Approximations for non-grey radiative transfer in numerical simulations
 of the solar photosphere

Vögler, A.; Bruls, J. H. M. J.; Schüßler, M.

doi:10.1051/0004-6361:20047043

Cited by 98 publications

(114 citation statements)

References 9 publications

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“…The construction of the opacity tables that are needed for the radiation transport relies on monochromatic MARCS opacities (Gustafsson et al 2008), computed in LTE and adopting the solar chemical composition of Grevesse & Sauval (1998), with the exception of the CNO abundances that were set to those of Asplund et al (2005). To speed up the calculations, opacities are grouped into 12 opacity bins according to the ideas of Nordlund (1982), and further elaboration by Ludwig et al (1994) and Vögler et al (2004). In the calculation of the tables, the continuum scattering opacity was added to the true absorption opacity.…”

Section: D Hydrodynamical Co5bold Model Atmospheresmentioning

confidence: 99%

The photospheric solar oxygen project

Steffen

Prakapavičius

Caffau

et al. 2015

A&A

109

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Context. The solar photospheric oxygen abundance is still widely debated. Adopting the solar chemical composition based on the "low" oxygen abundance, as determined with the use of three-dimensional (3D) hydrodynamical model atmospheres, results in a well-known mismatch between theoretical solar models and helioseismic measurements that is so far unresolved. Aims. We carry out an independent redetermination of the solar oxygen abundance by investigating the center-to-limb variation of the O i IR triplet lines at 777 nm in different sets of spectra.Methods. The high-resolution and high signal-to-noise solar center-to-limb spectra are analyzed with the help of detailed synthetic line profiles based on 3D hydrodynamical CO5BOLD model atmospheres and 3D non-LTE line formation calculations with NLTE3D. The idea is to exploit the information contained in the observations at different limb angles to simultaneously derive the oxygen abundance, A(O), and the scaling factor S H that describes the cross-sections for inelastic collisions with neutral hydrogen relative to the classical Drawin formula. Using the same codes and methods, we compare our 3D results with those obtained from the semiempirical Holweger-Müller model atmosphere as well as from different one-dimensional (1D) reference models. Results. With the CO5BOLD 3D solar model, the best fit of the center-to-limb variation of the triplet lines is obtained when the collisions by neutral hydrogen atoms are assumed to be efficient, i.e., when the scaling factor S H is between 1.2 and 1.8, depending on the choice of the observed spectrum and the triplet component used in the analysis. The line profile fits achieved with standard 1D model atmospheres (with fixed microturbulence, independent of disk position μ) are clearly of inferior quality compared to the 3D case, and give the best match to the observations when ignoring collisions with neutral hydrogen (S H = 0). The results derived with the Holweger-Müller model are intermediate between 3D and standard 1D. Conclusions. The analysis of various observations of the triplet lines with different methods yields oxygen abundance values (on a logarithmic scale where A(H) = 12) that fall in the range 8.74 < A(O) < 8.78, and our best estimate of the 3D non-LTE solar oxygen abundance is A(O) = 8.76 ± 0.02. All 1D non-LTE models give much lower oxygen abundances, by up to −0.15 dex. This is mainly a consequence of the assumption of a μ-independent microturbulence. An independent determination of the relevant collisional cross-sections is essential to substantially improve the accuracy of the oxygen abundance derived from the O i IR triplet.

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Section: D Hydrodynamical Co5bold Model Atmospheresmentioning

confidence: 99%

The photospheric solar oxygen project

Steffen

Prakapavičius

Caffau

et al. 2015

A&A

109

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“…The setup was the same as that used in Röhrbein et al (2011), except that we here used nongray radiative transport by opacity binning with 4 bins (Vögler et al 2004). The simulation domain was threaded by a prescribed vertical magnetic flux corresponding to a horizontally averaged vertical field of 200 G, which is roughly consistent with the value of B LOS inferred for the observation.…”

Section: Mhd Simulationmentioning

confidence: 99%

On the relation between continuum brightness and magnetic field in solar active regions

Danilović

Röhrbein

Cameron

et al. 2013

A&A

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Context. Variations of solar irradiance are mainly determined by the changing coverage of the visible solar disk with magnetic flux concentrations. The relationship between brightness and field strength is an important ingredient for models and reconstructions of irradiance variations. Aims. We assess the effect of limited observational resolution on the relationship between brightness and magnetic field by comparing comprehensive MHD simulations with observational results. Methods. Simulations of magnetoconvection representing the near-surface layers of a plage region were used to determine maps of the continuum brightness and Stokes profiles for the Fe  line at 630.22 nm. After convolving with instrumental profiles, synthetic observations of the magnetic field were generated by applying a Stokes inversion code. We compare the resulting relation between brightness and apparent vertical magnetic field to the corresponding outcome derived from real observations of a plage region with the Hinode satellite. Results. Consideration of the image smearing effects due to the limited resolution of the observations transform the largely monotonic relation between brightness and field strength at the original resolution of the simulations into a profile with a maximum at intermediate field strength, which is in good agreement with the observations. Conclusions. Considering the effect of limited observational resolution renders the relation between brightness and magnetic field from comprehensive MHD simulations consistent with observational results. This is a necessary prerequisite for the utilization of simulations for models and reconstruction of solar irradiance variations.

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“…The radiative transfer is based on monochromatic opacities from the MARCS stellar atmosphere package (Gustafsson et al 2008) which, to speed up the calculations, were grouped into five opacity bins (for details on the opacity binning approach see, e.g. Nordlund 1982;Ludwig 1992;Ludwig et al 1994;Vögler 2004). Solar elemental abundances as given by Grevesse & Sauval (1998) were assumed, except for carbon, nitrogen, and oxygen, for which the following values were used: A(C) = 8.41, A(N) = 7.8, and A(O) = 8.67 (Caffau et al 2008).…”

Section: Three-dimensional Hydrodynamical Model and 3d Snapshot Selecmentioning

confidence: 99%

Three-dimensional hydrodynamical CO⁵BOLD model atmospheres of red giant stars

Kučinskas¹,

Steffen²,

Ludwig³

et al. 2012

A&A

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Aims. We investigate the role of convection in the formation of atomic and molecular lines in the atmosphere of a red giant star. For this purpose we study the formation properties of spectral lines that belong to a number of astrophysically important tracer elements, including neutral and singly ionized atoms (Li Ba ii, and Eu ii), and molecules (CH, CO, C 2 , NH, CN, and OH).Methods. We focus our investigation on a prototypical red giant located close to the red giant branch (RGB) tip (T eff = 3660 K, log g = 1.0, [M/H] = 0.0). We used two types of model atmospheres, 3D hydrodynamical and classical 1D, calculated with the CO 5 BOLD and LHD stellar atmosphere codes, respectively. Both codes share the same atmospheric parameters, chemical composition, equation of state, and opacities, which allowed us to make a strictly differential comparison between the line formation properties predicted in 3D and 1D. The influence of convection on the spectral line formation was assessed with the aid of 3D-1D abundance corrections, which measure the difference between the abundances of chemical species derived with the 3D hydrodynamical and 1D classical model atmospheres. Results. We find that convection plays a significant role in the spectral line formation in this particular red giant. The derived 3D-1D abundance corrections rarely exceed ±0.1 dex when lines of neutral atoms and molecules are considered, which is in line with the previous findings for solar-metallicity red giants located on the lower RGB. The situation is different with lines that belong to ionized atoms, or to neutral atoms with high ionization potential. In both cases, the corrections for high-excitation lines (χ > 8 eV) may amount to Δ 3D−1D ∼ −0.4 dex. The 3D-1D abundance corrections generally show a significant wavelength dependence; in most cases they are smaller in the near-infrared, at 1600-2500 nm.

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Approximations for non-grey radiative transfer in numerical simulations of the solar photosphere

Cited by 98 publications

References 9 publications

The photospheric solar oxygen project

The photospheric solar oxygen project

On the relation between continuum brightness and magnetic field in solar active regions

Three-dimensional hydrodynamical CO⁵BOLD model atmospheres of red giant stars

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Approximations for non-grey radiative transfer in numerical simulations of the solar photosphere

Cited by 98 publications

References 9 publications

The photospheric solar oxygen project

The photospheric solar oxygen project

On the relation between continuum brightness and magnetic field in solar active regions

Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant stars

Contact Info

Product

Resources

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Three-dimensional hydrodynamical CO⁵BOLD model atmospheres of red giant stars