1,652 research outputs found
Differential rotation decay in the radiative envelopes of CP stars
Stars of spectral classes A and late B are almost entirely radiative. CP
stars are a slowly rotating subgroup of these stars. It is possible that they
possessed long-lived accretion disks in their T Tauri phase. Magnetic coupling
of disk and star leads to rotational braking at the surface of the star.
Microscopic viscosities are extremely small and will not be able to reduce the
rotation rate of the core of the star. We investigate the question whether
magneto-rotational instability can provide turbulent angular momentum
transport. We illuminate the question whether or not differential rotation is
present in CP stars. Numerical MHD simulations of thick stellar shells are
performed. An initial differential rotation law is subject to the influence of
a magnetic field. The configuration gives indeed rise to magneto-rotational
instability. The emerging flows and magnetic fields transport efficiently
angular momentum outwards. Weak dependence on the magnetic Prandtl number
(~0.01 in stars) is found from the simulations. Since the estimated time-scale
of decay of differential rotation is 10^7-10^8 yr and comparable to the
life-time of A stars, we find the braking of the core to be an ongoing process
in many CP stars. The evolution of the surface rotation of CP stars with age
will be an observational challenge and of much value for verifying the
simulations.Comment: 8 pages, 11 figures; submitted to Astron. & Astrophy
3D simulations of rising magnetic flux tubes in a compressible rotating interior: The effect of magnetic tension
Context: Long-term variability in solar cycles represents a challenging
constraint for theoretical models. Mean-field Babcock-Leighton dynamos that
consider non-instantaneous rising flux tubes have been shown to exhibit
long-term variability in their magnetic cycle. However a relation that
parameterizes the rise-time of non-axisymmetric magnetic flux tubes in terms of
stellar parameters is still missing. Aims: We aim to find a general
parameterization of the rise-time of magnetic flux tubes for solar-like stars.
Methods: By considering the influence of magnetic tension on the rise of
non-axisymmetric flux tubes, we predict the existence of a control parameter
referred as . This parameter is a measure of the
balance between rotational effects and magnetic effects (buoyancy and tension)
acting on the magnetic flux tube. We carry out two series of numerical
experiments (one for axisymmetric rise and one for non-axisymmetric rise) and
demonstrate that indeed controls the rise-time
of magnetic flux tubes. Results: We find that the rise-time follows a power law
of with an exponent that depends on the
azimuthal wavenumber of the magnetic flux loop. Conclusions: Compressibility
does not impact the rise of magnetic flux tubes, while non-axisymmetry does. In
the case of non-axisymmetric rise, the tension force modifies the force balance
acting on the magnetic flux tube. We identified the three independent
parameters required to predict the rise-time of magnetic flux tubes, that is,
the stellar rotation rate, the magnetic flux density of the flux tube, and its
azimuthal wavenumber. We combined these into one single relation that is valid
for any solar-like star. We suggest using this generalized relation to
constrain the rise-time of magnetic flux tubes in Babcock-Leighton dynamo
models.Comment: 18 pages, 15 figures, 6 tabula
Inconsistency of the Wolf sunspot number series around 1848
Aims. Sunspot number is a benchmark series in many studies, but may still
contain inhomogeneities and inconsistencies. In particular, an essential
discrepancy exists between the two main sunspot number series, Wolf (WSN) and
group (GSN) sunspot numbers, before 1848. The source of this discrepancy has so
far remained unresolved. However, the recently digitized series of solar
observations in 1825-1867 by Samuel Heinrich Schwabe, who was the primary
observer of the WSN before 1848, makes such an assessment possible. Methods. We
construct sunspot series, similar to WSN and GSN, but using only Schwabe's
data. These series, called WSN-S and GSN-S, respectively, were compared with
the original WSN and GSN series for the period 1835-1867 to look for possible
inhomogeneities. Results. We show that: (1) The GSN series is homogeneous and
consistent with the Schwabe data throughout the entire studied period; (2) The
WSN series decreases by roughly ~20% around 1848 caused by the change of the
primary observer from Schwabe to Wolf and an inappropriate individual
correction factor used for Schwabe in the WSN; (3) This implies a major
inhomogeneity in the WSN, which needs to be corrected by reducing its values by
20% before 1848; (4) The corrected WSN series is in good agreement with the GSN
series. This study supports the earlier conclusions that the GSN series is more
consistent and homogeneous in the earlier part than the WSN series.Comment: Published as: Leussu, R., I.G. Usoskin, R. Arlt and K. Mursula,
Inconsistency of the Wolf sunspot number series around 1848, Astron.
Astrophys., 559, A28, 201
Importance of second-order piezoelectric effects in zincblende semiconductors
We show that the piezoelectric effect that describes the emergence of an
electric field in response to a crystal deformation in III-V semiconductors
such as GaAs and InAs has strong contributions from second-order effects that
have been neglected so far. We calculate the second-order piezoelectric tensors
using density functional theory and obtain the piezoelectric field for
[111]-oriented InGaAs quantum wells of realistic dimensions and
concentration . We find that the linear and the quadratic piezoelectric
coefficients have the opposite effect on the field, and for large strains the
quadratic terms even dominate. Thus, the piezoelectric field turns out to be a
rare example of a physical quantity for which the first- and second-order
contributions are of comparable magnitude.Comment: 4 pages, 3 figures, Submitted to Phys. Rev. Let
Amplification and stability of magnetic fields and dynamo effect in young A stars
This study is concerned with the early evolution of magnetic fields and
differential rotation of intermediate-mass stars which may evolve into Ap
stars. We report on simulations of the interplay of differential rotation and
magnetic fields, the stability limits and non-linear evolution of such
configurations, and the prospects of dynamo action from the unstable cases. The
axisymmetric problem delivers a balance between field amplification and
back-reaction of the magnetic field on the differential rotation. The
non-axisymmetric case involves also the Tayler instability of the amplified
toroidal fields. We consider limits for field amplification and apply these to
young A stars. Apart from its application to Ap stars, the instability is
scrutinized for the fundamental possibility of a dynamo. We are not looking for
a dynamo as an explanation for the Ap star phenomenon. The kinetic helicity is
concentrated near the tangent cylinder of the inner sphere of the computational
domain and is negative in the northern hemisphere. This appears to be a
ubiquitous effect not special to the Tayler instability. The latter is actually
connected with a positive current helicity in the bulk of the spherical shell
giving rise to a small, but non-vanishing alpha-effect in non-linear evolution
of the instability.Comment: 13 pages, 14 figures, accepted by Mon. Not. R. Astro
Three-dimensional stability of the solar tachocline
The three-dimensional, hydrodynamic stability of the solar tachocline is
investigated based on a rotation profile as a function of both latitude and
radius. By varying the amplitude of the latitudinal differential rotation, we
find linear stability limits at various Reynolds numbers by numerical
computations. We repeated the computations with different latitudinal and
radial dependences of the angular velocity. The stability limits are all higher
than those previously found from two-dimensional approximations and higher than
the shear expected in the Sun. It is concluded that any part of the tachocline
which is radiative is hydrodynamically stable against small perturbations.Comment: 6 pages, 8 figures, accepted by Astron. & Astrophy
Hydrodynamic stability in accretion disks under the combined influence of shear and density stratification
The hydrodynamic stability of accretion disks is considered. The particular
question is whether the combined action of a (stable) vertical density
stratification and a (stable) radial differential rotation gives rise to a new
instability for nonaxisymmetric modes of disturbances. The existence of such an
instability is not suggested by the well-known Solberg-Hoiland criterion. It is
also not suggested by a local analysis for disturbances in general
stratifications of entropy and angular momentum which is presented in our
Section 2 confirming the results of the Solberg-Hoiland criterion also for
nonaxisymmetric modes within the frame of ideal hydrodynamics but only in the
frame of a short-wave approximation for small m. As a necessary condition for
stability we find that only conservative external forces are allowed to
influence the stable disk. As magnetic forces are never conservative, linear
disk instabilities should only exist in the magnetohydrodynamical regime which
indeed contains the magnetorotational instability as a much-promising
candidate. To overcome some of the used approximations in a numerical
approach,the equations of the compressible adiabatic hydrodynamics are
integrated imposing initial nonaxisymmetric velocity perturbations with m=1 to
m=200.
Only solutions with decaying kinetic energy are found. The system always
settles in a vertical equilibrium stratification according to pressure balance
with the gravitational potential of the central object. keywords: accretion
disks -- hydrodynamic instabilities -- turbulenceComment: 6 pages, 4 figures, 1 table, Astronomy and Astrophysics (subm.
The exceptional Herbig Ae star HD101412: The first detection of resolved magnetically split lines and the presence of chemical spots in a Herbig star
We obtained high-resolution, high signal-to-noise UVES and a few lower
quality HARPS spectra revealing the presence of resolved magnetically split
lines. HD101412 is the first Herbig Ae star for which the rotational Doppler
effect was found to be small in comparison to the magnetic splitting. The
measured mean magnetic field modulus varies from 2.5 to 3.5kG, while the mean
quadratic field was found to vary in the range of 3.5 to 4.8kG. To determine
the period of variations, we used radial velocity, equivalent width, line
width, and line asymmetry measurements of variable spectral lines of several
elements, as well as magnetic field measurements. The most pronounced
variability was detected for spectral lines of He I and the iron peak elements,
whereas the spectral lines of CNO elements are only slightly variable. From
spectral variations and magnetic field measurements we derived a potential
rotation period P_rot=13.86d, which has to be proven in future studies with a
larger number of observations. It is the first time that the presence of
element spots is detected on the surface of a Herbig Ae/Be star. Our previous
study of Herbig Ae stars revealed a trend towards stronger magnetic fields for
younger Herbig Ae stars, confirmed by statistical tests. This is in contrast to
a few other (non-statistical) studies claiming that magnetic Herbig Ae stars
are progenitors of the magnetic Ap stars. New developments in MHD theory show
that the measured magnetic field strengths are compatible with a current-driven
instability of toroidal fields generated by differential rotation in the
stellar interior. This explanation for magnetic intermediate-mass stars could
be an alternative to a frozen-in fossil field.Comment: 7 pages, 6 figures, 1 table, to appear in Astronomische Nachrichte
- …