6,945 research outputs found
Tides and angular momentum redistribution inside low-mass stars hosting planets: a first dynamical model
We introduce a general mathematical framework to model the internal transport
of angular momentum in a star hosting a close-in planetary/stellar companion.
By assuming that the tidal and rotational distortions are small and that the
deposit/extraction of angular momentum induced by stellar winds and tidal
torques are redistributed solely by an effective eddy-viscosity that depends on
the radial coordinate, we can formulate the model in a completely analytic way.
It allows us to compute simultaneously the evolution of the orbit of the
companion and of the spin and the radial differential rotation of the star. An
illustrative application to the case of an F-type main-sequence star hosting a
hot Jupiter is presented. The general relevance of our model to test more
sophisticated numerical dynamical models and to study the internal rotation
profile of exoplanet hosts, submitted to the combined effects of tides and
stellar winds, by means of asteroseismology are discussed.Comment: 32 pages, 10 figures, one table; accepted to Celestial Mechanics and
Dynamical Astronomy, special issue on tide
Magnetic energy cascade in spherical geometry: I. The stellar convective dynamo case
We present a method to characterize the spectral transfers of magnetic energy
between scales in simulations of stellar convective dynamos. The full triadic
transfer functions are computed thanks to analytical coupling relations of
spherical harmonics based on the Clebsch-Gordan coefficients. The method is
applied to mean field dynamo models as benchmark tests. From the
physical standpoint, the decomposition of the dynamo field into primary and
secondary dynamo families proves very instructive in the case.
The same method is then applied to a fully turbulent dynamo in a solar
convection zone, modeled with the 3D MHD ASH code. The initial growth of the
magnetic energy spectrum is shown to be non-local. It mainly reproduces the
kinetic energy spectrum of convection at intermediate scales. During the
saturation phase, two kinds of direct magnetic energy cascades are observed in
regions encompassing the smallest scales involved in the simulation. The first
cascade is obtained through the shearing of magnetic field by the large scale
differential rotation that effectively cascades magnetic energy. The second is
a generalized cascade that involves a range of local magnetic and velocity
scales. Non-local transfers appear to be significant, such that the net
transfers cannot be reduced to the dynamics of a small set of modes. The
saturation of the large scale axisymmetric dipole and quadrupole are detailed.
In particular, the dipole is saturated by a non-local interaction involving the
most energetic scale of the magnetic energy spectrum, which points out the
importance of the magnetic Prandtl number for large-scale dynamos.Comment: 21 pages, 14 figures, 1 table, accepted for publication in the
Astrophysical Journa
Characterizing the propagation of gravity waves in 3D nonlinear simulations of solar-like stars
The revolution of helio- and asteroseismology provides access to the detailed
properties of stellar interiors by studying the star's oscillation modes. Among
them, gravity (g) modes are formed by constructive interferences between
progressive internal gravity waves (IGWs), propagating in stellar radiative
zones. Our new 3D nonlinear simulations of the interior of a solar-like star
allows us to study the excitation, propagation, and dissipation of these waves.
The aim of this article is to clarify our understanding of the behavior of IGWs
in a 3D radiative zone and to provide a clear overview of their properties. We
use a method of frequency filtering that reveals the path of {individual}
gravity waves of different frequencies in the radiative zone. We are able to
identify the region of propagation of different waves in 2D and 3D, to compare
them to the linear raytracing theory and to distinguish between propagative and
standing waves (g modes). We also show that the energy carried by waves is
distributed in different planes in the sphere, depending on their azimuthal
wave number. We are able to isolate individual IGWs from a complex spectrum and
to study their propagation in space and time. In particular, we highlight in
this paper the necessity of studying the propagation of waves in 3D spherical
geometry, since the distribution of their energy is not equipartitioned in the
sphere.Comment: 14 pages, 12 figues, accepted by Astronomy & Astrophysic
So you want to run an experiment, now what? Some Simple Rules of Thumb for Optimal Experimental Design
Experimental economics represents a strong growth industry. In the past several decades the method has expanded beyond intellectual curiosity, now meriting consideration alongside the other more traditional empirical approaches used in economics. Accompanying this growth is an influx of new experimenters who are in need of straightforward direction to make their designs more powerful. This study provides several simple rules of thumb that researchers can apply to improve the efficiency of their experimental designs. We buttress these points by including empirical examples from the literature.
Estimating Column Density in Molecular Clouds with FIR and Sub-mm Emission Maps
We have used a numerical simulation of a turbulent cloud to synthesize maps
of the thermal emission from dust at a variety of far-IR and sub-mm
wavelengths. The average column density and external radiation field in the
simulation is well matched to clouds such as Perseus and Ophiuchus. We use
pairs of single-wavelength emission maps to derive the dust color temperature
and column density, and we compare the derived column densities with the true
column density. We demonstrate that longer wavelength emission maps yield less
biased estimates of column density than maps made towards the peak of the dust
emission spectrum. We compare the scatter in the derived column density with
the observed scatter in Perseus and Ophiuchus. We find that while in Perseus
all of the observed scatter in the emission-derived versus the
extinction-derived column density can be attributed to the flawed assumption of
isothermal dust along each line of sight, in Ophiuchus there is additional
scatter above what can be explained by the isothermal assumption. Our results
imply that variations in dust emission properties within a molecular cloud are
not necessarily a major source of uncertainty in column density measurements.Comment: Accepted to ApJ Letter
Understanding angular momentum transport in red giants: the case of KIC 7341231
Context. Thanks to recent asteroseismic observations, it has been possible to
infer the radial differential rotation profile of subgiants and red giants.
Aims. We want to reproduce through modeling the observed rotation profile of
the early red giant KIC 7341231 and constrain the physical mechanisms
responsible for angular momentum transport in stellar interiors.
Methods. We compute models of KIC 7341231 including a treatment of shellular
rotation and we compare the rotation profiles obtained with the one derived by
Deheuvels et al. (2012). We then modify some modeling parameters in order to
quantify their effect on the obtained rotation profile. Moreover, we mimic a
powerful angular momentum transport during the Main Sequence and study its
effect on the evolution of the rotation profile during the subgiant and red
giant phases.
Results. We show that meridional circulation and shear mixing alone produce a
rotation profile for KIC 7341231 too steep compared to the observed one. An
additional mechanism is then needed to increase the internal transport of
angular momentum. We find that this undetermined mechanism has to be efficient
not only during the Main Sequence but also during the much quicker subgiant
phase. Moreover, we point out the importance of studying the whole rotational
history of a star in order to explain its rotation profile during the red giant
evolution.Comment: 8 pages, 8 figures, 5 table
CP and related phenomena in the context of Stellar Evolution
We review the interaction in intermediate and high mass stars between their
evolution and magnetic and chemical properties. We describe the theory of
Ap-star `fossil' fields, before touching on the expected secular diffusive
processes which give rise to evolution of the field. We then present recent
results from a spectropolarimetric survey of Herbig Ae/Be stars, showing that
magnetic fields of the kind seen on the main-sequence already exist during the
pre-main sequence phase, in agreement with fossil field theory, and that the
origin of the slow rotation of Ap/Bp stars also lies early in the pre-main
sequence evolution; we also present results confirming a lack of stars with
fields below a few hundred gauss. We then seek which macroscopic motions
compete with atomic diffusion in determining the surface abundances of AmFm
stars. While turbulent transport and mass loss, in competition with atomic
diffusion, are both able to explain observed surface abundances, the interior
abundance distribution is different enough to potentially lead to a test using
asterosismology. Finally we review progress on the turbulence-driving and
mixing processes in stellar radiative zones.Comment: Proceedings of IAU GA in Rio, JD4 on Ap stars; 10 pages, 7 figure
- …