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KVA. Signatures of stellar surface structure Dainis Dravins - Lund Observatory www.astro.lu.se/~dainis. Stellar atmosphere theory classics… Unsöld (1938, 1968); Mihalas (1969, 1978). Some essential steps in model-atmosphere analysis for determining stellar abundances

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Signatures of stellar surface structure Dainis Dravins - Lund Observatory

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Signatures of stellar surface structure dainis dravins lund observatory

KVA

Signatures of stellar surface structure

Dainis Dravins - Lund Observatory

www.astro.lu.se/~dainis


Signatures of stellar surface structure dainis dravins lund observatory

Stellar atmosphere theory classics…

Unsöld (1938, 1968); Mihalas (1969, 1978)


Signatures of stellar surface structure dainis dravins lund observatory

Some essential steps in

model-atmosphere analysis

for determining stellar abundances

(Bengt Gustafsson)


Signatures of stellar surface structure dainis dravins lund observatory

ASSUMPTIONS FOR THE RADIATIVE PART OF STELLAR MODEL ATMOSPHERES


Signatures of stellar surface structure dainis dravins lund observatory

G.Worrall & A.M.Wilson: Can Astrophysical Abundances be Taken Seriously?, Nature 236, 15


Signatures of stellar surface structure dainis dravins lund observatory

Deducedquiet-Suntemperature distribution

Approximatedepthswherevariouscontinua and linesoriginate are marked

J.E.Vernazza, E.H.Avrett, 

R.Loeser: Structure of the solar chromosphere. III - Models of the EUV brightness components of the quiet-sun

ApJS45, 635


Signatures of stellar surface structure dainis dravins lund observatory

Paradigms of stellar atmosphere analyses

Craig & Brown (1986)


Signatures of stellar surface structure dainis dravins lund observatory

But…


Synthetic line profiles shifts

SYNTHETIC LINE PROFILES & SHIFTS

1-D models disagree with observations

(data from solar flux atlas)

M.Asplund, Å.Nordlund, R.Trampedach, C.AllendePrieto, R.F.Stein: Line formation in solar granulation.

I. Fe line shapes, shifts and asymmetries, Astron.Astrophys. 359, 729


Signatures of stellar surface structure dainis dravins lund observatory

ASSUMPTIONS FOR THE DYNAMIC PART OF STELLAR MODEL ATMOSPHERES


Signatures of stellar surface structure dainis dravins lund observatory

Real line formation


Observed solar granulation

OBSERVED SOLAR GRANULATION

Dutch Open Telescope (La Palma)


Simulated solar granulation

SIMULATED SOLAR GRANULATION

Hans-Günter Ludwig (Lund)


Signatures of stellar surface structure dainis dravins lund observatory

“Wiggly” spectral lines of solar granulation

“Wiggly" spectral lines in the solar photosphere inside and outside a region of activity, reflecting rising and sinking motions in granulation (wavelength increases to the right). The central part crosses a magnetically active region with reduced velocity amplitudes. (W.Mattig)


Signatures of stellar surface structure dainis dravins lund observatory

Spatially resolved line profiles of the Fe I 608.27 nm line (exc = 2.22 eV) in a 3-D solar simulation.

The thick red line denotes the spatially averaged profile.

The steeper temperature structures in upflows tend to make lines stronger (blue-shifted components).

M.Asplund: New Light on Stellar Abundance Analyses: Departures from LTE and Homogeneity, Ann.Rev.Astron.Astrophys. 43, 481


Signatures of stellar surface structure dainis dravins lund observatory

Spatially

resolved

line profiles

& bisectors

of solar

granulation

(modeled)

M.Asplund, Å.Nordlund,

R.Trampedach,

C.Allende Prieto, R.F.Stein:

Line Formation in Solar

Granulation. I.

Fe Line Shapes, Shifts and

Asymmetries,

Astron.Astrophys.359, 729


Synthetic line profiles shifts1

SYNTHETIC LINE PROFILES & SHIFTS

Good agreement for solar-type stars in 3-D

(no micro-, nor macroturbulence)

M.Asplund, Å.Nordlund, R.Trampedach, C.AllendePrieto, R.F.Stein: Line formation in solar granulation.

I. Fe line shapes, shifts and asymmetries, Astron.Astrophys. 359, 729


Changing stellar paradigms

CHANGING STELLAR PARADIGMS

  • RECENT PAST: ”Inversion” of line profiles; “any part of a profile corresponds to some height of formation”

  • Adjustable parameters, e.g., ”micro-” & ”macro-turbulence”

  • NOW: Stellar line profiles reflect statistical distribution of lateral inhomogeneities across stellar surfaces

  • Not possible,not even in principle, to ”invert” observed profiles into exact atmospheric parameters

  • Confrontation with theory through ”forward modeling”:

    numerical simulations of radiation-coupled stellar hydrodynamics, and computation of observables


Bisectors shifts line strength

BISECTORS & SHIFTS: Line-strength

Fe I 680.4

Fe I 627.1

Fe I 624.0 nm

Predicted (solid) and observed bisectors for differently strong solar lines; 3-D hydrodynamic modeling on an absolute velocity scale. (Classical 1D models produce vertical bisectors at zero absolute velocity.)

M.Asplund, Å.Nordlund, R.Trampedach, C.AllendePrieto, R.F.Stein: Line formation in solar granulation.

I. Fe line shapes, shifts and asymmetries, Astron.Astrophys. 359, 729


Stellar convection

STELLAR CONVECTION

Matthias Steffen (Potsdam) & Bernd Freytag (Uppsala)


Signatures of stellar surface structure dainis dravins lund observatory

Solar granulation at different depths

3-D models show change of flow topology with depth z (positive into the Sun).

The surface pattern consisting of lanes surrounding granules changes into a pattern of disconnected downdrafts.

R.F.Stein & Å.Nordlund: Topology of convection beneath the solar surface , Astrophys.J. 342, L95 & H.C.Spruit, Å.Nordlund, A.M.Title: Solar Convection, ARAA 28, 263


Signatures of stellar surface structure dainis dravins lund observatory

Solar granulation at 200 nm

3D radiation hydrodynamics simulation of solar surface convection M.Steffen & S.Wedemeyer

Quiet solar granulation at 200 nm

Quiet solar granulation at 445 nm


Signatures of stellar surface structure dainis dravins lund observatory

Effects of magnetic fields


Magnetic non magnetic granulation

MAGNETIC & NON-MAGNETIC GRANULATION

Difference in solar granulation between magnetic and non-magnetic regions. Continuum images of the same area, blackened out (left) where the average field strength is less than 75 G, and (right) where the field strength is larger than 75 G. (Swedish Vacuum Solar Telescope, La Palma)

H.C.Spruit, Å.Nordlund, A.M.Title: Solar Convection, Ann.Rev.Astron.Astrophys.28, 263 (1990)


Magnetic non magnetic bisectors

MAGNETIC & NON-MAGNETIC BISECTORS

Line bisectors gradually closer to an active region (dashed), compared to that of the quiet Sun. Positions relative to the Ca II K plage are indicated.

F.Cavallini, G.Ceppatelli, A.Righini, Astron.Astrophys.143, 116


Understanding stellar surfaces

UNDERSTANDINGSTELLARSURFACES

theory and observations interact about...

  • Spectral-line strengths

  • Spectral-line widths

  • Line-profile shapes

  • Line asymmetries and bisector patterns

  • Time variability in irradiance and spectrum

  • Stellar surface imaging

  • Relative & absolute wavelength shifts


Progress in science

PROGRESS IN SCIENCE

is driven by ...

  • Confrontation between theory and observation

  • Falsification of theoretical hypotheses

  • New observational measures requiring explanation


Progress in science1

PROGRESS IN SCIENCE

is notdriven by ...

  • Agreementbetween theory and observation

    (when they agree, not much new can be learned)


Progress in stellar physics

PROGRESSINSTELLARPHYSICS

Requires disagreement between

theory and observation !


Signatures of stellar surface structure dainis dravins lund observatory

Different stars


Signatures of stellar surface structure dainis dravins lund observatory

Fe I-line bisectors

in Sun and Procyon

(F5 IV-V)

[observed]

C.Allende Prieto, M.Asplund, R.J.García López, D.L.Lambert: Signatures of Convection in the Spectrum of Procyon: Fundamental Parameters and Iron Abundance, Astrophys.J.567, 544


Signatures of stellar surface structure dainis dravins lund observatory

Average bisectors for theoretical Fe I lines produced in the time-dependent hydrodynamical three-dimensional model atmosphere for lines of different strength.

Signatures of Convection in the Spectrum of Procyon: Fundamental Parameters and Iron Abundance

C.Allende Prieto, M.Asplund, R.J.García López, D.L.Lambert

Astrophys.J. 567, 544 (2002)


Signatures of stellar surface structure dainis dravins lund observatory

Hydrodynamic models: emperature and pressure distributions in a model of Procyon (Martin Asplund)


A type stellar convection

A-TYPE STELLAR CONVECTION

Bernd Freytag (Uppsala) & Matthias Steffen (Potsdam)


Signatures of stellar surface structure dainis dravins lund observatory

Hydrodynamicmodels: Temperature distributions in the Sun, and in a metal-poorstar.

Surface layers are much cooler in 3-D than in 1-D; expansion cooling dominates over radiative heating (effect of lines opposite to that in 1-D models). The zero-point in height corresponds to average continuum optical depth unity. Dashed: 1D hydrostatic model.


Signatures of stellar surface structure dainis dravins lund observatory

STELLAR CONVECTION – White dwarf vs. Red giant

Snapshots of emergent intensity during granular evolution on a 12,000 K white dwarf (left) and a 3,800 K red giant. Horizontal areas differ by dozen orders of magnitude: 7x7 km2 for the white dwarf, and 23x23 RSun2 for the giant. (Ludwig 2006)


Cool supergiant betelgeuse

Cool supergiant (”Betelgeuse”)

Bernd Freytag (Uppsala)


Signatures of stellar surface structure dainis dravins lund observatory

Stellar astrometric “flickering”

Two situations during granular evolution: At left a time when bright [red] elements are few, and the star is darker than average; At right, many bright elements make the star brighter.

Spatial imbalance of brighter and darker patches displace the photocenter [green dot] relative to the geometric center [blue dot].

(Ludwig 2006)


Signatures of stellar surface structure dainis dravins lund observatory

Limits to information content of stellar spectra?


Ultimate information content of stellar spectra

“ULTIMATE” INFORMATION CONTENT OF STELLAR SPECTRA ?

3-D models predict detailed line shapes and shifts

… but …

their predictions may not be verifiable due to:

  • Uncertain laboratory wavelengths

  • Absence of relevant stellar lines

  • Blends with stellar or telluric lines

  • Data noisy, low resolution, poor wavelengths

  • Line-broadening: rotation, oscillations


Signatures of stellar surface structure dainis dravins lund observatory

Absorption in the Earth’s atmosphere


Signatures of stellar surface structure dainis dravins lund observatory

Wavelength noise


Modeling spectra not only single lines

MODELING SPECTRA (not only single lines)

LTE solar 3-D spectra, assuming [O]=8.86 for two different van der Waals damping constant (black lines). Blue line: observed disk center FTS spectrum by Neckel (“Hamburg photosphere”), slightly blueshifted.

Hans-Günter Ludwig (2006)


O i line profiles shifts

O I LINE PROFILES & SHIFTS

777.41

O I 777.19

777.53

LTE solar 3-D hydrodynamic spectra, assuming [O]=8.86, for two different damping constants (black lines). Blue line: observed disk center FTS spectrum, slightly blueshifted.

Hans-Günter Ludwig (2006)


Signatures of stellar surface structure dainis dravins lund observatory

Limitsfromwavelengthnoise?

Bisectors of 54 Ti II lines at solar disk center from Jungfraujoch Atlas (grating spectrometer; left); and as recorded with the Kitt Peak FTS . Bisectors have similar shapes but differ in average lineshift, and scatter about their average.


Signatures of stellar surface structure dainis dravins lund observatory

Chromosphere & radio observations


Solar optical telescope sot on hinode solar b

Solar Optical Telescope (SOT) on Hinode/Solar-B

Corona

Chromosphrer

Temperature minimum

Photosphere

Magnetic

field


Signatures of stellar surface structure dainis dravins lund observatory

Solar Optical Telescope on board HINODE (Solar-B)

G-band (430nm) & Ca II H (397nm) movies


Signatures of stellar surface structure dainis dravins lund observatory

A view at the solar chromosphere with ALMA

3-D radiation hydrodynamics simulation of the non-magnetic solar atmosphere

M.Steffen, H.-G.Ludwig, S.Wedemeyer, H.Holweger, B.Freytag

Monochromatic image at 0.35 mm

Monochromatic image at 3 mm


Signatures of stellar surface structure dainis dravins lund observatory

Spatially resolved stellar spectroscopy


Signatures of stellar surface structure dainis dravins lund observatory

Solar granulation near the limb (upward on the image)

Filtergram at 488 nm; Swedish 1-m Solar Telescope on La Palma (G.Scharmer & M.G.Löfdahl)


Center to limb variation

Center-to-LimbVariation

Åke Nordlund

(Copenhagen)


Center to limb changes of solar spectral lines

Center-to-limb changes of solar spectral lines

Spatially and temporally averaged Fe I 608.2 profiles and bisectors at different viewing angles (center-to-limb distances).

Continuum intensity is normalized to that at disk center.

Thick solid lines represent disk-center.

Note the "limb effect": smaller blue-shift toward the limb

M.Asplund, Å.Nordlund, R.Trampedach, C.AllendePrieto, R.F.Stein: Line formation in solar granulation.

I. Fe line shapes, shifts and asymmetries, Astron.Astrophys. 359, 729


Signatures of stellar surface structure dainis dravins lund observatory

Center-to-limb line-profile changes

in Procyon

Evolution of spatially averaged line profiles and bisectors in the Procyon model, leading to the global averages.

Time variability increases toward the limb, and the limb effect has opposite sign from that on the Sun.

D.Dravins & Å.Nordlund

Stellar Granulation IV.

Line Formation in Inhomogeneous

Stellar Photospheres

A&A 228, 84


Spatially resolved stellar spectroscopy

SPATIALLY RESOLVED STELLAR SPECTROSCOPY

Future observational challenges include...

  • Center-to-limb changes of line profiles

  • Center-to-limb changes of line shifts

  • Center-to-limb changes of time variablity

  • Changes across stellar active regions


Work still to do

WORK STILL TO DO ...


Signatures of stellar surface structure dainis dravins lund observatory


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