17,752 research outputs found
Radiation-hydrodynamics simulations of surface convection in low-mass stars: connections to stellar structure and asteroseismology
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
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
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&
Advanced space information systems
Onboard computers for pre-transmission data processing, automation of data acquisition centers, and real time data relay to control centers for advanced space information system
Hydrodynamical model atmospheres and 3D spectral synthesis
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
The Uniqueness Problem of Sequence Product on Operator Effect Algebra
A quantum effect is an operator on a complex Hilbert space that satisfies
. We denote the set of all quantum effects by . In
this paper we prove, Theorem 4.3, on the theory of sequential product on which shows, in fact, that there are sequential products on which are not of the generalized L\"{u}ders form. This result answers a
Gudder's open problem negatively
Stellar granulation as seen in disk-integrated intensity. I. Simplified theoretical modeling
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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