wave pressure (e.g. Pijpers & Hearn 1989)
• Alfvén waves (e.g. Hartmann & MacGregor 1980)

... inspired by hydrostatic atmosphere models (steep density profile) and stationary wind models (time-independent dynamical structures) But: AGB atmospheres are highly dynamical due to convection and pulsation ... Rc

Dust: condensation distance

How to bridge the gap between the stellar photosphere and the condensation distance ? stellar pulsation → propagating atmospheric shock waves → extended atmosphere, periodic “ballistic” motion

• f mass layers

Rc

Dust: condensation distance

How to bridge the gap between the stellar photosphere and the condensation distance ? stellar pulsation → propagating atmospheric shock waves → extended atmosphere, periodic “ballistic” motion

• f mass layers

Rc

Effects of pulsation and shocks

movement of mass shells in a detailed dynamical model atmosphere

• uter atmospheric layers:

about 70 (50) percent of the time in the upper half (quarter) of the trajectory

⇒ levitation, extended

cool atmosphere formation and propagation of shock waves pulsation

Extended dynamical atmospheres

Spatial structure

• f a detailed

dynamical model (incl. frequency- dependent RT and dust formation) top: density and gas temperature bottom: partial pressures of various molecules Note the effects

• f shock waves.

Nowotny et al. (2010)

Dynamics of mass layers

time radial distance condensation distance mass layer accelerated by passing shock wave mass layer accelerated by radiation pressure

• n dust

dust-free pulsating atmosphere dust-driven

• utflow

“pulsation-enhanced dust-driven wind” scenario

Dynamics of mass layers

time radial distance condensation distance mass layer accelerated by passing shock wave mass layer accelerated by radiation pressure

• n dust

dust-free pulsating atmosphere dust-driven

• utflow

Epot Ekin

• bserved

velocities indicate maximum levitation distances

• f a few

stellar radii

• Form close to the star (at about 2-4 R*), i.e.

condensation distance < levitation distance

• Have high radiative cross sections (efficient

radiative acceleration of individual grains)

• Consist of abundant elements (sufficient total

momentum transfer to the gas)

Properties of wind-driving grains

Stellar spectrum Planck function

• Form close to the star (at about 2-4 R*), i.e.

condensation distance < levitation distance

• Have high radiative cross sections (efficient

radiative acceleration of individual grains)

• Consist of abundant elements (sufficient total

momentum transfer to the gas)

Properties of wind-driving grains

Studying the range of viable condensation temperatures and NIR optical grain properties using a dynamical model with detailed radiative transfer Bladh & Höfner (in prep.)

• Form close to the star (at about 2-4 R*), i.e.

condensation distance < levitation distance

• Have high radiative cross sections (efficient

radiative acceleration of individual grains)

• Consist of abundant elements (sufficient total

momentum transfer to the gas)

Properties of wind-driving grains

Studying the range of viable condensation temperatures and NIR optical grain properties using a dynamical model with detailed radiative transfer Bladh & Höfner (in prep.)

n

• w

i n d iron MgFeSiO4 Mg2SiO4

Radiative acceleration: basics

radiative / gravitational acceleration: κH L*

_______________________________

4 π c G M* critical value = 1 ⇒ critical flux mean opacity: κcrit= 4 π c G M* / L* > 1 < 1 Γ =

• Form close to the star (at about 2-4 R*), i.e.

condensation distance < levitation distance

• Have high radiative cross sections (efficient

radiative acceleration of individual grains)

• Consist of abundant elements (sufficient total

momentum transfer to the gas)

Properties of wind-driving grains

Chemistry: the simple picture

Abundance

• f C and O

M-type S-type C-type CO CO CO C/O ... changes during AGB evolution

available for carbon grains available for silicate grains

• Form close to the star (at about 2-4 R*), i.e.

condensation distance < levitation distance

• Have high radiative cross sections (efficient

radiative acceleration of individual grains)

• Consist of abundant elements (sufficient total

momentum transfer to the gas) Promising candidate for C/O<1: Mg2SiO4

Properties of wind-driving grains

Radiative pressure on Mg2SiO4

• Force:

radiation pressure (dust)

• Conditions:

set by shocks (pulsation)

• levitation
• temporal variations

“pulsation-enhanced dust-driven wind”

standard scenario (e.g. Woitke 2006): absorption by small grains (compared to λ)

stellar flux maximum ↔ κH Γ << 1: insufficient radiative pressure!

log Γ

Radiative pressure on Mg2SiO4

• Force:

radiation pressure (dust)

• Conditions:

set by shocks (pulsation)

• levitation
• temporal variations

“pulsation-enhanced dust-driven wind”

alternative scenario: µm-sized grains (scattering) Γ > 1

stellar flux maximum ↔ κH

log Γ

Dust-driven winds for C/O < 1

viable scenario for winds of pulsating M-type AGB stars detailed models with frequency-dependent radiative transfer and non-equilibrium dust condensation (Mg2SiO4) wind driven by Fe-free, micron-sized silicate grains Höfner 2008 (A&A 491, L1)

• models (Höfner 2008)

+ observations (Olofsson et al. 2002, Gonzalez Delgado 2003)

• pacity of Mg2SiO4

as a function of wavelength and grain radius black contour marks where radiative pressure exceeds gravity for a typical AGB star stellar flux maximum alternative scenario: µm-sized grains (scattering) comparison of mass loss rates and wind velocities with

• bservations

Γ > 1 log Γ

Dust-driven winds for C/O < 1

Testing the scenario: NIR photometry (Bladh et al., in prep.)

} models

Radiative pressure: M- vs. C-stars

C/O < 1: Fe-free silicates scattering does the trick for grain radius > 0.1 micron C/O > 1: amorphous carbon absorption dominates, even small grains can drive winds

Γ > 1 Γ > 1

• C/O > 1: winds driven by carbon grains,

good agreement of detailed non-grey models with observations

• C/O < 1: non-grey effects on temperature

→ Fe-free silicates close to the star → too low radiative pressure for small grains (absorption) → winds possibly driven by Fe-free silicates > 0.1 µm (scattering)

• More observational constraints for models!

Summary: dust-driven winds

• Collect suitable stones (lots of them ...)
• Accelerate them to high velocity
• Aim high and let them collide with the upper

end of the giant ... Sounds familiar?

How to kill a giant