2,126 research outputs found
Impact of rotation and disc lifetime on pre-main sequence lithium depletion of solar-type stars
Aims: We study the influence of rotation and disc lifetime on lithium
depletion of pre-main sequence (PMS) solar-type stars. Methods: The impact of
rotational mixing and of the hydrostatic effects of rotation on lithium
abundances are investigated by computing non-rotating and rotating PMS models
that include a comprehensive treatment of shellular rotation. The influence of
the disc lifetime is then studied by comparing the lithium content of PMS
rotating models experiencing different durations of the disc-locking phase
between 3 and 9 Myr. Results: The surface lithium abundance at the end of the
PMS is decreased when rotational effects are included. During the beginning of
the lithium depletion phase, only hydrostatic effects of rotation are at work.
This results in a decrease in the lithium depletion rate for rotating models
compared to non-rotating ones. When the convective envelope recedes from the
stellar centre, rotational mixing begins to play an important role due to
differential rotation near the bottom of the convective envelope. This mixing
results in a decrease in the surface lithium abundance with a limited
contribution from hydrostatic effects of rotation, which favours lithium
depletion during the second part of the PMS evolution. The impact of rotation
on PMS lithium depletion is also found to be sensitive to the duration of the
disc-locking phase. When the disc lifetime increases, the PMS lithium abundance
of a solar-type star decreases owing to the higher efficiency of rotational
mixing in the radiative zone. A relationship between the surface rotation and
lithium abundance at the end of the PMS is then obtained: slow rotators on the
zero-age main sequence are predicted to be more lithium-depleted than fast
rotators due to the increase in the disc lifetime.Comment: 8 pages, 11 figures, A&
Asteroseismology of the visual binary 70 Ophiuchi
Convection in stars excites resonant acoustic waves. The frequencies of these
oscillations depend on the sound speed inside the star, which in turn depends
on density, temperature, gas motion, and other properties of the stellar
interior. Therefore, analysis of the oscillations provides an unrivaled method
to probe the internal structure of a star. Solar-like oscillations in the
primary of the visual binary 70 Ophiuchi are investigated. 70 Ophiuchi A was
observed with the Harps spectrograph mounted on the 3.6-m telescope at the ESO
La Silla Observatory (Chile) during 6 nights in July 2004 allowing us to
collect 1758 radial velocity measurements with a standard deviation of about
1.39 m s-1. The power spectrum of the high precision velocity time series
clearly presents several identifiable peaks between 3 and 6 mHz showing
regularity with a large spacing of Delta_nu = 161.7 +- 0.3 uHz. Fourteen
individual modes were identified with amplitudes in the range 11 to 14 cm s-1.Comment: 5 pages, A&A in pres
Orbital Stability of Planets in Binary Systems: A New Look at Old Results
About half of all known stellar systems with Sun-like stars consist of two or
more stars, significantly affecting the orbital stability of any planet in
these systems. This observational evidence has prompted a large array of
theoretical research, including the derivation of mathematically stringent
criteria for the orbital stability of planets in stellar binary systems, valid
for the "coplanar circular restricted three-body problem". In the following, we
use these criteria to explore the validity of results from previous theoretical
studies.Comment: 3 pages, 1 figure; submitted to: Exoplanets: Detection, Formation and
Dynamics, IAU Symposium 249, eds. Y.-S. Sun, S. Ferraz-Mello, and J.-L. Zhou
(Cambridge: Cambridge University Press
Massive star models with magnetic braking
Magnetic fields at the surface of a few early-type stars have been directly
detected. These fields have magnitudes between a few hundred G up to a few kG.
In one case, evidence of magnetic braking has been found. We investigate the
effects of magnetic braking on the evolution of rotating (=200 km s) 10 M stellar models at solar metallicity during
the main-sequence (MS) phase. The magnetic braking process is included in our
stellar models according to the formalism deduced from 2D MHD simulations of
magnetic wind confinement by ud-Doula and co-workers. Various assumptions are
made regarding both the magnitude of the magnetic field and of the efficiency
of the angular momentum transport mechanisms in the stellar interior. When
magnetic braking occurs in models with differential rotation, a strong and
rapid mixing is obtained at the surface accompanied by a rapid decrease in the
surface velocity. Such a process might account for some MS stars showing strong
mixing and low surface velocities. When solid-body rotation is imposed in the
interior, the star is slowed down so rapidly that surface enrichments are
smaller than in similar models with no magnetic braking. In both kinds of
models (differentially or uniformly rotating), magnetic braking due to a field
of a few 100 G significantly reduces the angular momentum of the core during
the MS phase. This reduction is much greater in solid-body rotating models.Comment: 4 pages, 4 figures, accepted for publication as a Letter in Astronomy
and Astrophysic
Remote balance weighs accurately amid high radiation
Commercial beam-type balance, modified and outfitted with electronic controls and digital readout, can be remotely controlled for use in high radiation environments. This allows accurate weighing of breeder-reactor fuel pieces when they are radioactively hot
Can rotation explain the multiple main sequence turn-offs of Magellanic Cloud star clusters?
Many intermediate age star clusters in the Magellanic Clouds present multiple
main sequence turn-offs (MMSTO), which challenge the classical idea that star
formation in such objects took place over short timescales. It has been
recently suggested that the presence of fast rotators among main sequence stars
could be the cause of such features (Bastian & de Mink 2009), hence relaxing
the need for extended periods of star formation. In this letter, we compute
evolutionary tracks and isochrones of models with and without rotation. We find
that, for the same age and input physics, both kinds of models present
turn-offs with an almost identical position in the colour-magnitude diagrams.
As a consequence, a dispersion of rotational velocities in coeval ensembles of
stars could not explain the presence of MMSTOs. We construct several synthetic
colour-magnitude diagrams for the different kinds of tracks and combinations of
them. The models that best reproduce the morphology of observed MMSTOs are
clearly those assuming a significant spread in the stellar ages - as long as
~400 Myr - added to a moderate amount of convective core overshooting. Only
these models produce the detailed "golf club" shape of observed MMSTOs. A
spread in rotational velocities alone cannot do anything similar. We also
discuss models involving a mixture of stars with and without overshooting, as
an additional scenario to producing MMSTOs with coeval populations. We find
that they produce turn-offs with a varying extension in the CMD direction
perpendicular to the lower main sequence, which are clearly not present in
observed MMSTOs.Comment: To appear in MNRAS Letters. Figs. 2 and 3 are in colou
Electronic gating circuit and ultraviolet laser excitation permit improved dosimeter sensitivity
Standard dosimeter reader, modified by adding an electronic gating circuit to trigger the intensity level photomultiplier, increases readout sensitivity of photoluminescent dosimeter systems. The gating circuit is controlled by a second photomultiplier which senses a short ultraviolet pulse from a laser used to excite the dosimeter
Discriminating between overshooting and rotational mixing in massive stars: any help from asteroseismology?
Chemical turbulent mixing induced by rotation can affect the internal
distribution of mu near the energy-generating core of main-sequence stars,
having an effect on the evolutionary tracks similar to that of overshooting.
However, this mixing also leads to a smoother chemical composition profile near
the edge of the convective core, which is reflected in the behaviour of the
buoyancy frequency and, therefore, in the frequencies of gravity modes. We show
that for rotational velocities typical of main-sequence B-type pulsating stars,
the signature of a rotationally induced mixing significantly perturbs the
spectrum of gravity modes and mixed modes, and can be distinguished from that
of overshooting. The cases of high-order gravity modes in Slowly Pulsating B
stars and of low-order g modes and mixed modes in beta Cephei stars are
discussed.Comment: 6 pages, 4 figures, Comm. in Asteroseismology, Contribution to the
Proceedings of the 38th LIAC, HELAS-ESTA, BAG, 200
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