On the Scale of Photospheric Convection

Freytag, B.; Holweger, H.; Steffen, M.; Ludwig, Hans-Günter

doi:10.1007/978-3-540-69398-7_38

Cited by 253 publications

(366 citation statements)

References 3 publications

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“…Early calculations of 2D model atmospheres have shown that granules typically have an horizontal size about 10 times the local pressure scale height, which is also a measure of the vertical extent of the granules (Freytag et al 1997). In Fig.…”

Section: Characteristic Sizementioning

confidence: 99%

Granulation properties of giants, dwarfs, and white dwarfs from the CIFIST 3D model atmosphere grid

Tremblay

Ludwig

Freytag

et al. 2013

A&A

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Three-dimensional model atmospheres for giants, dwarfs, and white dwarfs, computed with the CO5BOLD code and part of the CIFIST grid, have been used for spectroscopic and asteroseismic studies. Unlike existing plane-parallel 1D structures, these simulations predict the spatially and temporally resolved emergent intensity so that granulation can be analysed, which provides insights on how convective energy transfer operates in stars. The wide range of atmospheric parameters of the CIFIST 3D simulations (3600 < T eff (K) < 13 000 and 1 < log g < 9) allows the comparison of convective processes in significantly different environments. We show that the relative intensity contrast is correlated with both the Mach and Péclet numbers in the photosphere. The horizontal size of granules varies between 3 and 10 times the local pressure scale height, with a tight correlation between the factor and the Mach number of the flow. Given that convective giants, dwarfs, and white dwarfs cover the same range of Mach and Péclet numbers, we conclude that photospheric convection operates in a very similar way in those objects.

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Section: Characteristic Sizementioning

confidence: 99%

Granulation properties of giants, dwarfs, and white dwarfs from the CIFIST 3D model atmosphere grid

Tremblay

Ludwig

Freytag

et al. 2013

A&A

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“…From physical considerations in the formation, transport and dissipation of convective cells, one can assume that the vertical size of the cells will be of the order of the local pressure scale height. However, calculations of stellar RHD models have shown that granular patterns have larger horizontal dimensions of the order of 10 times the local value of H p (Freytag et al 1997). …”

Section: Characteristic Length Scalesmentioning

confidence: 99%

Pure-hydrogen 3D model atmospheres of cool white dwarfs

Tremblay

Ludwig

Steffen

et al. 2013

A&A

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A sequence of pure-hydrogen CO5BOLD 3D model atmospheres of DA white dwarfs is presented for a surface gravity of log g = 8 and effective temperatures from 6000 to 13 000 K. We show that convective properties, such as flow velocities, characteristic granulation size and intensity contrast of the granulation patterns, change significantly over this range. We demonstrate that these 3D simulations are not sensitive to numerical parameters unlike the 1D structures that considerably depend on the mixing-length parameters. We conclude that 3D spectra can be used directly in the spectroscopic analyses of DA white dwarfs. We confirm the result of an earlier preliminary study that 3D model spectra provide a much better characterization of the mass distribution of white dwarfs and that shortcomings of the 1D mixing-length theory are responsible for the spurious high-log g determinations of cool white dwarfs. In particular, the 1D theory is unable to account for the cooling effect of the convective overshoot in the upper atmospheres.

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“…We wrote a proportionality only since we want to investigate the differential functional dependence on the fundamental parameters here. Freytag et al (1997) argue on the basis of numerical simulations that l gran is proportional to the pressure scale height at the stellar surface H surf…”

Section: Uncertainties In the Fundamental Parametersmentioning

confidence: 99%

Hydrodynamical simulations of convection-related stellar micro-variability

Ludwig¹,

Samadi

Steffen

et al. 2009

A&A

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Context. Local-box hydrodynamical model atmospheres provide statistical information about a star's emergent radiation field which allows one to predict the level of its granulation-related micro-variability. Space-based photometry is now sufficiently accurate to test model predictions. Aims. We aim to model the photometric granulation background of HD 49933 as well as the Sun, and compare the predictions to the measurements obtained by the CoRoT and SOHO satellite missions. Methods. We construct hydrodynamical model atmospheres representing HD 49933 and the Sun, and use a previously developed scaling technique to obtain the observable disk-integrated brightness fluctuations. We further performed exploratory magnetohydrodynamical simulations to gauge the impact of small scale magnetic fields on the synthetic light-curves. Results. We find that the granulation-related brightness fluctuations depend on metallicity. We obtain a satisfactory correspondence between prediction and observation for the Sun, validating our approach. For HD 49933, we arrive at a significant over-estimation by a factor of two to three in total power. Locally generated magnetic fields are unlikely to be responsible, otherwise existing fields would need to be rather strong to sufficiently suppress the granulation signal. Presently suggested updates on the fundamental stellar parameters do not improve the correspondence; however, an ad-hoc increase of the HD 49933 surface gravity by about 0.2 dex would eliminate most of the discrepancy. Conclusions. We diagnose a puzzling discrepancy between the predicted and observed granulation background in HD 49933, with only rather ad-hoc ideas for remedies at hand.

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On the Scale of Photospheric Convection

Cited by 253 publications

References 3 publications

Granulation properties of giants, dwarfs, and white dwarfs from the CIFIST 3D model atmosphere grid

Granulation properties of giants, dwarfs, and white dwarfs from the CIFIST 3D model atmosphere grid

Pure-hydrogen 3D model atmospheres of cool white dwarfs

Hydrodynamical simulations of convection-related stellar micro-variability

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