5,456 research outputs found
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&
Dependence of Gas Phase Abundances in the ISM on Column Density
Sightlines through high- and intermediate-velocity clouds allow measurements
of ionic gas phase abundances, A, at very low values of HI column density,
N(HI). Present observations cover over 4 orders of magnitude in N(HI).
Remarkably, for several ions we find that the A vs N(HI) relation is the same
at high and low column density and that the abundances have a relatively low
dispersion (factors of 2-3) at any particular N(HI). Halo gas tends to have
slightly higher values of A than disk gas at the same N(HI), suggesting that
part of the dispersion may be attributed to the environment. We note that the
dispersion is largest for NaI; using NaI as a predictor of N(HI) can lead to
large errors. Important implications of the low dispersions regarding the
physical nature of the ISM are: (a) because of clumping, over sufficiently long
pathlengths N(HI) is a reasonable measure of the_local_ density of_most_ of the
H atoms along the sight line; (b) the destruction of grains does not mainly
take place in catastrophic events such as strong shocks, but is a continuous
function of the mean density; (c) the cycling of the ions becoming attached to
grains and being detached must be rapid, and the two rates must be roughly
equal under a wide variety of conditions; (d) in gas that has a low average
density the attachment should occur within denser concentrations
Fast scan infrared detection and measuring instrument monthly progress report, oct. 1-31, 1964
Optics, scan, detector, and product design of fast scan infrared detection and measuring instrumen
Impact of the frequency dependence of tidal Q on the evolution of planetary systems
Context. Tidal dissipation in planets and in stars is one of the key physical
mechanisms that drive the evolution of planetary systems.
Aims. Tidal dissipation properties are intrisically linked to the internal
structure and the rheology of studied celestial bodies. The resulting
dependence of the dissipation upon the tidal frequency is strongly different in
the cases of solids and fluids.
Methods. We compute the tidal evolution of a two-body coplanar system, using
the tidal quality factor's frequency-dependencies appropriate to rocks and to
convective fluids.
Results. The ensuing orbital dynamics comes out smooth or strongly erratic,
dependent on how the tidal dissipation depends upon frequency.
Conclusions. We demonstrate the strong impact of the internal structure and
of the rheology of the central body on the orbital evolution of the tidal
perturber. A smooth frequency-dependence of the tidal dissipation renders a
smooth orbital evolution while a peaked dissipation can furnish erratic orbital
behaviour.Comment: Accepted for publication as a letter in Astronomy And Astrophysic
Scaling laws to understand tidal dissipation in fluid planetary regions and stars I - Rotation, stratification and thermal diffusivity
Tidal dissipation in planets and stars is one of the key physical mechanisms
driving the evolution of star-planet and planet-moon systems. Several
signatures of its action are observed in planetary systems thanks to their
orbital architecture and the rotational state of their components. Tidal
dissipation inside the fluid layers of celestial bodies are intrinsically
linked to the dynamics and the physical properties of the latter. This complex
dependence must be characterized. We compute the tidal kinetic energy
dissipated by viscous friction and thermal diffusion in a rotating local fluid
Cartesian section of a star/planet/moon submitted to a periodic tidal forcing.
The properties of tidal gravito-inertial waves excited by the perturbation are
derived analytically as explicit functions of the tidal frequency and local
fluid parameters (i.e. the rotation, the buoyancy frequency characterizing the
entropy stratification, viscous and thermal diffusivities) for periodic normal
modes. The sensitivity of the resulting possibly highly resonant dissipation
frequency-spectra to a control parameter of the system is either important or
negligible depending on the position in the regime diagram relevant for
planetary and stellar interiors. For corresponding asymptotic behaviors of
tidal gravito-inertial waves dissipated by viscous friction and thermal
diffusion, scaling laws for the frequencies, number, width, height and contrast
with the non-resonant background of resonances are derived to quantify these
variations. We characterize the strong impact of the internal physics and
dynamics of fluid planetary layers and stars on the dissipation of tidal
kinetic energy in their bulk. We point out the key control parameters that
really play a role and demonstrate how it is now necessary to develop ab-initio
modeling for tidal dissipation in celestial bodies.Comment: 24 pages, 14 figures, accepted for publication in Astronomy &
Astrophysic
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
The surface signature of the tidal dissipation of the core in a two-layer planet
Tidal dissipation, which is directly linked to internal structure, is one of
the key physical mechanisms that drive systems evolution and govern their
architecture. A robust evaluation of its amplitude is thus needed to predict
evolution time for spins and orbits and their final states. The purpose of this
paper is to refine recent model of the anelastic tidal dissipation in the
central dense region of giant planets, commonly assumed to retain a large
amount of heavy elements, which constitute an important source of dissipation.
The previous paper evaluated the impact of the presence of the static fluid
envelope on the tidal deformation of the core and on the associated anelastic
tidal dissipation, through the tidal quality factor Qc. We examine here its
impact on the corresponding effective anelastic tidal dissipation, through the
effective tidal quality factor Qp. We show that the strength of this mechanism
mainly depends on mass concentration. In the case of Jupiter- and Saturn-like
planets, it can increase their effective tidal dissipation by, around, a factor
2.4 and 2 respectively. In particular, the range of the rheologies compatible
with the observations is enlarged compared to the results issued from previous
formulations. We derive here an improved expression of the tidal effective
factor Qp in terms of the tidal dissipation factor of the core Qc, without
assuming the commonly used assumptions. When applied to giant planets, the
formulation obtained here allows a better match between the an elastic core's
tidal dissipation of a two-layer model and the observations.Comment: 5 pages, 2 figures, Accepted for publication in Astronomy &
Astrophysic
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