3,424 research outputs found

### Dynamical Tide in Solar-Type Binaries

Circularization of late-type main-sequence binaries is usually attributed to
turbulent convection, while that of early-type binaries is explained by
resonant excitation of g modes. We show that the latter mechanism operates in
solar-type stars also and is at least as effective as convection, despite
inefficient damping of g modes in the radiative core. The maximum period at
which this mechanism can circularize a binary composed of solar-type stars in
10 Gyr is as low as 3 days, if the modes are damped by radiative diffusion only
and g-mode resonances are fixed; or as high as 6 days, if one allows for
evolution of the resonances and for nonlinear damping near inner turning
points. Even the larger theoretical period falls short of the observed
transition period by a factor two.Comment: 17 pages, 2 postscript figures, uses aaspp4.sty. Submitted to Ap

### 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&

### A Robust Measure of Tidal Circularization in Coeval Binary Populations: The solar-type spectroscopic Binary Population in The Open Cluster M35

We present a new homogeneous sample of 32 spectroscopic binary orbits in the
young (~ 150 Myr) main-sequence open cluster M35. The distribution of orbital
eccentricity vs. orbital period (e-log(P)) displays a distinct transition from
eccentric to circular orbits at an orbital period of ~ 10 days. The transition
is due to tidal circularization of the closest binaries. The population of
binary orbits in M35 provide a significantly improved constraint on the rate of
tidal circularization at an age of 150 Myr. We propose a new and more robust
diagnostic of the degree of tidal circularization in a binary population based
on a functional fit to the e-log(P) distribution. We call this new measure the
tidal circularization period. The tidal circularization period of a binary
population represents the orbital period at which a binary orbit with the most
frequent initial orbital eccentricity circularizes (defined as e = 0.01) at the
age of the population. We determine the tidal circularizationperiod for M35 as
well as for 7 additional binary populations spanning ages from the pre
main-sequence (~ 3 Myr) to late main-sequence (~ 10 Gyr), and use Monte Carlo
error analysis to determine the uncertainties on the derived circularization
periods. We conclude that current theories of tidal circularization cannot
account for the distribution of tidal circularization periods with population
age.Comment: 37 pages, 9 figures, to be published in The Astrophysical Journal,
February 200

### Influence of the Tachocline on Solar Evolution

Recently helioseismic observations have revealed the presence of a shear
layer at the base of the convective zone related to the transition from
differential rotation in the convection zone to almost uniform rotation in the
radiative interior, the tachocline. At present, this layer extends only over a
few percent of the solar radius and no definitive explanations have been given
for this thiness. Following Spiegel and Zahn (1992, Astron. Astrophys.), who
invoke anisotropic turbulence to stop the spread of the tachocline deeper in
the radiative zone as the Sun evolves, we give some justifications for their
hypothesis by taking into account recent results on rotating shear instability
(Richard and Zahn 1999, Astron. Astrophys.). We study the impact of the
macroscopic motions present in this layer on the Sun's structure and evolution
by introducing a macroscopic diffusivity $D_T$ in updated solar models. We find
that a time dependent treatment of the tachocline significantly improves the
agreement between computed and observed surface chemical species, such as the
$^7$Li and modify the internal structure of the Sun (Brun, Turck-Chi\`eze and
Zahn, 1999, in Astrophys. J.).Comment: to appear in Annals of the New York Academy of Sciences, vol 898.
Postscript file, 9 pages and 5 figures New Email Address for A. S. Brun:
[email protected]

### Standard Solar models in the Light of New Helioseismic Constraints II. Mixing Below the Convective Zone

In previous work, we have shown that recent updated standard solar models
cannot reproduce the radial profile of the sound speed at the base of the
convective zone (CZ) and fail to predict the Li7 depletion. In parallel,
helioseismology has shown that the transition from differential rotation in the
CZ to almost uniform rotation in the radiative solar interior occurs in a
shallow layer called the tachocline. This layer is presumably the seat of large
scale circulation and of turbulent motions. Here, we introduce a macroscopic
transport term in the structure equations, which is based on a hydrodynamical
description of the tachocline proposed by Spiegel and Zahn, and we calculate
the mixing induced within this layer. We discuss the influence of different
parameters that represent the tachocline thickness, the Brunt-Vaissala
frequency at the base of the CZ, and the time dependence of this mixing process
along the Sun's evolution. We show that the introduction of such a process
inhibits the microscopic diffusion by about 25%. Starting from models including
a pre-main sequence evolution, we obtain: a) a good agreement with the observed
photospheric chemical abundance of light elements such as He3, He4, Li7 and
Be9, b) a smooth composition gradient at the base of the CZ, and c) a
significant improvement of the sound speed square difference between the
seismic sun and the models in this transition region, when we allow the
phostospheric heavy element abundance to adjust, within the observational
incertitude, due to the action of this mixing process. The impact on neutrino
predictions is also discussed.Comment: 15 pages, 7 figures, to be published in ApJ (used emulateapj style
for latex2e). New email for A. S. Brun: [email protected]

### The New Transiting Planet OGLE-TR-56b: Orbit and Atmosphere

Motivated by the identification of the very close-in extrasolar giant planet
OGLE-TR-56b, we explore the implications of its existence on problems of tidal
dissipation, planet migration, and atmospheric stability. The small orbit of
OGLE-TR-56b makes the planet an interesting test particle case for tidal
dissipation in stellar convection zones. We show that it favors prescriptions
of suppressed convective eddy viscosity. Precise timing of the transits of
OGLE-TR-56b might place interesting constraints on stellar convection theory,
if orbital period change is detected in the near future.Comment: 12 pages, 1 figure, submitted to ApJ

### Post Main Sequence Orbital Circularization of Binary Stars in the Large and Small Magellanic Clouds

We present results from a study of the orbits of eclipsing binary stars (EBs)
in the Magellanic Clouds. The samples comprise 4510 EBs found in the Large
Magellanic Cloud (LMC) by the MACHO project, 2474 LMC EBs found by the OGLE-II
project (of which 1182 are also in the MACHO sample), 1380 in the Small
Magellanic Cloud (SMC) found by the MACHO project, and 1317 SMC EBs found by
the OGLE-II project (of which 677 are also in the MACHO sample); we also
consider the EROS sample of 79 EBs in the bar of the LMC. Statistics of the
phase differences between primary and secondary minima allow us to infer the
statistics of orbital eccentricities within these samples. We confirm the
well-known absence of eccentric orbit in close binary stars. We also find
evidence for rapid circularization in longer period systems when one member
evolves beyond the main sequence, as also found by previous studies.Comment: 37 pages, 16 figures, accepted for publication in ApJ. Added a new
reference and updated information on on line materia

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