1,880 research outputs found
Solar radius and luminosity variations induced by the internal dynamo magnetic fields
Although the occurrence of solar irradiance variations induced by magnetic
surface features (e.g., sunspots, faculae, magnetic network) is generally
accepted, the existence of intrinsic luminosity changes due to the internal
magnetic fields is still controversial. This additional contribution is
expected to be accompanied by radius variations, and to be potentially
significant for the climate of the Earth. We aim to constrain theoretically the
radius and luminosity variations of the Sun that are due to the effect of the
variable magnetic fields in its interior associated with the dynamo cycle. We
have extended a one-dimensional stellar evolution code to include several
effects of the magnetic fields on the interior structure. We investigate
different magnetic configurations, based on both observational constraints and
on the output of state-of-the-art mean field dynamo models. We explore both
step-like and simply periodic time dependences of the magnetic field peak
strength. We find that the luminosity and radius variations are in anti-phase
and in phase, respectively, with the magnetic field strength. For peak magnetic
field strengths of the order of tens of kilogauss, luminosity variations
ranging between 10^{-6} and 10^{-3} (in modulus) and radius variations between
10^{-6} and 10^{-5} are obtained. Modest but significant radius variations (up
to 10^{-5} in relative terms) are obtained for magnetic fields of realistic
strength and geometry, providing a potentially observable signature of the
intrinsic variations. Establishing their existence in addition to the accepted
surface effects would have very important implications for the understanding of
solar-induced long-term trends on climate.Comment: 18 pages, 7 figures; accepted for publication in Astronomische
Nachrichte
Angular momentum transport efficiency in post-main sequence low-mass stars
Context. Using asteroseismic techniques, it has recently become possible to
probe the internal rotation profile of low-mass (~1.1-1.5 Msun) subgiant and
red giant stars. Under the assumption of local angular momentum conservation,
the core contraction and envelope expansion occurring at the end of the main
sequence would result in a much larger internal differential rotation than
observed. This suggests that angular momentum redistribution must be taking
place in the interior of these stars. Aims. We investigate the physical nature
of the angular momentum redistribution mechanisms operating in stellar
interiors by constraining the efficiency of post-main sequence rotational
coupling. Methods. We model the rotational evolution of a 1.25 Msun star using
the Yale Rotational stellar Evolution Code. Our models take into account the
magnetic wind braking occurring at the surface of the star and the angular
momentum transport in the interior, with an efficiency dependent on the degree
of internal differential rotation. Results. We find that models including a
dependence of the angular momentum transport efficiency on the radial
rotational shear reproduce very well the observations. The best fit of the data
is obtained with an angular momentum transport coefficient scaling with the
ratio of the rotation rate of the radiative interior over that of the
convective envelope of the star as a power law of exponent ~3. This scaling is
consistent with the predictions of recent numerical simulations of the
Azimuthal Magneto-Rotational Instability. Conclusions. We show that an angular
momentum transport process whose efficiency varies during the stellar evolution
through a dependence on the level of internal differential rotation is required
to explain the observed post-main sequence rotational evolution of low-mass
stars.Comment: 8 pages, 6 figures; accepted for publication in Astronomy &
Astrophysic
Searching for a link between the presence of chemical spots on the surface of HgMn stars and their weak magnetic fields
We present the results of mapping the HgMn star AR Aur using the Doppler
Imaging technique for several elements and discuss the obtained distributions
in the framework of a magnetic field topology.Comment: 2 pages, 1 figure, to appear in Proceedings of IAU Symposium 259
"Cosmic Magnetic Fields: from Planets, to Stars and Galaxies", Tenerife,
Spain, November 3-7, 200
Sunspot areas and tilt angles for solar cycles 7-10
Extending the knowledge about the properties of solar cycles into the past is
essential for understanding the solar dynamo. This paper aims at estimating
areas of sunspots observed by Schwabe in 1825-1867 and at calculating the tilt
angles of sunspot groups. The sunspot sizes in Schwabe's drawings are not to
scale and need to be converted into physical sunspot areas. We employed a
statistical approach assuming that the area distribution of sunspots was the
same in the 19th century as it was in the 20th century. Umbral areas for about
130,000 sunspots observed by Schwabe were obtained, as well as the tilt angles
of sunspot groups assuming them to be bipolar. There is, of course, no polarity
information in the observations. The annually averaged sunspot areas correlate
reasonably with sunspot number. We derived an average tilt angle by attempting
to exclude unipolar groups with a minimum separation of the two alleged
polarities and an outlier rejection method which follows the evolution of each
group and detects the moment it turns unipolar at its decay. As a result, the
tilt angles, although displaying considerable scatter, place the leading
polarity on average 5.85+-0.25 closer to the equator, in good agreement with
tilt angles obtained from 20th-century data sets. Sources of uncertainties in
the tilt angle determination are discussed and need to be addressed whenever
different data sets are combined. The sunspot area and tilt angle data are
provided online.Comment: accepted for publication in Astron. & Astrophy
A Waveguide for Bose-Einstein Condensates
We report on the creation of Bose-Einstein condensates of Rb in a
specially designed hybrid, dipole and magnetic trap. This trap naturally allows
the coherent transfer of matter waves into a pure dipole potential waveguide
based on a doughnut beam. Specifically, we present studies of the coherence of
the ensemble in the hybrid trap and during the evolution in the waveguide by
means of an autocorrelation interferometer scheme. By monitoring the expansion
of the ensemble in the waveguide we observe a mean field dominated acceleration
on a much longer time scale than in the free 3D expansion. Both the
autocorrelation interference and the pure expansion measurements are in
excellent agreement with theoretical predictions of the ensemble dynamics
Finite-temperature behavior of the Bose polaron
We consider a mobile impurity immersed in a Bose gas at finite temperature.
Using perturbation theory valid for weak coupling between the impurity and the
bosons, we derive analytical results for the energy and damping of the impurity
for low and high temperatures, as well as for temperatures close to the
critical temperature for Bose-Einstein condensation. These results show
that the properties of the impurity vary strongly with temperature. In
particular, the energy exhibits a non-monotonic behavior close to , and
the damping rises sharply close to . We argue that this behaviour is
generic for impurities immersed in an environment undergoing a phase transition
that breaks a continuous symmetry. Finally, we discuss how these effects can be
detected experimentally.Comment: 10 pages and 6 figure
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