dynamics in atmospheres and outflows of evolved …...dynamics in atmospheres and outflows of...

Dynamics

in atmospheres and outflows of

evolved stars

Elvire De BeckWouter Vlemmings, Theo Khouri, Matthias Maercker, Hans Olofsson

Department of Space, Earth and Environment, Chalmers University of Technology Onsala Space Observatory

The Dynamical Universe for All Lund Observatory, 5th - 6th February 2018

Quick intro

Surface temperature

Lum

inos

ity c

ompa

red

to th

e su

n

• Evolved stars in this talk = Post-Main-Sequence Giants

• < 8 Msun AGB: asymptotic giant branch stars

• >8 Msun RSG: red supergiant stars

• Cool surface ~ 3000K • Luminous ~ 100 - 100,000 Lsun • Mass loss ~ 10-8-10-3 Msun/yr

Dynamics of AGB starsReview: Mass loss of stars on the asymptotic giant branch Mechanisms, models and measurements Hoefner & Olofsson, 2018, A&ARv 26:1

Evolution and appearance on AGB strongly affected by range of dynamical processes • large-scale convective flows

>> transport of newly formed chemical elements to the surface • stellar pulsations

>> trigger shock waves in extended stellar atmosphere

• dust grain formation in upper atmosphere >> acceleration through scattering/absorption and collisions with gas

• massive outflows of gas and dust

These lead to • enrichment of ISM • evolution from giant

to white dwarf

Mass loss of stars on the asymptotic... Page 5 of 92 1

Fig. 1 A schematic figure of an AGB star and its circumstellar environment, including regions and processesof relevance to this review. The stellar radius (R∗) of a typical AGB star is of the order 1 AU (above 100solar radii). By courtesy of S. Liljegren

grated light of galaxies. Estimates suggest that AGB stars are a major source of dustin the Milky Way (e.g., Tielens 2005), but their contribution to dust production in theearly universe is still debated (Michałowski 2015, and references therein). To assesthe effects of AGB stars on their host galaxies requires, among other things, an under-standing of how the mass-loss process depends on stellar metallicity. This will affectthe mass return, not only directly, but also through the fact that a less efficient massloss may lead to a significant fraction of the ≈ 3 − 8 M⊙ stars ending as supernovaeof type 1.5, with consequences for the early chemical evolution. Mass loss also deter-mines the maximum luminosity reached by an AGB star. Current estimates suggestthat AGB stars dominate the mid-IR light in young to moderate-age galaxies with lowamounts of diffuse dust (Villaume et al. 2015, and references therein). Any errors inour understanding of the mass loss thus propagate into errors in our understanding ofthe chemical evolution of the universe and the evolution of galaxies.

1.2 Early indications of mass loss

The first evidence of (substantial) post-main-sequence mass loss came through obser-vations of the binary system α Her by Deutsch (1956). Circumstellar absorption lineswere seen in the spectrum of the companion star, suggesting circumstellar gas as faraway as 105 R⊙ from the M giant. With an estimated outflow velocity of 10 km s−1,the gas velocity is well above the escape velocity at this point. The estimated mass-lossrate was 3 × 10−8 M⊙ year−1. In a study of similar systems, Reimers (1975) derivedan empirical relation between mass-loss rate and stellar characteristics (luminosity,mass, and radius) that bears his name.

There were also indirect indications. Auer and Woolf (1965) found that the Hyadescluster contains about a dozen white dwarfs (WDs); each one must have a mass belowthe Chandrasekhar limit of 1.4 M⊙. But, the Hyades cluster has stars with a massof 2 M⊙ that are still on the main sequence (MS). Consequently, the white dwarfprogenitors must have lost at least 0.6 M⊙ during their post-MS evolution.

123

Dynamics of AGB stars

Observations of asymmetries and inhomogeneities • short-lived & small-scale: photosphere and dust-forming layers • long-lived & large-scale: circumstellar envelope

High-angular resolution observations give information on e.g. dust condensation • location • chemical composition • size

“These are essential constraints for building realistic models

of wind acceleration and developing a predictive theory of

mass loss for AGB stars, which is a crucial ingredient of stellar and galactic chemical

evolution models.”

Review: Mass loss of stars on the asymptotic giant branch Mechanisms, models and measurements Hoefner & Olofsson, 2018, A&ARv 26:1

Mass loss of stars on the asymptotic... Page 5 of 92 1

Fig. 1 A schematic figure of an AGB star and its circumstellar environment, including regions and processesof relevance to this review. The stellar radius (R∗) of a typical AGB star is of the order 1 AU (above 100solar radii). By courtesy of S. Liljegren

grated light of galaxies. Estimates suggest that AGB stars are a major source of dustin the Milky Way (e.g., Tielens 2005), but their contribution to dust production in theearly universe is still debated (Michałowski 2015, and references therein). To assesthe effects of AGB stars on their host galaxies requires, among other things, an under-standing of how the mass-loss process depends on stellar metallicity. This will affectthe mass return, not only directly, but also through the fact that a less efficient massloss may lead to a significant fraction of the ≈ 3 − 8 M⊙ stars ending as supernovaeof type 1.5, with consequences for the early chemical evolution. Mass loss also deter-mines the maximum luminosity reached by an AGB star. Current estimates suggestthat AGB stars dominate the mid-IR light in young to moderate-age galaxies with lowamounts of diffuse dust (Villaume et al. 2015, and references therein). Any errors inour understanding of the mass loss thus propagate into errors in our understanding ofthe chemical evolution of the universe and the evolution of galaxies.

1.2 Early indications of mass loss

The first evidence of (substantial) post-main-sequence mass loss came through obser-vations of the binary system α Her by Deutsch (1956). Circumstellar absorption lineswere seen in the spectrum of the companion star, suggesting circumstellar gas as faraway as 105 R⊙ from the M giant. With an estimated outflow velocity of 10 km s−1,the gas velocity is well above the escape velocity at this point. The estimated mass-lossrate was 3 × 10−8 M⊙ year−1. In a study of similar systems, Reimers (1975) derivedan empirical relation between mass-loss rate and stellar characteristics (luminosity,mass, and radius) that bears his name.

There were also indirect indications. Auer and Woolf (1965) found that the Hyadescluster contains about a dozen white dwarfs (WDs); each one must have a mass belowthe Chandrasekhar limit of 1.4 M⊙. But, the Hyades cluster has stars with a massof 2 M⊙ that are still on the main sequence (MS). Consequently, the white dwarfprogenitors must have lost at least 0.6 M⊙ during their post-MS evolution.

123

Observations: large scales

Large scales = circumstellar envelope (CSE) >> spherical

• entire outflow • shells

Olofsson et al. (2000) CO (J=1-0), PdBI

TT Cyg, detached shell

Castro Carrizo et al. (2010) CO (J=2-1), PdBI

IK Tau, normal mass loss

~17”(~0.02pc)




De Beck & Olofsson (2018) APEX spectral survey at 0.8-1.9 mm R Dor, normal mass loss

• main, smooth, spherical component • up to 4 extra components in the outflow

• one at velocities >> wind expansion velocity • shells / rings / spiral / … ? -20 -10 0 10 20

Velocity [km s-1]

SiO, v=1(4-3)

SiO, v=1(5-4)

SiO, v=1(7-6)

SiO, v=2(4-3)

SiO, v=2(6-5)

SiO, v=2(7-6)

SiO, v=3(4-3)

29SiO, v=1(4-3)

30SiO, v=1(4-3)

30SiO, v=1(8-7)

-20 -10 0 10 20Velocity [km s-1]

Nor

mal

ised

inte

nsity

2-13-24-3

2-13-24-3

-20 -15 -10 -5 0

-20 -10 0 10 20Velocity [km s-1]

Nor

mal

ised

inte

nsity

4-35-46-57-68-7

4-35-46-57-68-7

-15 -10 -5 0

-20 -10 0 10 20Velocity [km s-1]

Nor

mal

ised

inte

nsity

65-5467-5655-4456-4554-4344-3345-34

65-5467-5655-4456-4554-4344-3345-34

0 5 10 15 20

-37 -35.5 -34 -32.5 -31

-29.5 -28 -26.5 -25 -23.5

-22

Dec

. offs

et [a

rcse

c]

-20.5 -19 -17.5 -16

-14.5 -13 -11.5 -10 -8.5

0

0.5

1

1.5-7

-20-1001020-20

-10

0

10

20 -5.5 -4

R.A. offset [arcsec]

-2.5 -1




>> not so spherical • binary interaction

Maercker et al. (2012, 2014, 2016) R Scl, detached shell source

CO(J=3-2), ALMADust-scattered stellar light, PolCor

• previously unknown binary companion ~60 AU separation

• from spiral windings:information on changes in velocity and mass loss on evolutionary timescales

• Dust and gas dynamics comparable?




>> not so spherical • binary interaction

Maercker et al. (2012, 2014, 2016) R Scl, detached shell source

CO(J=3-2), ALMADust-scattered stellar light, PolCor

• previously unknown binary companion ~60 AU separation

• from spiral windings:information on changes in velocity and mass loss on evolutionary timescales

• Dust and gas dynamics comparable?R.A. offset [arcsec]

Dec

. off

set [

arcs

ec]

−20−1001020

−20

−15

−10

−5

0

5

10

15

20

0

0.2

0.4

0.6

0.8

1

R.A. offset [arcsec]

Dec

. off

set [

arcs

ec]

−20−1001020

−20

−15

−10

−5

0

5

10

15

20

0

0.5

1

1.5

2




>> not so spherical • binary interaction • disk

43.0 km/s

East Offset [arcsec]

Offset[arcsec]

10 5 0 −5 −10

5

0

−5

Jy/beam

0.2

0.4

0.6

0.8

1

1.2

1.4

1.650.0 km/s


Offset[arcsec]

5 0 −5

10

5

0

−5

−10

Jy/beam

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

Ramstedt et al. (2014) Mira, 0.5” (~45 AU) binary separation

CO(J=3-2), ALMA, “bubble” in the AGB wind

Kervella et al. (2014, 2015, 2016) L2 Pup

ALMA, VLT (NACO/SPHERE), edge-on disk

Observations: small scales

• Small scales • upper atmospheric layers (warm molecular layer)

i.e. before wind acceleration

Khouri et al. (2016) CO(v=1, J=3-2), ALMA

• inverse P-Cygni profiles indicate infall motion

• multiple epochs reflect changes of the upper atmosphere

• temperature • density • motion


• Small scales • upper atmospheric layers

i.e. before wind acceleration • dust formation region

Khouri et al. (2016), SPHERE/ZIMPOL, R Dor

• Topvariable morphology in continuum = changing opacity of TiO in extended atmosphere

• Bottompolarised light from ~annular region around central star.

• density profile steeper than constant-v wind • upper limit for the d/g ~2×10-4 at 5.0 R ,

consistent with minimum required by wind-driving models


• Small scales • upper atmospheric layers

i.e. before wind acceleration • dust formation region • surface imaging

Paladini et al. (2018, Nature) π1 Gruis, VLTI/PIONIER

surface granulation due to convection

submillimeter wavelength observations probe closer to the photosphere than

observations at most other wavelengths. Dust emission, molecules and atmostpheric

absorption affect the measurements in significant parts of the near-infrared and optical

regimes (20).

Fig. 1. Brightness temperature map of the surface of the AGB star W Hya as observed

with ALMA at 338 GHz. Contours are drawn at 227, 907, 1814, 2268, 2722, and

3194 K. The rms noise in the image is 18 K and the peak is 3560 K. The image is

centered at RA(J2000) = 13h49m01.939s and Dec(J2000) = -28d22m 04.484s. The map

was produced using uniform weighting of the interferometric visibilities and was

restored with a circular beam (shown in the bottom left) of 17 mas (~1.7 au),

corresponding to the minor axis of the 17x24 mas dirty beam. The red ellipse

indicates the size of the fitted, uniform, stellar disk (51x56.5 mas, PA 65.7 degrees)

with a brightness temperature of 2495 ± 125 K. The white circle denotes the stellar

photosphere estimated from near-infrared observations (19). The effective

temperature of the photosphere is ~2500 K (18).

Vlemmings et al. (2017, Nature Astro.) W Hya, ALMA

heating of the upper atmosphere due to shocks chromosphere?

Conclusions

Mass loss dominates appearance and evolution on AGB >> critical to have predictive description as input for models of

• stellar evolution • galactic chemical evolution

>> strongly dependent on variety of dynamical processes

Observations show • asymmetries and inhomogeneities

• on large spatial scales & long timescales • on small spatial scales & short timescales

• dust location, size, composition • surface structure for stars other than our Sun (!)

constraining • convection and pulsation motions • dust growth • wind acceleration • mass-loss rates

and heading for a deeper understanding of the dynamics of evolved stars.

submillimeter wavelength observations probe closer to the photosphere than

observations at most other wavelengths. Dust emission, molecules and atmostpheric

absorption affect the measurements in significant parts of the near-infrared and optical

regimes (20).

Fig. 1. Brightness temperature map of the surface of the AGB star W Hya as observed

with ALMA at 338 GHz. Contours are drawn at 227, 907, 1814, 2268, 2722, and

3194 K. The rms noise in the image is 18 K and the peak is 3560 K. The image is

centered at RA(J2000) = 13h49m01.939s and Dec(J2000) = -28d22m 04.484s. The map

was produced using uniform weighting of the interferometric visibilities and was

restored with a circular beam (shown in the bottom left) of 17 mas (~1.7 au),

corresponding to the minor axis of the 17x24 mas dirty beam. The red ellipse

indicates the size of the fitted, uniform, stellar disk (51x56.5 mas, PA 65.7 degrees)

with a brightness temperature of 2495 ± 125 K. The white circle denotes the stellar

photosphere estimated from near-infrared observations (19). The effective

temperature of the photosphere is ~2500 K (18).

43.0 km/s


Offset[arcsec]

10 5 0 −5 −10

5

0

−5

Jy/beam

0.2

0.4

0.6

0.8

1

1.2

1.4

1.650.0 km/s


Offset[arcsec]

5 0 −5

10

5

0

−5

−10

Jy/beam

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

Theoretical models

surface granulation due to convectionA&A proofs: manuscript no. aaagb3dfirstgrid

-600

-400

-200

0

200

400

600

y [R

O • ]

t= 0.0 d

t= 28.9 d

t= 57.9 d

t= 86.8 d

t= 112.9 d

-600-400-200 0 200 400 600

x [RO • ]

-600

-400

-200

0

200

400

600

y [R

O • ]

t= 141.8 d

-600-400-200 0 200 400 600

x [RO • ]

t= 170.7 d

-600-400-200 0 200 400 600

x [RO • ]

t= 199.7 d

-600-400-200 0 200 400 600

x [RO • ]

t= 228.6 d

-600-400-200 0 200 400 600

x [RO • ]

t= 257.5 d

Fig. 3. Time sequence (for the standard model st28gm06n26) of bolometric intensity maps, where black corresponds to zero intensity. Thesnapshots are about 1 month apart to demonstrate the relative short timescale for changes of the small surface features.

Fig. 4. Plot (for the standard model st28gm06n26) of radial velocity vradial(r, t) for all grid points in one column from the center to the side of thecomputational box for the entire simulation time. Blue indicates outward and red inward flow.

Fig. 5. Examples for the standard model st28gm06n26: Left: The spherically averaged radial velocity hvradiali(r, t) for the full run time and radialdistance. The di↵erent colors show the average vertical velocity at that time and radial distance. Right: Part of the velocity field from the rightimage, indicated with the rectangle with mass-shell movements plotted as iso-mass contour lines.

Article number, page 6 of 14

B. Freytag et al.: Global 3D radiation-hydrodynamics models of AGB stars.

-500

0

500

y [R

O • ]

-500

0

500

y [R

O • ]

-500

0

500

y [R

O • ]

-500

0

500

y [R

O • ]

-500

0

500

y [R

O • ]

-500 0 500

x [RO • ]

-500

0

500

y [R

O • ]

-500 0 500

x [RO • ]

-500 0 500

x [RO • ]

-500 0 500

x [RO • ]

t= 0.0 d

t= 92.6 d

t= 185.2 d

t= 277.8 d

t= 370.3 d

-500 0 500

x [RO • ]

t= 463.0 d

Fig. 2. Time sequences (for the extended model st28gm06n25) of temperature slices, density slices, entropy slices, pseudo-streamlines (integratedover about 3 months), and bolometric intensity maps, where black corresponds to zero intensity. The snapshots are nearly 3 months apart (seethe counter in the right-most panels), so that the entire sequence covers about one pulsational cycle. The color scales are kept the same for allsnapshots. This figure can be directly compared to Fig. 1 of Freytag & Höfner (2008).

Article number, page 5 of 14

Freytag et al. (2017) star-in-a-box models

temperature density

dynamics in atmospheres and outflows of evolved …...dynamics in atmospheres and outflows of...

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