6,082 research outputs found
The variation of tidal dissipation in the convective envelope of low-mass stars along their evolution
Since 1995, more than 1500 exoplanets have been discovered around a large
diversity of host stars (from M- to A-type stars). Tidal dissipation in stellar
convective envelopes is a key actor that shapes the orbital architecture of
short-period systems. Our objective is to understand and evaluate how tidal
dissipation in the convective envelope of low-mass stars (from M to F types)
depends on their mass, evolutionary stage and rotation. Using a simplified
two-layer assumption, we compute analytically the frequency-averaged tidal
dissipation in their convective envelope. This dissipation is due to the
conversion into heat of the kinetic energy of tidal non wave-like/equilibrium
flow and inertial waves because of the viscous friction applied by turbulent
convection. Using grids of stellar models allows us to study the variation of
the dissipation as a function of stellar mass and age on the Pre-Main-Sequence
and on the Main-Sequence for stars with masses spanning from to
. As shown by observations, tidal dissipation in stars varies
over several orders of magnitude as a function of stellar mass, age and
rotation. During their Pre-Main-Sequence, all low-mass stars have an increase
of the frequency-averaged tidal dissipation for a fixed angular velocity in
their convective envelope until they reach a critical aspect and mass ratios.
Next, the dissipation evolves on the Main Sequence to an asymptotic value that
becomes maximum for K-type stars and that decreases by several
orders of magnitude with increasing stellar mass. Finally, the rotational
evolution of low-mass stars strengthens the importance of tidal dissipation
during the Pre-Main-Sequence for star-planet and multiple star systems.Comment: 5 pages, 4 figures, accepted for publication as a Letter in Astronomy
& Astrophysic
Grain alignment by ferromagnetic impurities
The observed wavelength dependence of linear polarization, and its variation from region to region can be explained by the following assumptions. Interstellar grains resemble interplanetary grains, in that they are composed of collections of small particles coagulated together into elongated masses. A fraction of the small particles are ferromagnetic. Presumably these are either metallic Fe or magnetite, Fe3O4. If and only if a large grain contains one or more magnetic particles is the grain aligned in the galactic magnetic field. The magnetic particles stick only to silicate grains because of chemical similarities, or (equivalently) any pure carbon grains in the diffuse interstellar medium (ISM) are too spherical to produce polarization. Grains in dense regions, such as the outer parts of molecular clouds, are larger than those in the diffuse ISM because of coagulation of the grains rather than accretion of icy mantles. These regions are known to have larger than normal values of lambda (max), the wavelength of the maximum of linear polarization. The above assumptions are sufficient to allow the calculation of the wavelength dependence of the polarization
Interstellar and cometary dust
Aspects of interstellar dust which are known from direct observation will be discussed. Some specific difficulties that various theories have in explaining the observations will be presented. Several theoretical interpretations which have been advanced will be discussed, highlighting first their similarities and then their differences. Also discussed will be the author's ideas about the conditions of interstellar dust throughout its life cycle, from origin to incorporation in pre-cometary ices. Dust is primarily observed by its effects on the spectra of background stars, so observations at optical and ultraviolet (UV) wavelengths are confined to the diffuse interstellar medium (ISM) or to the outer regions of dense clouds. Within this somewhat limited range of environments there are very few lines of sight which show any evidence for icy mantles, but there are major variations in the wavelength dependence of the extinction. In the infrared region of the spectrum, it is possible to observe a few stellar sources deeply embedded within molecular clouds
Transport and mixing in the radiation zones of rotating stars: I-Hydrodynamical processes
The purpose of this paper is to improve the modelization of the rotational
mixing which occurs in stellar radiation zones, through the combined action of
the thermally driven meridional circulation and of the turbulence generated by
the shear of differential rotation. The turbulence is assumed to be
anisotropic, due to the stratification, with stronger transport in the
horizontal directions than in the vertical. The main difference with the former
treatments by Zahn (1992) and Maeder & Zahn (1998) is that we expand here the
departures from spherical symmetry to higher order, and include explicitly the
differential rotation in latitude, to first order. This allows us to treat
simultaneously the bulk of a radiation zone and its tachocline(s). Moreover, we
take fully into account the non-stationarity of the problem, which will enable
us to tackle the rapid phases of evolution. The system of partial differential
equations, which govern the transport of angular momentum, heat and chemical
elements, is written in a form which makes it ready to implement in a stellar
evolution code. Here the effect of a magnetic field is deliberately ignored; it
will be included in forthcoming papers.Comment: 16 pages, no figures, accepted for publication in A&
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
Understanding tidal dissipation in gaseous giant planets from their core to their surface
Tidal dissipation in planetary interiors is one of the key physical
mechanisms that drive the evolution of star-planet and planet-moon systems.
Tidal dissipation in planets is intrinsically related to their internal
structure. In particular, fluid and solid layers behave differently under tidal
forcing. Therefore, their respective dissipation reservoirs have to be
compared. In this work, we compute separately the contributions of the
potential dense rocky/icy core and of the convective fluid envelope of gaseous
giant planets, as a function of core size and mass. We demonstrate that in
general both mechanisms must be taken into account.Comment: 2 pages, 2 figures, CoRoT Symposium 3 / Kepler KASC-7 joint meeting,
Toulouse, July 2014; To be published by EPJ Web of Conference
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