An Observational take on 3D + NLTE effects

1. An Observational take on 3D + NLTE effects C. Allende Prieto MSSL, University College London

2. Chemical Abundances from the Continuum C. Allende Prieto MSSL, University College London

3. Not alone … Lars Koesterke (TACC, Austin) David Lambert (McDonald Observatory) Martin Asplund (MPA) Ivan Hubeny (Univ. of Arizona) Sultana Nahar (Ohio State) Manuel Bautista (IVIC, Caracas)

4. Spectroscopic Abundance Determination • Input parameters for a stellar atmosphere: Teff, logg, [M/H] but in general (H,He,Li,Be,B,C, …) • Typically Teff is determined from the continuum, sometimes logg, but all other parameters from lines • For most stars: continuum shaped by H, H- in optical/IR, but other elements become important in the UV

5. Computing Absolute Fluxes • Opacities • Equation of State • Model atmosphere code

6. Opacities: an update • Line opacities: Kurucz compilations • Continuum opacities: TOPBASE for C, Mg, Al, Si, Ca

7. MgI Σσi ni / N = Σσigi exp(-Ei /K/T)

8. Energy adjustment

9. Opacities: an update • Line opacities: Kurucz compilations • Continuum opacities: TOPBASE for C, N, O, Mg, Al, Si, Ca • Continuum opacities: Iron Project for FeI (Bautista 1996,1997), FeII (Nahar & Pradhan 1996, 1999)

10. Computing Absolute Fluxes • Opacities • Equation of State • Model atmosphere code

11. Synspec upgrades • Partition functions from Irwin (1981) • Chemical equilibrium: Subroutines from Uffe Jørgensen (molecular data from Tsuji 1973, not partition functions) • Electron density recomputed for consistency

12. Computing Absolute Fluxes • Opacities • Equation of State • Model atmosp. code

13. Model atmosphere code • Atlas9 newodf (ξ=2 km/s) grid • Linux port of Atlas9 (Sbordone & Bonifacio 2006) • Newly developed tools for changing the chemical composition and recomputing models

14. Important elements • Electron donors: Fe, Mg, Si, Ca • Line opacity: Fe-peak, O (OH), C (CH, CO), Mg, H • Bound-free free-free opacity (H, H-, Fe, Mg, Si, Al)

15. The “trick” • Keep the T-ρ structure • Recompute Chemical equilibrium and recalculate the electron density and the radiative transfer for each composition

16. 0.2 dex

17. He – consistent model -0.2 +0.2

18. A poor trick!

19. Synspec vs Atlas: continuum

20. Synspec vs Atlas: continuum

21. But a small effect, anyway + 5 K

22. Metals

23. Mg

24. Are the continuum opacities that different?

25. M/H, Mg/H, C/H, O/H • 3x3x3x3 grid • Interpolation to find the best match to the SOLSTICE-UARS/SUSIM solar spectrum • The best match takes place for [M/H] = - 0.18 [Mg/H]= +0.17 [C/H] = +0.12 [O/H] = +0.07

29. 3D effects? • New 3D code Assεtt (Koesterke 2007) • Handles arbitrarily complex opacities • Electron and Rayleigh (H) scattering • High accuracy interpolations (cubic Bezier splines) Too large a job for the full spectrum with 100 snapshots and δλ ~ Doppler

30. 3D

31. 2000 2000

32. Summary • The UV continuum contains valuable abundance information • Computing absolute fluxes requires a consistent model atmosphere calculation • 3D effects appear to be modest in the continua (~ 5% at most) • Our preliminary calculations can reproduce fairly well the solar spectrum in the regions 200-260 nm and 300-400 nm with modest abundance adjustments, but too much absorption in the region around 280 nm