123 research outputs found
Seismic diagnostics of red giants: first comparison with stellar models
The clear detection with CoRoT and KEPLER of radial and non-radial solar-like
oscillations in many red giants paves the way to seismic inferences on the
structure of such stars. We present an overview of the properties of the
adiabatic frequencies and frequency separations of radial and non-radial
oscillation modes for an extended grid of models. We highlight how their
detection allows a deeper insight into the internal structure and evolutionary
state of red giants. In particular, we find that the properties of dipole modes
constitute a promising seismic diagnostic tool of the evolutionary state of
red-giant stars. We compare our theoretical predictions with the first 34 days
of KEPLER data and predict the frequency diagram expected for red giants in the
COROT exofield in the galactic center direction.Comment: 14 pages, 5 figures, accepted in ApJ
The kink-type instability of toroidal stellar magnetic fields with thermal diffusion
The stability of toroidal magnetic fields in rotating radiative stellar zones
is studied for realistic values of both the Prandtl numbers. The two considered
models for the magnetic geometry represent fields with odd and even symmetry
with respect to the equator. In the linear theory in Boussinesq approximation
the resulting complex eigenfrequency (including growth rate and drift rate) are
calculated for a given radial wavenumber of a nonaxisymmetric perturbation with
m=1. The ratio of the Alfven frequency, \Omega_A, to the rate of the basic
rotation, \Omega, controls the eigenfrequency of the solution. For strong
fields with \Omega_A > \Omega the solutions do not feel the thermal diffusion.
The growth rate runs with \Omega_A and the drift rate is close to -\Omega so
that the magnetic pattern will rest in the laboratory system. For weaker fields
with \Omega_A < \Omega the growth rate strongly depends on the thermal
conductivity. For fields with dipolar parity and for typical values of the heat
conductivity the resulting very small growth rates are almost identical with
those for vanishing gravity. For fields with dipolar symmetry the differential
rotation of any stellar radiative zone (like the solar tachocline) is shown as
basically stabilizing the instability independent of the sign of the shear.
Finally, the current-driven kink-type instability of a toroidal background
field is proposed as a model for the magnetism of Ap stars. The recent
observation of a lower magnetic field treshold of about 300 Gauss for Ap stars
is understood as corresponding to the minimum magnetic field producing the
instability.Comment: 11 pages, 7 figures, acc. for publicatio
Masses of subgiant stars from asteroseismology using the coupling strengths of mixed modes
Since few decades, asteroseismology, the study of stellar oscillations,
enables us to probe the interiors of stars with great precision. It allows
stringent tests of stellar models and can provide accurate radii, masses and
ages for individual stars. Of particular interest are the mixed modes that
occur in subgiant solar-like stars since they can place very strong constraints
on stellar ages. Here we measure the characteristics of the mixed modes,
particularly the coupling strength, using a grid of stellar models for stars
with masses between 0.9 and 1.5 M_{\odot}. We show that the coupling strength
of the mixed modes is predominantly a function of stellar mass and
appears to be independent of metallicity. This should allow an accurate mass
evaluation, further increasing the usefulness of mixed modes in subgiants as
asteroseismic tools.Comment: 7 pages, 4 figures, 1 table, Accepted (ApJL
The Tayler instability of toroidal magnetic fields in a columnar gallium experiment
The nonaxisymmetric Tayler instability of toroidal magnetic fields due to
axial electric currents is studied for conducting incompressible fluids between
two coaxial cylinders without endplates. The inner cylinder is considered as so
thin that even the limit of R_in \to 0 can be computed. The magnetic Prandtl
number is varied over many orders of magnitudes but the azimuthal mode number
of the perturbations is fixed to m=1. In the linear approximation the critical
magnetic field amplitudes and the growth rates of the instability are
determined for both resting and rotating cylinders. Without rotation the
critical Hartmann numbers do {\em not} depend on the magnetic Prandtl number
but this is not true for the growth rates. For given product of viscosity and
magnetic diffusivity the growth rates for small and large magnetic Prandtl
number are much smaller than those for Pm=1. For gallium under the influence of
a magnetic field at the outer cylinder of 1 kG the resulting growth time is 5
s. The minimum electric current through a container of 10 cm diameter to excite
the kink-type instability is 3.20 kA. For a rotating container both the
critical magnetic field and the related growth times are larger than for the
resting column.Comment: 7 pages, 9 figures, submitted to Astron. Nach
Tayler instability of toroidal magnetic fields in MHD Taylor-Couette flows
The nonaxisymmetric 'kink-type' Tayler instability (TI) of toroidal magnetic
fields is studied for conducting incompressible fluids of uniform density
between two infinitely long cylinders rotating around the same axis. It is
shown that for resting cylinders the critical Hartmann number for the unstable
modes does not depend on Pm. By rigid rotation the instability is suppressed
where the critical ratio of the rotation velocity and the Alfven velocity of
the field (only) slightly depends on the magnetic Prandtl number Pm. For Pm=1
the rotational quenching of TI takes its maximum. Rotation laws with negative
shear (i.e. d\Omega/dR<0) strongly destabilize the toroidal field if the
rotation is not too fast. For sufficiently high Reynolds numbers of rotation
the suppression of the nonaxisymmetric magnetic instability always dominates.
The angular momentum transport of the instability is anticorrelated with the
shear so that an eddy viscosity can be defined which proves to be positive. For
negative shear the Maxwell stress of the perturbations remarkably contributes
to the angular momentum transport. We have also shown the possibility of
laboratory TI experiments with a wide-gap container filled with fluid metals
like sodium or gallium. Even the effect of the rotational stabilization can be
reproduced in the laboratory with electric currents of only a few kAmp.Comment: 9 pages, 11 figures, sub
Magnetic fields of Ap stars as a result of an instability
Ap star magnetism is often attributed to fossil magnetic fields which have
not changed much since the pre-main-sequence epoch of the stars. Stable
magnetic field configurations are known which could persist probably for the
entire main-sequence life of the star, but they may not show the complexity and
diversity exhibited by the Ap stars observed. We suggest that the Ap star
magnetism is not a result of stable configurations, but is the result of an
instability based on strong toroidal magnetic fields buried in the stars. The
highly nonaxisymmetric remainders of the instability are reminiscent of the
diversity of fields seen on Ap stars. The strengths of these remnant magnetic
fields is actually between a few per cent up to considerable fractions of the
internal toroidal field; this means field strengths of the order of kGauss
being compatible with what is observed. The magnetic fields emerge at the
surface rather quickly; rough estimates deliver time-scales of the order of a
few years. Since rotation stabilizes the instability, normal A stars may still
host considerable, invisible toroidal magnetic fields.Comment: 7 pages, 8 figures, submitted to Astron. Nachr., color figures at
http://www.aip.de/People/rarlt/a
Stellar ages from asteroseismology
Asteroseismology provides powerful means to probe stellar interiors. The
oscillations frequencies are closely related to stellar interior properties via
the density and sound speed profiles. Since these are tightly linked with the
mass and evolutionary state, we can expect to determine the age and mass of a
star from the comparison of its oscillation spectrum with predictions of
stellar models. Such a comparison suffers both from the problems we face when
modeling a particular star (as the uncertainties on global parameters and
chemical composition) and from our misunderstanding of processes at work in
stellar interiors (as the transport processes that may lead to core mixing and
affect the model ages). For stars where observations have provided precise and
numerous oscillation frequencies together with accurate global parameters and
additional information (as the radius or the mass if the star is in a binary
system, the interferometric radius or the mean density if the star is an
exoplanet host), we can also expect to better constrain the physical
description of the stellar structure and to get a more reliable age estimation.
After a survey of stellar pulsations, we present some seismic diagnostics that
can be used to infer the age of a star as well as their limitations. We then
illustrate the ability of asteroseismology to scrutinize stellar interiors on
the basis of a few exemples. In the years to come, extended very precise
asteroseismic observations are expected, in photometry or in spectroscopy, from
ground-based (HARPS, CORALIE, ELODIE, UVES, UCLES, SIAMOIS, SONG) or spatial
devices (MOST, CoRoT, WIRE, Kepler, PLATO). This will considerably enlarge the
sample of stars eligible to asteroseismic age determination and should allow to
estimate the age of individual stars with a 10-20% accuracy.Comment: 10 pages, 15 figures, Proc. of the IAU Symp. 258 "The Ages of Stars",
Baltimore USA 13-17 Oct 2008, eds D. Soderblom et al., CUP in pres
Elementary model of internal electromagnetic pinch-type instability
We analyse numerically a pinch-type instability in a semi-infinite planar
layer of inviscid conducting liquid bounded by solid walls and carrying a
uniform electric current. Our model is as simple as possible but still captures
the salient features of the instability which otherwise may be obscured by the
technical details of more comprehensive numerical models and laboratory
experiments. Firstly, we show the instability in liquid metals, which are
relatively poor conductors, differs significantly from the
astrophysically-relevant Tayler instability. In liquid metals, the instability
develops on the magnetic response time scale, which depends on the conductivity
and is much longer than the Alfv\'en time scale, on which the Tayler
instability develops in well conducting fluids. Secondly, we show that this
instability is an edge effect caused by the curvature of the magnetic field,
and its growth rate is determined by the linear current density and independent
of the system size. Our results suggest that this instability may affect future
liquid metal batteries when their size reaches a few meters.Comment: 14 pages, 5 figures (to appear in J Fluid Mech
Nonaxisymmetric linear instability of cylindrical magnetohydrodynamic Taylor-Couette flow
We consider the nonaxisymmetric modes of instability present in Taylor-Couette flow under the application of helical magnetic fields, mainly for magnetic Prandtl numbers close to the inductionless limit, and conduct a full examination of marginal stability in the resulting parameter space. We allow for the azimuthal magnetic field to be generated by a combination of currents in the inner cylinder and fluid itself and introduce a parameter governing the relation between the strength of these currents. A set of governing eigenvalue equations for the nonaxisymmetric modes of instability are derived and solved by spectral collocation with Chebyshev polynomials over the relevant parameter space, with the resulting instabilities examined in detail. We find that by altering the azimuthal magnetic field profiles the azimuthal magnetorotational instability, nonaxisymmetric helical magnetorotational instability, and Tayler instability yield interesting dynamics, such as different preferred mode types and modes with azimuthal wave number m>1 . Finally, a comparison is given to the recent WKB analysis performed by Kirillov et al. [Kirillov, Stefani, and Fukumoto, J. Fluid Mech. 760, 591 (2014)] and its validity in the linear regime
Asteroseismic Investigation of Known Planet Hosts in the Kepler Field
In addition to its great potential for characterizing extra-solar planetary
systems the Kepler mission is providing unique data on stellar oscillations. A
key aspect of Kepler asteroseismology is the application to solar-like
oscillations of main-sequence stars. As an example we here consider an initial
analysis of data for three stars in the Kepler field for which planetary
transits were known from ground-based observations. For one of these, HAT-P-7,
we obtain a detailed frequency spectrum and hence strong constraints on the
stellar properties. The remaining two stars show definite evidence for
solar-like oscillations, yielding a preliminary estimate of their mean
densities.Comment: Astrophysical Journal Letters, in the pres
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