5,364 research outputs found
A polarity reversal in the large-scale magnetic field of the rapidly rotating Sun HD 190771
Aims. We investigate the long-term evolution of the large-scale photospheric
magnetic field geometry of the solar-type star HD 190771. With fundamental
parameters very close to those of the Sun except for a shorter rotation period
of 8.8 d, HD 190771 provides us with a first insight into the specific impact
of the rotation rate in the dynamo generation of magnetic fields in 1
stars.
Methods. We use circularly polarized, high-resolution spectra obtained with
the NARVAL spectropolarimeter (Observatoire du Pic du Midi, France) and compute
cross-correlation line profiles with high signal-to-noise ratio to detect
polarized Zeeman signatures. From three phase-resolved data sets collected
during the summers of 2007, 2008, and 2009, we model the large-scale
photospheric magnetic field of the star by means of Zeeman-Doppler imaging and
follow its temporal evolution.
Results. The comparison of the magnetic maps shows that a polarity reversal
of the axisymmetric component of the large-scale magnetic field occurred
between 2007 and 2008, this evolution being observed in both the poloidal and
toroidal magnetic components. Between 2008 and 2009, another type of global
evolution occured, characterized by a sharp decrease of the fraction of
magnetic energy stored in the toroidal component. These changes were not
accompanied by significant evolution in the total photospheric magnetic energy.
Using our spectra to perform radial velocity measurements, we also detect a
very low-mass stellar companion to HD 190771.Comment: Accepted by Astronomy and Astrophysics (Letter to the Editor
Magnetic fields and differential rotation on the pre-main sequence I: The early-G star HD 141943 - brightness and magnetic topologies
Spectroscopic and spectropolarimetric observations of the pre-main sequence
early-G star HD 141943 were obtained at four observing epochs (in 2006, 2007,
2009 and 2010). The observations were undertaken at the 3.9-m Anglo-Australian
Telescope using the UCLES echelle spectrograph and the SEMPOL
spectropolarimeter visitor instrument. Brightness and surface magnetic field
topologies were reconstructed for the star using the technique of least-squares
deconvolution to increase the signal-to-noise of the data.
The reconstructed brightness maps show that HD 141943 had a weak polar spot
and a significant amount of low latitude features, with little change in the
latitude distribution of the spots over the 4 years of observations. The
surface magnetic field was reconstructed at three of the epochs from a high
order (l <= 30) spherical harmonic expansion of the spectropolarimetric
observations. The reconstructed magnetic topologies show that in 2007 and 2010
the surface magnetic field was reasonably balanced between poloidal and
toroidal components. However we find tentative evidence of a change in the
poloidal/toroidal ratio in 2009 with the poloidal component becoming more
dominant. At all epochs the radial magnetic field is predominantly
non-axisymmetric while the azimuthal field is predominantly axisymmetric with a
ring of positive azimuthal field around the pole similar to that seen on other
active stars.Comment: 18 pages, 17 figures, accepted by MNRA
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
Eosinophilic ulcer of the tongue - Case report.
Eosinophilic ulcer of the oral mucosa is a rare, self-limiting, chronic and benign lesion of unknown pathogenesis that affects the oral mucosa. We present the case of a 65 year-old Caucasian female with a fivemonth history of a painful ulcer on the lateral side of her tongue. The ulcer was not adhered to the underlying structures and there was no evidence of regional lymph node involvement. Laboratory examinations and X-rays revealed no abnormalities. Topical treatments had been performed without any improvement. Histopathological examination showed an ulcerated surface and mixed inflammatory infiltrate with several eosinophils extending into the mucosa and submucosa. No cellular atypia was observed. Based on the patient-s history and mucosal biopsy, a final diagnosis of eosinophilic ulcer of the oral mucosa was made
Girsanov reweighting for metadynamics simulations
Metadynamics is a computational method to explore the phase space of a molecular system. Gaussian functions are added along relevant coordinates on the fly during a molecular-dynamics simulation to force the system to escape from minima in the potential energy function. The dynamics in the resulting trajectory are however unphysical and cannot be used directly to estimate dynamical prop- erties of the system. Girsanov reweighting is a recent method used to construct the Markov State Model (MSM) of a system subjected to an external perturbation. With the combination of these two techniques—metadynamics/Girsanov-reweighting—the unphysical dynamics in a metadynam- ics simulation can be reweighted to obtain the MSM of the unbiased system. We demonstrate the method on a one-dimensional diffusion process, alanine dipeptide, and the hexapeptide Val-Gly- Val-Ala-Pro-Gly (VGVAPG). The results are in excellent agreement with the MSMs obtained from direct unbiased simulations of these systems. We also apply metadynamics/Girsanov-reweighting to a β-hairpin peptide, whose dynamics is too slow to efficiently explore its phase space by direct simulation
Three-dimensional Simulations of Accretion to Stars with Complex Magnetic Fields
Disk accretion to rotating stars with complex magnetic fields is investigated
using full three-dimensional magnetohydrodynamic (MHD) simulations. The studied
magnetic configurations include superpositions of misaligned dipole and
quadrupole fields and off-centre dipoles. The simulations show that when the
quadrupole component is comparable to the dipole component, the magnetic field
has a complex structure with three major magnetic poles on the surface of the
star and three sets of loops of field lines connecting them. A significant
amount of matter flows to the quadrupole "belt", forming a ring-like hot spot
on the star. If the maximum strength of the magnetic field on the star is
fixed, then we observe that the mass accretion rate, the torque on the star,
and the area covered by hot spots are several times smaller in the
quadrupole-dominant cases than in the pure dipole cases. The influence of the
quadrupole component on the shape of the hot spots becomes noticeable when the
ratio of the quadrupole and dipole field strengths , and
becomes dominant when . In the case of an off-centre dipole
field, most of the matter flows through a one-armed accretion stream, forming a
large hot spot on the surface, with a second much smaller secondary spot. The
light curves may have simple, sinusoidal shapes, thus mimicking stars with pure
dipole fields. Or, they may be complex and unusual. In some cases the light
curves may be indicators of a complex field, in particular if the inclination
angle is known independently. We also note that in the case of complex fields,
magnetospheric gaps are often not empty, and this may be important for the
survival of close-in exosolar planets.Comment: 13 pages, 21 figures, accepted for publication in MNRA
Surface magnetic fields on two accreting T Tauri stars: CV Cha and CR Cha
We have produced brightness and magnetic field maps of the surfaces of CV Cha
and CR Cha: two actively accreting G and K-type T Tauri stars in the Chamaeleon
I star-forming cloud with ages of 3-5 Myr. Our magnetic field maps show
evidence for strong, complex multi-polar fields similar to those obtained for
young rapidly rotating main sequence stars. Brightness maps indicate the
presence of dark polar caps and low latitude spots -- these brightness maps are
very similar to those obtained for other pre-main sequence and rapidly rotating
main sequence stars.
Only two other classical T Tauri stars have been studied using similar
techniques so far: V2129 Oph and BP Tau. CV Cha and CR Cha show magnetic field
patterns that are significantly more complex than those recovered for BP Tau, a
fully convective T Tauri star.
We discuss possible reasons for this difference and suggest that the
complexity of the stellar magnetic field is related to the convection zone;
with more complex fields being found in T Tauri stars with radiative cores
(V2129 Oph, CV Cha and CR Cha). However, it is clearly necessary to conduct
magnetic field studies of T Tauri star systems, exploring a wide range of
stellar parameters in order to establish how they affect magnetic field
generation, and thus how these magnetic fields are likely to affect the
evolution of T Tauri star systems as they approach the main sequence.Comment: Accepted for publication by MNRAS: 15 pages, 11 figure
Alginate-metal cation interactions: Macromolecular approach
Alginates are a broad family of linear (unbranched) polysaccharides derived from brown seaweeds and some bacteria. Despite having only two monomers, i.e. β-D-mannuronate (M) and its C5 epimer α-L-guluronate (G), their blockwise arrangement in oligomannuronate (.MMM.), oligoguluronate (.GGG.), and polyalternating (.MGMG.) blocks endows it with a rather complex interaction pattern with specific counterions and salts. Classic polyelectrolyte theories well apply to alginate as polyanion in the interaction with monovalent and non-gelling divalent cations. The use of divalent gelling ions, such as Ca2+, Ba2+ or Sr2+, provides thermostable homogeneous or heterogeneous hydrogels where the block composition affects both macroscopic and microscopic properties. The mechanism of alginate gelation is still explained in terms of the original egg-box model, although over the years some novel insights have been proposed. In this review we summarize several decades of research related to structure-functionships in alginates in the presence of non-gelling and gelling cations and present some novel applications in the field of self-assembling nanoparticles and use of radionuclides
Surface differential rotation and prominences of the Lupus post T Tauri star RX J1508.6-4423
We present in this paper a spectroscopic monitoring of the Lupus post T Tauri star RX J1508.6-4423 carried out at two closely separated epochs (1998 May 06 and 10) with the UCL Echelle Spectrograph on the 3.9-m Anglo-Australian Telescope. Applying least-squares convolution and maximum entropy image reconstruction techniques to our sets of spectra, we demonstrate that this star features on its surface a large cool polar cap with several appendages extending to lower latitudes, as well as one spot close to the equator. The images reconstructed at both epochs are in good overall agreement, except for a photospheric shear that we interpret in terms of latitudinal differential rotation. Given the spot distribution at the epoch of our observations, differential rotation could only be investigated between latitudes 15° and 60°. We find in particular that the observed differential rotation is compatible with a solar-like law (i.e., with rotation rate decreasing towards high latitudes proportionally to sin 2l, where l denotes the latitude) in this particular latitude range. Assuming that such a law can be extrapolated to all latitudes, we find that the equator of RX J1508.6-4423 does one more rotational cycle than the pole every 50 ±10 d, implying a photospheric shear 2 to 3 times stronger than that of the Sun. We also discover that the Hα emission profile of RX J1508.6-4423 is most of the time double-peaked and strongly modulated with the rotation period of the star. We interpret this rotationally modulated emission as being caused by a dense and complex prominence system, the circumstellar distribution of which is obtained through maximum entropy Doppler tomography. These maps show in particular that prominences form a complete and inhomogeneous ring around the star, precisely at the corotation radius. We use the total Hα and Hβ emission flux to estimate that the mass of the whole prominence system is about 10 20g. From our observation that the whole cloud system surrounding the star is regenerated in less than 4 d, we conclude that the braking time-scale of RX J1508.6-4423 is shorter than 1 Gyr, and that prominence expulsion is thus likely to contribute significantly to the rotational spindown of young low-mass stars
HD 46375: seismic and spectropolarimetric analysis of a young Sun hosting a Saturn-like planet
HD 46375 is known to host a Saturn-like exoplanet orbiting at 0.04 AU from
its host star. Stellar light reflected by the planet was tentatively identified
in the 34-day CoRoT run acquired in October-November 2008. We constrain the
properties of the magnetic field of HD 46375 based on spectropolarimetric
observations with the NARVAL spectrograph at the Pic du Midi observatory. In
addition, we use a high-resolution NARVAL flux spectrum to contrain the
atmospheric parameters. With these constraints, we perform an asteroseismic
analysis and modelling of HD 46375 using the frequencies extracted from the
CoRoT light curve. We used Zeeman Doppler imaging to reconstruct the magnetic
map of the stellar surface. In the spectroscopic analysis we fitted isolated
lines using 1D LTE atmosphere models. This analysis was used to constrain the
effective temperature, surface gravity, and chemical composition of the star.
To extract information about the p-mode oscillations, we used a technique based
on the envelope autocorrelation function (EACF). From the Zeeman Doppler
imaging observations, we observe a magnetic field of ~5 gauss. From the
spectral analysis, HD 46375 is inferred to be an unevolved K0 type star with
high metallicity [Fe/H]=+0.39. Owing to the relative faintness of the star
(m_hip=8.05), the signal-to-noise ratio is too low to identify individual
modes. However, we measure the p-mode excess power and large separation Delta
nu_0=153.0 +/- 0.7 muHz. We are able do constrain the fundamental parameters of
the star thanks to spectrometric and seismic analyses. We conclude that HD
46375 is similar to a young version of Alpha-CenB. This work is of special
interest because of its combination of exoplanetary science and
asteroseismology, which are the subjects of the current Kepler mission and the
proposed PLATO mission.Comment: Accepted in Astronomy & Astrophysics. 8 pages, 9 figure
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