3,786 research outputs found
Dynamo Processes in the T Tauri star V410 Tau
We present new brightness and magnetic images of the weak-line T Tauri star
V410 Tau, made using data from the NARVAL spectropolarimeter at Telescope
Bernard Lyot (TBL). The brightness image shows a large polar spot and
significant spot coverage at lower latitudes. The magnetic maps show a field
that is predominantly dipolar and non-axisymmetric with a strong azimuthal
component. The field is 50% poloidal and 50% toroidal, and there is very little
differential rotation apparent from the magnetic images.
A photometric monitoring campaign on this star has previously revealed V-band
variability of up to 0.6 magnitudes but in 2009 the lightcurve is much flatter.
The Doppler image presented here is consistent with this low variability.
Calculating the flux predicted by the mapped spot distribution gives an
peak-to-peak variability of 0.04 magnitudes. The reduction in the amplitude of
the lightcurve, compared with previous observations, appears to be related to a
change in the distribution of the spots, rather than the number or area.
This paper is the first from a Zeeman-Doppler imaging campaign being carried
out on V410 Tau between 2009-2012 at TBL. During this time it is expected that
the lightcurve will return to a high amplitude state, allowing us to ascertain
whether the photometric changes are accompanied by a change in the magnetic
field topology.Comment: 12 pages, 11 figures, accepted by MNRA
On the Response Function Technique for Calculating the Random-Phase Approximation Correlation Energy
We develop a scheme to exactly evaluate the correlation energy in the
random-phase approximation, based on linear response theory. It is demonstrated
that our formula is completely equivalent to a contour integral representation
recently proposed by Donau et al. being numerically more efficient for
realistic calculations. Numerical examples are presented for pairing
correlations in rapidly rotating nuclei.Comment: 4 pages, 4 figure
Solar Wind and its Evolution
By using our previous results of magnetohydrodynamical simulations for the
solar wind from open flux tubes, I discuss how the solar wind in the past is
different from the current solar wind. The simulations are performed in fixed
one-dimensional super-radially open magnetic flux tubes by inputing various
types of fluctuations from the photosphere, which automatically determines
solar wind properties in a forward manner. The three important parameters which
determine physical properties of the solar wind are surface fluctuation,
magnetic field strengths, and the configuration of magnetic flux tubes.
Adjusting these parameters to the sun at earlier times in a qualitative sense,
I infer that the quasi-steady-state component of the solar wind in the past was
denser and slightly slower if the effect of the magneto-centrifugal force is
not significant. I also discuss effects of magneto-centrifugal force and roles
of coronal mass ejections.Comment: 6 pages, 1 figure, Earth, Planets, & Space in press (based on 5th
Alfven Conference) correction of discussion on a related pape
The large-scale magnetic field and poleward mass accretion of the classical T Tauri star TW Hya
We report here results of spectropolarimetric observations of the ~8Myr
classical TTauri star (cTTS) TWHya carried out with ESPaDOnS at the
Canada-France-Hawaii Telescope (CFHT) in the framework of the `Magnetic
Protostars and Planets' (MaPP) programme, and obtained at 2 different epochs
(2008 March and 2010 March). Obvious Zeeman signatures are detected at all
times, both in photospheric lines and in accretion-powered emission lines.
Significant intrinsic variability and moderate rotational modulation is
observed in both photospheric and accretion proxies.
Using tomographic imaging, we reconstruct maps of the large-scale field, of
the photospheric brightness and of the accretion-powered emission at the
surface of TWHya at both epochs. We find that the magnetic topology is mostly
poloidal and axisymmetric with respect to the rotation axis of the star, and
that the octupolar component of the large-scale field (2.5-2.8kG at the pole)
largely dominates the dipolar component. This large-scale field topology is
characteristic of partly-convective stars, supporting the conclusion (from
evolutionary models) that TWHya already hosts a radiative core. We also show
that TWHya features a high-latitude photospheric cool spot overlapping with the
main magnetic pole (and producing the observed radial velocity fluctuations);
this is also where accretion concentrates most of the time, although accretion
at lower latitudes is found to occur episodically.
We propose that the relatively rapid rotation of TWHya (with respect to
AATau-like cTTSs) directly reflects the weakness of the large-scale dipole, no
longer capable of magnetically disrupting the accretion disc up to the
corotation radius (at which the Keplerian period equals the stellar rotation
period). We therefore conclude that TWHya is in a phase of rapid spin-up as its
large-scale dipole field progressively vanishes.Comment: MNRAS in pres
The first magnetic maps of a pre-main sequence binary star system - HD 155555
We present the first maps of the surface magnetic fields of a pre-main
sequence binary system. Spectropolarimetric observations of the young, 18 Myr,
HD 155555 (V824 Ara, G5IV + K0IV) system were obtained at the Anglo-Australian
Telescope in 2004 and 2007. Both datasets are analysed using a new binary
Zeeman Doppler imaging (ZDI) code. This allows us to simultaneously model the
contribution of each component to the observed circularly polarised spectra.
Stellar brightness maps are also produced for HD 155555 and compared to
previous Doppler images. Our radial magnetic maps reveal a complex surface
magnetic topology with mixed polarities at all latitudes. We find rings of
azimuthal field on both stars, most of which are found to be non-axisymmetric
with the stellar rotational axis. We also examine the field strength and the
relative fraction of magnetic energy stored in the radial and azimuthal field
components at both epochs. A marked weakening of the field strength of the
secondary star is observed between the 2004 and 2007 epochs. This is
accompanied by an apparent shift in the location of magnetic energy from the
azimuthal to radial field. We suggest that this could be indicative of a
magnetic activity cycle. We use the radial magnetic maps to extrapolate the
coronal field (by assuming a potential field) for each star individually - at
present ignoring any possible interaction. The secondary star is found to
exhibit an extreme tilt (~75 deg) of its large scale magnetic field to that of
its rotation axis for both epochs. The field complexity that is apparent in the
surface maps persists out to a significant fraction of the binary separation.
Any interaction between the fields of the two stars is therefore likely to be
complex also. Modelling this would require a full binary field extrapolation.Comment: 17 pages, 12 figures, accepted for publication in MNRA
Temperature dependence of spatially heterogeneous dynamics in a model of viscous silica
Molecular dynamics simulations are performed to study spatially heterogeneous
dynamics in a model of viscous silica above and below the critical temperature
of the mode coupling theory, . Specifically, we follow the evolution
of the dynamic heterogeneity as the temperature dependence of the transport
coefficients shows a crossover from non-Arrhenius to Arrhenius behavior when
the melt is cooled. It is demonstrated that, on intermediate time scales, a
small fraction of oxygen and silicon atoms are more mobile than expected from a
Gaussian approximation. These highly mobile particles form transient clusters
larger than that resulting from random statistics, indicating that dynamics are
spatially heterogeneous. An analysis of the clusters reveals that the mean
cluster size is maximum at times intermediate between ballistic and diffusive
motion, and the maximum size increases with decreasing temperature. In
particular, the growth of the clusters continues when the transport
coefficients follow an Arrhenius law. These findings imply that the structural
relaxation in silica cannot be understood as a statistical bond breaking
process. Though the mean cluster sizes for silica are at the lower end of the
spectrum of values reported in the literature, we find that spatially
heterogeneous dynamics in strong and fragile glass formers are similar on a
qualitative level. However, different from results for fragile liquids, we show
that correlated particle motion along quasi one-dimensional, string-like paths
is of little importance for the structural relaxation in this model of silica,
suggesting that string-like motion is suppressed by the presence of covalent
bonds.Comment: 13 pages, 11 figure
Complex magnetic topology and strong differential rotation on the low-mass T Tauri star V2247 Oph
From observations collected with the ESPaDOnS spectropolarimeter at the
Canada-France-Hawaii Telescope, we report the detection of Zeeman signatures on
the low-mass classical TTauri star (cTTS) V2247Oph. Profile distortions and
circular polarisation signatures detected in photospheric lines can be
interpreted as caused by cool spots and magnetic regions at the surface of the
star. The large-scale field is of moderate strength and highly complex;
moreover, both the spot distribution and the magnetic field show significant
variability on a timescale of only one week, as a likely result of strong
differential rotation. Both properties make V2247Oph very different from the
(more massive) prototypical cTTS BPTau; we speculate that this difference
reflects the lower mass of V2247Oph.
During our observations, V2247Oph was in a low-accretion state, with emission
lines showing only weak levels of circular polarisation; we nevertheless find
that excess emission apparently concentrates in a mid-latitude region of strong
radial field, suggesting that it is the footpoint of an accretion funnel.
The weaker and more complex field that we report on V2247Oph may share
similarities with those of very-low-mass late-M dwarfs and potentially explain
why low-mass cTTSs rotate on average faster than intermediate mass ones. These
surprising results need confirmation from new independent data sets on V2247Oph
and other similar low-mass cTTSs.Comment: MNRAS (in press) - 12 pages, 9 figure
Effects of the Bacterial Extract OM-85 on Phagocyte Functions and the Stress Response
The effects of the bacterial extract OM-85 on the respiratory burst,
intracellular calcium and the stress response have been investigated
in human peripheral blood monocytes from normal donors. Activation
of the respiratory burst during bacterial phagocytosis has been
previously associated with heat shock/stress proteins synthesis.
Whereas OM-85 stimulated superoxide production and increased
Ca2+ mobilization, it fared to induce synthesis of
classical HSPs. The lack of stress protein induction was observed
even in the presence of iron which potentiates both oxidative injury
and stress protein induction during bacterial phagocytosis. However
OM-85 induced a 75–78 kDa protein, which is likely to be a
glucose regulated protein (GRP78), and enhanced intracellular
expression of interleukin-lβ precursor
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