Signatures of stellar surface structure

Dainis Dravins - Lund Observatory

www.astro.lu.se/~dainis

KVA

Stellar atmosphere theory classics…

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

Some essential steps in model-atmosphere analysisfor determining stellar abundances

(Bengt Gustafsson)

ASSUMPTIONS FOR THE RADIATIVE PART OF STELLAR MODEL ATMOSPHERESTE Thermodynamic

EquilibriumLTE Local Thermodynamic

EquilibriumKE Kinetic Equilibrium

One single temperature T determines all properties of gas and radiation

Radiation field: Isotropic, given by the Planck function

Information needed: No further knowledge needed (and none more can be obtained)

One local temperature in each spatial point determines:

Source function = Planck function

Excitation: Boltzmann equation

Ionization: Saha equation

Radiation field: Equation of radiative transfer (depends on conditions along the photon mean-free-path)

Information needed: Local values for kinetic temperature. Chemical composition and laboratory data for opacities of various elements as function of pressure, temperature, and wavelength

One local temperature in each spatial point determines:

Electron velocities: Maxwell-Boltzmann distribution

Excitation: For each energy level: statistical equilibrium between exciting & de-exciting processes

Ionization: Statistical equilibrium between ionizing & recombining processes

Radiation field: Equation of radiative transfer, coupled to equilibrium equations for excitation & ionization

Information needed: As for LTE, plus data for atomic processes such as photoexcitation cross sections; collisional [de]excitation & ionization; spontaneous & stimulated emission, free-free emission & absorption; radiative & dielectronic recombination, for different species, for different electron energies, as function of wavelength, etc.

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

Deduced quiet-Sun temperature distribution

Approximate depths where various continua and lines originate 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-sunApJS 45, 635

Paradigms of stellar atmosphere analyses

Craig & Brown (1986)

But…

SYNTHETIC LINE PROFILES & SHIFTS

1-D models disagree with observations(data from solar flux atlas)

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

ASSUMPTIONS FOR THE DYNAMIC PART OF STELLAR MODEL ATMOSPHERES

Classical model atmosphere Hydrodynamic simulations

Vertical structure of temperature and pressure from assumed convective heat exchange over a mixing-length. Pressure follows from gas density and temperature

Atmosphere horizontally homogenous, also no time variability

Spectral line broadening: Assumed [often isotropic] “macroturbulence”

Spectral-line strengths: Assumed [often isotropic] “microturbulence”

Spectral-line shapes & shifts: Not modeled

Comparison to observations: Model parameters adjusted ad hoc to agree with observations

Vertical structure of temperature and pressure fromtime-dependent 3-dimensional hydrodynamic simulations, coupled to radiative transfer. Pressure now also includes contributions from turbulence and shock waves

Atmosphere horizontally inhomogenous, parameters depend on lateral position, and also evolve with time

Spectral-line broadening: Largely follows from the calculated RMS velocity amplitudes

Spectral-line strengths: Largely follow from calculated velocity and temperature gradients

Spectral-line shapes & shifts: Arising from correlations between velocity, temperature, and local line strength

Comparison to observations: No adjustable physical parameters. Temporally and spatially averaged simulation sequences predict various stellar properties. If do not agree with observations, the physical, mathematical and numerical model approximations have to be adjusted

Real line formation

OBSERVED SOLAR GRANULATION

Dutch Open Telescope (La Palma)

SIMULATED SOLAR GRANULATION

Hans-Günter Ludwig (Lund)

“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)

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

Spatiallyresolved

line profiles& bisectors

of solargranulation

(modeled)M.Asplund, Å.Nordlund,

R.Trampedach,C.Allende Prieto, R.F.Stein:

Line Formation in Solar Granulation. I.

Fe Line Shapes, Shifts andAsymmetries,

Astron.Astrophys. 359, 729

SYNTHETIC LINE PROFILES & SHIFTS

Good agreement for solar-type stars in 3-D (no micro-, nor macroturbulence)

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

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

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

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.)

Fe I 680.4Fe I 627.1Fe I 624.0 nm

STELLAR CONVECTION

Matthias Steffen (Potsdam) & Bernd Freytag (Uppsala)

Solar granulatio

n 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

Solar granulation at 200 nm3D radiation hydrodynamics simulation of solar surface convection

M.Steffen & S.Wedemeyer

Quiet solar granulation at 200 nm Quiet solar granulation at 445 nm

Effects of magnetic fields

MAGNETIC & NON-MAGNETIC GRANULATION

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

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)

MAGNETIC & NON-MAGNETIC BISECTORS

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

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.

UNDERSTANDING STELLAR

SURFACES

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 SCIENCEPROGRESS IN SCIENCE

is driven by ...

Confrontation between theory and observation

Falsification of theoretical hypotheses

New observational measures requiring explanation

PROGRESS IN SCIENCE

is not driven by ...

Agreement between theory and observation

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

PROGRESS IN STELLAR

PHYSICS

Requires disagreement between

theory and observation !

Different stars

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

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 AbundanceC.Allende Prieto, M.Asplund, R.J.García López, D.L.LambertAstrophys.J. 567, 544 (2002)

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

A-TYPE STELLAR CONVECTION

Bernd Freytag (Uppsala) & Matthias Steffen (Potsdam)

Hydrodynamic models: Temperature distributions in the Sun, and in a metal-poor star.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.

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 RSun

2 for the giant. (Ludwig 2006)

Cool supergiant (”Betelgeuse”)

Bernd Freytag (Uppsala)

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)

Limits to 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

Absorption in the Earth’s atmosphere

Wavelength noise

MODELING SPECTRA (not only single lines)

Hans-Günter Ludwig (2006)

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.

O I LINE PROFILES & SHIFTS

Hans-Günter Ludwig (2006)

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.

O I 777.19 777.41 777.53

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.

Limits from wavelength noise ?

Chromosphere & radio

observations

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

Corona

Magneticfield

Chromosphrer

Temperature minimum

Photosphere

Solar Optical Telescope on board HINODE (Solar-B)G-band (430nm) & Ca II H (397nm) movies

A view at the solar chromosphere with ALMA3-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

Spatially resolved stellar

spectroscopy

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

Åke Nordlund(Copenhagen)

Center-to-limb changes of solar spectral lines

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

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

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.NordlundStellar Granulation IV. Line Formation in InhomogeneousStellar PhotospheresA&A 228, 84

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 ...

…