994 research outputs found
Investigating variation of latitudinal stellar spot rotation and its relation to the real stellar surface rotation
In this work the latitude dependent stellar spot rotation is investigated
based on dynamo models. The maps of the magnetic pressure at the surface from
the dynamo calculations are treated similarly to the temperature maps obtained
using Doppler imaging techniques. A series of snapshots from the dynamo models
are cross-correlated to obtain the shift of the magnetic patterns at each
latitude and time point. The surface differential rotation patterns obtained
from the snapshots of the dynamo calculations show in all studied cases
variability over the activity cycle. In the models using only the large scale
dynamo field the measured rotation patterns are only at times similar to the
input rotation law. This is due to the spot motion being mainly determined by
the geometric properties of the large scale dynamo field. In the models with
additional small scale magnetic field the surface differential rotation
measured from the model follows well the input rotation law. The results imply
that the stellar spots caused by the large scale dynamo field are not
necessarily tracing the stellar differential rotation, whereas the spots formed
from small scale fields trace well the surface flow patterns. It can be
questioned whether the large spots observed in active stars could be caused by
small scale fields. Therefore, it is not clear that the true stellar surface
rotation can be recovered using measurements of large starspots, which are
currently the only ones that can be observed.Comment: 8 pages, 10 figures, accepted for publication in A&
Delayed Babcock-Leighton dynamos in the diffusion-dominated regime
Context. Solar dynamo models of Babcock-Leighton type typically assume the
rise of magnetic flux tubes to be instantaneous. Solutions with
high-magnetic-diffusivity have too short periods and a wrong migration of their
active belts. Only the low-diffusivity regime with advective meridional flows
is usually considered. Aims. In the present paper we discuss these assumptions
and applied a time delay in the source term of the azimuthally averaged
induction equation. This delay is set to be the rise time of magnetic flux
tubes which supposedly form at the tachocline. We study the effect of the
delay, which adds to the spacial non-locality a non-linear temporal one, in the
advective but particularly in the diffusive regime. Methods. Fournier et al.
(2017) obtained the rise time according to stellar parameters such as rotation,
and the magnetic field strength at the bottom of the convection zone. These
results allowed us to constrain the delay in the mean-field model used in a
parameter study. Results. We identify an unknown family of solutions. These
solutions self-quench, and exhibit longer periods than their non-delayed
counterparts. Additionally, we demonstrate that the non-linear delay is
responsible for the recover of the equatorward migration of the active belts at
high turbulent diffusivities. Conclusions. By introducing a non-linear temporal
non-locality (the delay) in a Babcock-Leighton dynamo model, we could obtain
solutions quantitatively comparable to the solar butterfly diagram in the
diffusion-dominated regime.Comment: 11 pages, 10 Figure
Seeking large-scale magnetic fields in a pure-disk dwarf galaxy NGC 2976
It is still unknown how magnetic field-generation mechanisms could operate in
low-mass dwarf galaxies. Here, we present a detailed study of a nearby
pure-disk dwarf galaxy NGC 2976. Unlike previously observed dwarf objects, this
galaxy possesses a clearly defined disk. For the purpose of our studies, we
performed deep multi-frequency polarimetric observations of NGC 2976 with the
VLA and Effelsberg radio telescopes. Additionally, we supplement them with
re-imaged data from the WSRT-SINGS survey. The magnetic field morphology
discovered in NGC 2976 consists of a southern polarized ridge. This structure
does not seem to be due to just a pure large-scale dynamo process (possibly
cosmic-ray driven) at work in this object, as indicated by the RM data and
dynamo number calculations. Instead, the field of NGC 2976 is modified by past
gravitational interactions and possibly also by ram pressure inside the M 81
galaxy group environment. The estimates of total (7 muG) and ordered (3 muG)
magnetic field strengths, as well as degree of field order (0.46), which is
similar to those observed in spirals, suggest that tidally generated magnetized
gas flows can further enhance dynamo action in the object. NGC 2976 is
apparently a good candidate for the efficient magnetization of its
neighbourhood. It is able to provide an ordered (perhaps also regular) magnetic
field into the intergalactic space up to a distance of about 5 kpc. Tidal
interactions (and possibly also ram pressure) can lead to the formation of
unusual magnetic field morphologies (like polarized ridges) in galaxies out of
the star-forming disks, which do not follow any observed component of the
interstellar medium (ISM), as observed in NGC 2976. These galaxies are able to
provide ordered magnetic fields far out of their main disks.Comment: 16 page
Structural fluctuations and quantum transport through DNA molecular wires: a combined molecular dynamics and model Hamiltonian approach
Charge transport through a short DNA oligomer (Dickerson dodecamer) in
presence of structural fluctuations is investigated using a hybrid
computational methodology based on a combination of quantum mechanical
electronic structure calculations and classical molecular dynamics simulations
with a model Hamiltonian approach. Based on a fragment orbital description, the
DNA electronic structure can be coarse-grained in a very efficient way. The
influence of dynamical fluctuations arising either from the solvent
fluctuations or from base-pair vibrational modes can be taken into account in a
straightforward way through time series of the effective DNA electronic
parameters, evaluated at snapshots along the MD trajectory. We show that charge
transport can be promoted through the coupling to solvent fluctuations, which
gate the onsite energies along the DNA wire
Parity properties of an advection-dominated solar \alpha^2\Om-dynamo
We have developed a high-precision code which solves the kinematic dynamo
problem both for given rotation law and meridional flow in the case of a low
eddy diffusivity of the order of cm/s known from the sunspot
decay. All our models work with an \alf-effect which is positive (negative) in
the northern (southern) hemisphere. It is concentrated in radial layers located
either at the top or at the bottom of the convection zone. We have also
considered an \alf-effect uniformly distributed in all the convection zone. In
the present paper the main attention is focused on i) the parity of the
solution, ii) the form of the butterfly diagram and iii) the phase relation of
the resulting field components. If the helioseismologically derived internal
solar rotation law is considered, a model without meridional flow of high
magnetic Reynolds number (corresponding to low eddy diffusivity) fails in all
the three issues in comparison with the observations. However, a meridional
flow with equatorial drift at the bottom of the convection zone of few meters
by second can indeed enforce the equatorward migration of the toroidal magnetic
field belts similar to the observed butterfly diagram but, the solution has
only a dipolar parity if the (positive) \alf-effect is located at the base of
the convection zone rather than at the top. We can, therefore, confirm the main
results of a similar study by Dikpati & Gilman (2001).Comment: 9 pages, 16 figures, to appear on Astronomy and Astrophysic
Spin-1/2 Heisenberg-Antiferromagnet on the Kagome Lattice: High Temperature Expansion and Exact Diagonalisation Studies
For the spin- Heisenberg antiferromagnet on the Kagom\'e lattice
we calculate the high temperature series for the specific heat and the
structure factor. A comparison of the series with exact diagonalisation studies
shows that the specific heat has further structure at lower temperature in
addition to a high temperature peak at . At the
structure factor agrees quite well with results for the ground state of a
finite cluster with 36 sites. At this temperature the structure factor is less
than two times its value and depends only weakly on the wavevector
, indicating the absence of magnetic order and a correlation length of
less than one lattice spacing. The uniform susceptibility has a maximum at
and vanishes exponentially for lower temperatures.Comment: 15 pages + 5 figures, revtex, 26.04.9
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