signatures of stellar surface structure dainis dravins - lund observatory dainis kva
Post on 19-Dec-2015
218 views
TRANSCRIPT
Signatures of stellar surface structure
Dainis Dravins - Lund Observatory
www.astro.lu.se/~dainis
KVA
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.
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
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
“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
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
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)
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: 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)
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)
“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
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 ?
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
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 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