Granulation in K-type dwarf stars

Ramírez, Iván; Prieto, C. Allende; Koesterke, L.; Lambert, David L.; Asplund, Martin

doi:10.1051/0004-6361/200911741

Cited by 47 publications

(41 citation statements)

References 36 publications

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“…Stein & Nordlund 1998;Carlsson et al 2004;Danilovic et al 2008;Pereira et al 2009b;Nordlund et al 2009;Wedemeyer-Böhm & Rouppe van der Voort 2009) and other late-type stars (e.g. Allende Prieto et al 2002;Ramírez et al 2009;Chiavassa et al 2010). Theoretical predictions of photospheric temperature stratifications from 3D model stellar atmospheres have been tested in a number of cases against observed centre-to-limb variations, showing in general excellent agreement with the measurements (Aufdenberg et al 2005;Bigot et al 2006;Koesterke et al 2008;Pereira et al 2009a;Chiavassa et al 2010).…”

Section: Introductionmentioning

confidence: 68%

Three-dimensional surface convection simulations of metal-poor stars

Collet

Hayek

Asplund

et al. 2011

A&A

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Context. Three-dimensional (3D) radiative hydrodynamic model atmospheres of metal-poor late-type stars are characterized by cooler upper photospheric layers than their one-dimensional counterparts. This property of 3D model atmospheres can dramatically affect the determination of elemental abundances from temperature-sensitive spectral features, with profound consequences on galactic chemical evolution studies. Aims. We investigate whether the cool surface temperatures predicted by 3D model atmospheres of metal-poor stars can be ascribed to approximations in the treatment of scattering during the modelling phase. Methods. We use the Bifrost code to construct 3D model atmospheres of metal-poor stars and test three different ways to handle scattering in the radiative transfer equation. As a first approach, we solve iteratively the radiative transfer equation for the general case of a source function with a coherent scattering term, treating scattering in a correct and consistent way. As a second approach, we solve the radiative transfer equation in local thermodynamic equilibrium approximation, neglecting altogether the contribution of continuum scattering to extinction in the optically thin layers; this has been the default mode in our previous 3D modelling as well as in present Stagger-Code models. As our third and final approach, we treat continuum scattering as pure absorption everywhere, which is the standard case in the 3D modelling by the CO 5 BOLD collaboration. Results. For all simulations, we find that the second approach produces temperature structures with cool upper photospheric layers very similar to the case in which scattering is treated correctly. In contrast, treating scattering as pure absorption leads instead to significantly hotter and shallower temperature stratifications. The main differences in temperature structure between our published models computed with the Stagger-and Bifrost codes and those generated with the CO 5 BOLD code can be traced to the different treatments of scattering. Conclusions. Neglecting the contribution of continuum scattering to extinction in optically thin layers provides a good approximation to the full, iterative solution of the radiative transfer equation in metal-poor stellar surface convection simulations, and at a much lower computational cost. Our results also demonstrate that the cool temperature stratifications predicted for metal-poor late-type stars by previous models by our collaboration are not an artifact of the approximated treatment of scattering.

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Section: Introductionmentioning

confidence: 68%

Three-dimensional surface convection simulations of metal-poor stars

Collet

Hayek

Asplund

et al. 2011

A&A

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“…The 3D models are capable of accurately reproducing the surface structure of the observed solar granulation with their strongly inhomogeneous surface A&A 517, A49 (2010) intensities (Stein & Nordlund 1998). The velocity fields predicted by the 3D simulations lead to a close match with both the observed spectral line bisectors and the broadening of their profiles in the atmospheres of different stars (e.g., Dravins & Nordlund 1990;Asplund et al 2000;Allende Prieto et al 2002;Ramírez et al 2009). Recently, impressive agreement between a new synthetic 3D model and solar observations has been found in a detailed comparison of spectral line shifts, equivalent widths and center-to-limb variations for normalized line profiles (Pereira et al 2009a,b).…”

Section: Introductionmentioning

confidence: 81%

Radiative transfer with scattering for domain-decomposed 3D MHD simulations of cool stellar atmospheres

Hayek

Asplund

Carlsson

et al. 2010

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174

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Aims. We present the implementation of a radiative transfer solver with coherent scattering in the new BIFROST code for radiative magneto-hydrodynamical (MHD) simulations of stellar surface convection. The code is fully parallelized using MPI domain decomposition, which allows for large grid sizes and improved resolution of hydrodynamical structures. We apply the code to simulate the surface granulation in a solar-type star, ignoring magnetic fields, and investigate the importance of coherent scattering for the atmospheric structure. Methods. A scattering term is added to the radiative transfer equation, requiring an iterative computation of the radiation field. We use a short-characteristics-based Gauss-Seidel acceleration scheme to compute radiative flux divergences for the energy equation. The effects of coherent scattering are tested by comparing the temperature stratification of three 3D time-dependent hydrodynamical atmosphere models of a solar-type star: without scattering, with continuum scattering only, and with both continuum and line scattering. Results. We show that continuum scattering does not have a significant impact on the photospheric temperature structure for a star like the Sun. Including scattering in line-blanketing, however, leads to a decrease of temperatures by about 350 K below log 10 τ 5000 < ∼ −4. The effect is opposite to that of 1D hydrostatic models in radiative equilibrium, where scattering reduces the cooling effect of strong LTE lines in the higher layers of the photosphere. Coherent line scattering also changes the temperature distribution in the high atmosphere, where we observe stronger fluctuations compared to a treatment of lines as true absorbers.

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“…for exoplanet detection and characterisation). As this section has shown, the many different mechanisms that determine the convective blueshift for individual spectral lines can only be reproduced by comprehensive 3D calculations (also see Ramírez et al 2009;Allende Prieto et al 2013). …”

Section: Centre-to-limb Variation Of Line Profilesmentioning

confidence: 99%

Three-dimensional simulations of near-surface convection in main-sequence stars

Beeck

Cameron

Reiners

et al. 2013

A&A

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Context. The atmospheres of cool main-sequence stars are structured by convective flows from the convective envelope that penetrate the optically thin layers and lead to structuring of the stellar atmospheres analogous to solar granulation. The flows have considerable influence on the 3D structure of temperature and pressure and affect the profiles of spectral lines formed in the photosphere. Aims. For the set of six 3D radiative (M)HD simulations of cool main-sequence stars described in the first paper of this series, we analyse the near-surface layers. We aim at describing the properties of granulation of different stars and at quantifying the effects on spectral lines of the thermodynamic structure and flows of 3D convective atmospheres. Methods. We detected and tracked granules in brightness images from the simulations to analyse their statistical properties, as well as their evolution and lifetime. We calculated spatially resolved spectral line profiles using the line synthesis code SPINOR. To enable a comparison to stellar observations, we implemented a numerical disc-integration, which includes (differential) rotation. Results. Although the stellar parameters change considerably along the model sequence, the properties of the granules are very similar. The impact of the 3D structure of the atmospheres on line profiles is measurable in disc-integrated spectra. Line asymmetries caused by convection are modulated by stellar rotation. Conclusions. The 3D structure of cool stellar atmospheres as shaped by convective flows has to be taken into account when using photospheric lines to determine stellar parameters.

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Granulation in K-type dwarf stars

Cited by 47 publications

References 36 publications

Three-dimensional surface convection simulations of metal-poor stars

Three-dimensional surface convection simulations of metal-poor stars

Radiative transfer with scattering for domain-decomposed 3D MHD simulations of cool stellar atmospheres

Three-dimensional simulations of near-surface convection in main-sequence stars

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