1. Signatures of stellar surface structure Dainis Dravins - Lund Observatory www.astro.lu.se/~dainis KVA

2. Stellar atmosphere theory classics… Unsöld (1938, 1968); Mihalas (1969, 1978)

3. Some essential steps in model-atmosphere analysis for determining stellar abundances (Bengt Gustafsson)

4. ASSUMPTIONS FOR THE RADIATIVE PART OF STELLAR MODEL ATMOSPHERES TE Thermodynamic Equilibrium LTE Local Thermodynamic Equilibrium KE 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.

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

6. 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-sun ApJS 45, 635

7. Paradigms of stellar atmosphere analyses Craig & Brown (1986)

8. But…

9. 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

10. ASSUMPTIONS FOR THE DYNAMIC PART OF STELLAR MODEL ATMOSPHERES Classical model atmosphereHydrodynamic 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 from time-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

11. Real line formation

12. OBSERVED SOLAR GRANULATION Dutch Open Telescope (La Palma)

13. SIMULATED SOLAR GRANULATION Hans-Günter Ludwig (Lund)

14. “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)

15. 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

16. 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

17. 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

18. 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 even in principle  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

19. 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 680.4 Fe I 627.1 Fe I 624.0 nm

20. STELLAR CONVECTION Matthias Steffen (Potsdam) & Bernd Freytag (Uppsala)

21. 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

22. Solar granulation at 200 nm 3D radiation hydrodynamics simulation of solar surface convection M.Steffen & S.Wedemeyer Quiet solar granulation at 200 nmQuiet solar granulation at 445 nm

23. Effects of magnetic fields

24. 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)

25. 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.

26. 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

27. PROGRESS IN SCIENCE is driven by...  Confrontation between theory and observation  Falsification of theoretical hypotheses  New observational measures requiring explanation

28. PROGRESS IN SCIENCE is not driven by...  Agreement between theory and observation (when they agree, not much new can be learned)

29. PROGRESS IN STELLAR PHYSICS Requires disagreement between theory and observation !

30. Different stars

31. 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

32. 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)

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

34. A-TYPE STELLAR CONVECTION Bernd Freytag (Uppsala) & Matthias Steffen (Potsdam)

35. 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.

36. 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 km 2 for the white dwarf, and 23x23 R Sun 2 for the giant. (Ludwig 2006)

37. Cool supergiant (”Betelgeuse”) Bernd Freytag (Uppsala)

38. 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)

39. Limits to information content of stellar spectra ?

40. “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

41. Absorption in the Earth’s atmosphere

45. Wavelength noise

46. 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.

47. 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

48. 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 ?

49. Chromosphere & radio observations

50. Solar Optical Telescope (SOT) on Hinode/Solar-B Corona Magnetic field Chromosphrer Temperature minimum Photosphere

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

52. 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 mmMonochromatic image at 3 mm

54. Spatially resolved stellar spectroscopy

55. 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)

56. Center-to- Limb Variation Åke Nordlund (Copenhagen)

57. 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

58. 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

59. 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

60. WORK STILL TO DO...

61. …