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

Abstract

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.

Dynamics of small-scale convective motions

“…The brightness enhancement of CBPs is the result of a side-wall heating in the interior of the vortices (see e.g. Beeck et al 2013). CBPs have a horizontal diameter of ∼ 80 km which is consistent with results in Moll et al (2011).…”

“…The number of fragments rises with increasing granular diameter. In their MHD simulations, Beeck et al (2013) label the small cells as erroneous detections. In paper I we showed that small granules strongly contribute to the total number of detected granules, and hence, have a major influence on the dynamics of the population of all granules.…”

The Rossiter-McLaughlin effect reloaded: Probing the 3D spin-orbit geometry, differential stellar rotation, and the spatially-resolved stellar spectrum of star-planet systems

“…For example, towards the limb some convective flows will be be hidden behind granules, which may result in a reduction in the line broadening induced by the LOS velocities and could decrease the FWHM (Beeck et al 2015b). However, one could also expect an increase in the FWHM because of an increasing contribution of the flows orthogonal to the granule peaks, since these flows have a higher variability than those parallel (Beeck et al 2013).…”

The nature of solar brightness variations

“…To test whether this full apparatus was required, we performed this detailed analysis for the small (9 Mm in both horizontal directions, 3 Mm in the vertical direction) G2V solar simulations described in Beeck et al 2013b 20 . To save time for the purposes of the test, we used four snapshots separated in time by 19 minutes (making the radiative output essentially uncorrelated for these small boxes as the granulation structure has changed completely between any two snapshots).…”

“…This is a feasible task since a) reliable simulations of granulation patterns on main sequence stars are gradually becoming available 20,30 , b) new methods for extrapolating the magneticallydriven brightness variations from the Sun to Sun-like stars with various levels of magnetic activity observed at arbitrary angles between rotational axis and line-of-sight have been recently developed 6,28 .…”