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

  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

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

  6. 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

  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.AllendePrieto, 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

  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.AllendePrieto, 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 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 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

  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 nm Quiet solar granulation at 445 nm

  23. Effects of magnetic fields

  24. 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)

  25. 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

  26. 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

  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 notdriven by ... • Agreementbetween theory and observation (when they agree, not much new can be learned)

  29. PROGRESSINSTELLARPHYSICS 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. 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.

  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 km2 for the white dwarf, and 23x23 RSun2 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

  42. Wavelength noise

  43. 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)

  44. 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)

  45. 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.

  46. Chromosphere & radio observations

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

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