arXiv subjects

B. Freytag

Publications and source records attributed to B. Freytag.

60 records · Page 4Linked to original sources

arXiv2005

3D Simulation of Convection and Spectral Line Formation in A-type Stars

We present first realistic numerical simulations of 3D radiative convection in the surface layers of main sequence A-type stars with Teff = 8000 K and 8500 K, log g = 4.4 and 4.0, recently performed with the CO5BOLD radiation hydrodynamics code. The resulting models are used to investigate the structure of the H+HeI and the HeII convection zones in comparison with the predictions of local and non-local convection theories, and to determine the amount of "overshoot" into the stable layers below the HeII convection zone. The simulations also predict how the topology of the photospheric granulation pattern changes from solar to A-type star convection. The influence of the photospheric temperature fluctuations and velocity fields on the shape of spectral lines is demonstrated by computing synthetic line profiles and line bisectors for some representative examples, allowing us to confront the 3D model results with observations.

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arXiv2005

Carbon monoxide in the solar atmosphere I. Numerical method and two-dimensional models

The radiation hydrodynamic code CO5BOLD has been supplemented with the time-dependent treatment of chemical reaction networks. Advection of particle densities due to the hydrodynamic flow field is also included. The radiative transfer is treated frequency-independently, i.e. grey, so far. The upgraded code has been applied to two-dimensional simulations of carbon monoxide (CO) in the non-magnetic solar photosphere and low chromosphere. For this purpose a reaction network has been constructed, taking into account the reactions which are most important for the formation and dissociation of CO under the physical conditions of the solar atmosphere. The network has been strongly reduced to 27 reactions, involving the chemical species H, H2, C, O, CO, CH, OH, and a representative metal. The resulting CO number density is highest in the cool regions of the reversed granulation pattern at mid-photospheric heights and decreases strongly above. There, the CO abundance stays close to a value of 8.3 on the usual logarithmic abundance scale with [H]=12 but is reduced in hot shock waves which are a ubiquitous phenomenon of the model atmosphere. For comparison, the corresponding equilibrium densities have been calculated, based on the reaction network but also under assumption of instantaneous chemical equilibrium by applying the Rybicki & Hummer (RH) code by Uitenbroek (2001). Owing to the short chemical timescales, the assumption holds for a large fraction of the atmosphere, in particular the photosphere. In contrast, the CO number density deviates strongly from the corresponding equilibrium value in the vicinity of chromospheric shock waves. Simulations with altered reaction network clearly show that the formation channel via hydroxide (OH) is the most important one under the conditions of the solar atmosphere.

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arXiv2004

Dynamic model atmospheres of cool giants

Cool giant stars are highly dynamical objects, and complex micro-physical processes play an important role in their extended atmospheres and winds. The interpretation of observations, and in particular of high-resolution IR spectra, requires realistic self-consistent model atmospheres. Current dynamical models include rather detailed micro-physics, and the resulting synthetic spectra compare reasonably well with observations. A transition from qualitative to quantitative modelling is taking place at present. We give an overview of existing dynamical model atmospheres for AGB stars, discussing recent advances and current trends in modelling, including 3D 'star-in-a-box' models. When comparing synthetic spectra and other observable properties resulting from dynamical models with observations we focus on the near- and mid-IR wavelength range.

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arXiv2002

Does Betelgeuse have a magnetic field?

Recent numerical simulations by Freytag et al. of the outer convection envelope of the cool super-giant Betelgeuse, have shown that the fluctuations in the star's apparent luminosity may be caused by giant cell convection. These simulations bring forth the possibility of addressing another question; namely whether stars such as Betelgeuse may harbor magnetic activity. Taking the detailed numerical simulations of the star at face value, we have applied a kinematic dynamo analysis, to study whether the flow field of the super-giant may be able to amplify a weak seed magnetic field. We do indeed find a positive exponential growth rate of the magnetic energy. The possible Betelgeusian dynamo may be characterized as belonging to the class of so-called ``local small-scale dynamos'' (i.e. dynamos where rotation is not dominant), but this is a less meaningful designation in the case of Betelgeuse, since the field is both global and large-scale.

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arXiv1999

A calibration of the mixing-length for solar-type stars based on hydrodynamical simulations - I. Methodical aspects and results for solar metallicity

Based on detailed 2D numerical radiation hydrodynamics (RHD) calculations of time-dependent compressible convection, we have studied the dynamics and thermal structure of the convective surface layers of solar-type stars. The RHD models provide information about the convective efficiency in the superadiabatic region at the top of convective envelopes and predict the asymptotic value of the entropy of the deep, adiabatically stratified layers. This information is translated into an effective mixing-length parameter alpha suitable to construct standard stellar structure models. We validate the approach by a detailed comparison to helioseismic data. The grid of RHD models for solar metallicity comprises 58 simulation runs with a helium abundance of Y=0.28 in the range of effective temperatures 4300K < Teff < 7100K and gravities 2.54 < logg < 4.74. We find a moderate, nevertheless significant variation of alpha between about 1.3 for F-dwarfs and 1.75 for K-subgiants with a dominant dependence on Teff. In the close neighbourhood of the Sun we find a plateau where alpha remains almost constant. The internal accuracy of the calibration of alpha is estimated to be +/- 0.05 with a possible systematic bias towards lower values. An analogous calibration of the convection theory of Canuto & Mazzitelli gives a different temperature dependence but a similar variation of the free parameter. For the first time, values for the gravity-darkening exponent beta are derived independently of mixing-length theory: beta=0.07...0.10. We show that our findings are consistent with constraints from stellar stability considerations and provide compact fitting formulae for the calibrations.

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arXiv1998

Lithium depletion in the sun: a study of mixing based on hydrodynamical simulations

Based on radiation hydrodynamics modeling of stellar convection zones, a diffusion scheme has been devised describing the downward penetration of convective motions beyond the Schwarzschild boundary (overshoot) into the radiative interior. This scheme of exponential diffusive overshoot has already been successfully applied to AGB stars. Here we present an application to the Sun in order to determine the time scale and depth extent of this additional mixing, i.e. diffusive overshoot at the base of the convective envelope. We calculated the associated destruction of lithium during the evolution towards and on the main-sequence. We found that the slow-mixing processes induced by the diffusive overshoot may lead to a substantial depletion of lithium during the Sun's main-sequence evolution.

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