15,098 research outputs found

    Radiation-hydrodynamics simulations of surface convection in low-mass stars: connections to stellar structure and asteroseismology

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    Radiation-hydrodynamical simulations of surface convection in low-mass stars can be exploited to derive estimates of i) the efficiency of the convective energy transport in the stellar surface layers; ii) the convection-related photometric micro-variability. We comment on the universality of the mixing-length parameter, and point out potential pitfalls in the process of its calibration which may be in part responsible for the contradictory findings about its variability across the Hertzsprung-Russell digramme. We further comment on the modelling of the photometric micro-variability in HD49933 - one of the first main COROT targets.Comment: 6 pages, 5 figures, Proceedings paper of IAU Symposium 25

    Knowing the gap - intermediate information in tournaments

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    Intermediate information is often available to competitors in dynamic tournaments. We develop two simple tournament models with two stages: one with intermediate information on subjects’ relative positions after the first stage, one without. In our models, equilibrium behavior in both stages is not changed by intermediate information. We test our formal analysis using data from laboratory experiments. We find no difference between average first and second stage efforts. With intermediate information, however, subjects adjust their effort to a higher extent. Subjects who lead tend to lower their second stage effort, subjects who lag still try to win the tournament. Overall, intermediate information does not endanger the effectiveness of rank-order tournaments: incentives do neither break down nor does a rat race arise. We also briefly investigate costly intermediate information

    3D hydrodynamical CO5BOLD model atmospheres of red giant stars: I. Atmospheric structure of a giant located near the RGB tip

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    We investigate the character and role of convection in the atmosphere of a prototypical red giant located close to the red giant branch (RGB) tip with atmospheric parameters, Teff=3660K, log(g)=1.0, [M/H]=0.0. Differential analysis of the atmospheric structures is performed using the 3D hydrodynamical and 1D classical atmosphere models calculated with the CO5BOLD and LHD codes, respectively. All models share identical atmospheric parameters, elemental composition, opacities and equation-of-state. We find that the atmosphere of this particular red giant consists of two rather distinct regions: the lower atmosphere dominated by convective motions and the upper atmosphere dominated by wave activity. Convective motions form a prominent granulation pattern with an intensity contrast (~18%) which is larger than in the solar models (~15%). The upper atmosphere is frequently traversed by fast shock waves, with vertical and horizontal velocities of up to Mach ~2.5 and ~6.0, respectively. The typical diameter of the granules amounts to ~5Gm which translates into ~400 granules covering the whole stellar surface. The turbulent pressure in the giant model contributes up to ~35% to the total (i.e., gas plus turbulent) pressure which shows that it cannot be neglected in stellar atmosphere and evolutionary modeling. However, there exists no combination of the mixing-length parameter and turbulent pressure that would allow to satisfactorily reproduce the 3D temperature-pressure profile with 1D atmosphere models based on a standard formulation of mixing-length theory.Comment: 13 pages, 18 figures, accepted for publication in A&

    Hydrodynamical model atmospheres and 3D spectral synthesis

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    We discuss three issues in the context of three-dimensional (3D) hydrodynamical model atmospheres for late-type stars, related to spectral line shifts, radiative transfer in metal-poor 3D models, and the solar oxygen abundance. We include a brief overview about the model construction, taking the radiation-hydrodynamics code CO5BOLD (COnservative COde for the COmputation of COmpressible COnvection in a BOx of L Dimensions with L=2,3) and the related spectral synthesis package Linfor3D as examples.Comment: 6 pages, 2 figures, to appear in the Proceedings of the ESO/Lisbon/Aveiro Workshop "Precision Spectroscopy in Astrophysics", eds. L. Pasquini, M. Romaniello, N.C. Santos, and A. Correi

    Stellar granulation as seen in disk-integrated intensity. I. Simplified theoretical modeling

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    The solar granulation is known for a long time to be a surface manifestation of convection. Thanks to the current space-borne missions CoRoT and Kepler, it is now possible to observe in disk-integrated intensity the signature of this phenomena in a growing number of stars. The space-based photometric measurements show that the global brightness fluctuations and the lifetime associated with granulation obeys characteristic scaling relations. We thus aim at providing a simple theoretical modeling to reproduce these scaling relations and subsequently at inferring the physical properties of granulation properties across the HR diagram. We develop a simple 1D theoretical model that enable us to test any prescription concerning the time-correlation between granules. The input parameters of the model are extracted from 3D hydrodynamical models of the surface layers of stars, and the free parameters involved in the model are calibrated with solar observations. Two different prescriptions for representing the eddy time-correlation in the Fourier space are compared: a Lorentzian and an exponential form. Finally, we compare our theoretical prediction with a 3D radiative hydrodynamical (RHD) numerical modeling of stellar granulation (ab-initio approach). Provided that the free parameters are appropriately adjusted, our theoretical model satisfactorily reproduces the shape and the amplitude of the observed solar granulation spectrum. The best agreement is obtained with an exponential form. Furthermore, our theoretical model results in granulation spectra that consistently agree with the these calculated on the basis of the ab-initio approach with two 3D RHD models. Comparison between theoretical granulation spectra calculated with the present model and high precision photometry measurements of stellar granulation is undertaken in a companion paper.Comment: 10 pages, 2 figures, accepted for publication in A&
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