462 research outputs found
A Physical Limit to the Magnetic Fields of T Tauri Stars
Recent estimates of magnetic field strengths in T Tauri stars yield values
--. In this paper, I present an upper limit to the
photospheric values of by computing the equipartition values for different
surface gravities and effective temperatures. The values of derived from
the observations exceed this limit, and I examine the possible causes for this
discrepancy
Magnetic field topology and chemical spot distributions of the Ap star HD119419
Analysis of high-resolution spectropolarimetric observations of early-type
magnetic stars is currently the most advanced method of obtaining detailed
information on their surface magnetic field topologies and horizontal spot
distributions. In this study we analyse a new set of high-quality four Stokes
parameter observations of the magnetic Ap star HD119419 - a member of the 14
Myr old Lower Cen-Cru association - for the purpose of studying the surface
field topology and mapping the chemical abundance spots. We made use of the
circular and linear polarisation data collected for HD119419 with the HARPSpol
instrument at the ESO 3.6-m telescope. These observations were analysed with a
multi-line magnetic diagnostic technique and modelled in detail with a Magnetic
Doppler imaging code. We present a new set of mean longitudinal magnetic field
measurements and derive a revised stellar rotational period. We also
redetermine the basic stellar atmospheric parameters. Our four Stokes parameter
magnetic inversions reveal a moderately complex surface field topology with a
mean field strength of 18 kG and a maximum local strength of 24 kG. A poloidal
dipolar component dominates the magnetic energy spectrum of the surface field
in HD119419. However, significant contributions of the higher-order spherical
harmonic components are also present. We show that the dipole plus quadrupole
part of the reconstructed field geometry is incapable of reproducing the
observed amplitudes and shapes of the Stokes Q and U profiles. The chemical
abundance distributions of Fe, Cr, Ti, and Nd, derived self-consistently with
the magnetic field geometry, are characterised by large abundance gradients and
a lack of clear correlation with the magnetic field structure. This full Stokes
vector analysis of HD119419 extends the modern hot-star magnetic mapping
investigations to an open cluster Ap star with a well-determined age.Comment: 15 pages, 11 figures; accepted for publication in A&
Magnetic Field Measurement with Ground State Alignment
Observational studies of magnetic fields are crucial. We introduce a process
"ground state alignment" as a new way to determine the magnetic field direction
in diffuse medium. The alignment is due to anisotropic radiation impinging on
the atom/ion. The consequence of the process is the polarization of spectral
lines resulting from scattering and absorption from aligned atomic/ionic
species with fine or hyperfine structure. The magnetic field induces precession
and realign the atom/ion and therefore the polarization of the emitted or
absorbed radiation reflects the direction of the magnetic field. The atoms get
aligned at their low levels and, as the life-time of the atoms/ions we deal
with is long, the alignment induced by anisotropic radiation is susceptible to
extremely weak magnetic fields (G). In fact,
the effects of atomic/ionic alignment were studied in the laboratory decades
ago, mostly in relation to the maser research. Recently, the atomic effect has
been already detected in observations from circumstellar medium and this is a
harbinger of future extensive magnetic field studies. A unique feature of the
atomic realignment is that they can reveal the 3D orientation of magnetic
field. In this article, we shall review the basic physical processes involved
in atomic realignment. We shall also discuss its applications to
interplanetary, circumstellar and interstellar magnetic fields. In addition,
our research reveals that the polarization of the radiation arising from the
transitions between fine and hyperfine states of the ground level can provide a
unique diagnostics of magnetic fields in the Epoch of Reionization.Comment: 30 pages, 12 figures, chapter in Lecture Notes in Physics "Magnetic
Fields in Diffuse Media". arXiv admin note: substantial text overlap with
arXiv:1203.557
The multi-component field topology of sunspot penumbrae - A diagnostic tool for spectropolarimetric measurements
Context: Sunspot penumbrae harbor highly structured magnetic fields and
flows. The moving flux tube model offers an explanation for several observed
phenomena, e.g. the Evershed effect and bright penumbral grains.
Aims: A wealth of information can be extracted from spectropolarimetric
observations. In order to deduce the structure of the magnetic field in sunspot
penumbrae, detailed forward modeling is necessary. On the one hand, it gives
insight into the sensitivity of various spectral lines to different physical
scenarios. On the other hand, it is a very useful tool to guide inversion
techniques. In this work, we present a generalized 3D geometrical model that
embeds an arbitrarily shaped flux tube in a stratified magnetized atmosphere.
Methods: The new semi-analytical geometric model serves as a frontend for a
polarized radiative transfer code. The advantage of this model is that it
preserves the discontinuities of the physical parameters across the flux tube
boundaries. This is important for the detailed shape of the emerging Stokes
Profiles and the resulting net circular polarization (NCP).
Results: (a) The inclination of downflows in the outer penumbra must be
shallower than approximately 15 degrees. (b) Observing the limb-side NCP of
sunspots in the Fe I 1564.8 nm line offers a promising way to identify a
reduced magnetic field strength in flow channels. (c) The choice of the
background atmosphere can significantly influence the shape of the Stokes
profiles, but does not change the global characteristics of the resulting NCP
curves for the tested atmospheric models.Comment: 12 pages, 14 figure
Global Distribution of Non-algal Particles From Ocean Color Data and Implications for Phytoplankton Biomass Detection
In the last few decades, phytoplankton biomass has been commonly studied from space. However, satellite analysis of non-algal particles (NAPs), including heterotrophic bacteria and viruses, is relatively recent. In this work, we estimate the backscattering coefficient associated with the NAP fraction that does not covary with chlorophyll based on satellite particulate backscattering coefficient and chlorophyll (bbpNAP). bbpNAP is computed at 100-km resolution using 19 years of monthly satellite data. We find clear differences in bbpNAP between northern and southern oceans. High bbpNAP values are found in the Arctic and Southern Oceans, the North Atlantic area influenced by the Gulf Stream current, as well as shelf regions (i.e., Patagonian shelf) affected by upwelling regimes. Low correlation between chlorophyll and backscattering prevents precise bbpNAP estimations in oligotrophic areas (e.g., subtropical gyres). These bbpNAP estimations lead to a reduction to half in satellite-based phytoplankton biomass estimates respect to previously published results
Association between plasma omentin-1 levels in type 2 diabetic patients and peripheral artery disease.
BACKGROUND: Type-2 diabetes mellitus is one of the major risk factors of atherosclerosis, particularly in peripheral artery disease (PAD). Several studies have documented a correlation between omentin-1 serum levels, atherosclerosis, and cardiovascular diseases. However, a clear link between circulating omentin-1 and PAD in diabetic patients has yet to be established. The aim of this study was to investigate the potential role of omentin-1 in PAD in type-2 diabetic patients.
METHODS: In this cross-sectional study, we analyzed omentin-1 serum levels by ELISA in 600 type-2 diabetic patients with (n = 300) and without (n = 300) PAD at Fontaine's stage II, III, or IV.
RESULTS: We found that omentin-1 serum levels were significantly lower in diabetic patients with PAD than in diabetic controls (29.46 vs 49.24 ng/mL, P < 0.001) and that the levels gradually decreased in proportion to disease severity (P < 0.05). The association between omentin-1 levels and PAD remained significant after adjusting for major risk factors in a multivariate analysis.
CONCLUSIONS: Our results suggest that omentin-1 is reduced in type 2 diabetic patients with PAD and that omentin-1 levels are related to disease severity
The pecular magnetic field morphology of the white dwarf WD 1953-011: evidence for a large-scale magnetic flux tube?
We present and interpret new spectropolarimetric observations of the magnetic
white dwarf WD 1953-011. Circular polarization and intensity spectra of the
H spectral line demonstrate the presence of two-component magnetic
field in the photosphere of this star. The geometry consists of a weak, large
scale component, and a strong, localized component. Analyzing the rotationally
modulated low-field component, we establish a rotation period days. Modeling the measured magnetic observables, we find that the
low-field component can be described by the superposition of a dipole and
quadrupole. According to the best-fit model, the inclination of the stellar
rotation axis with respect to the line of sight is , and
the angle between the rotation axis and the dipolar axis is . The dipole strength at the pole is about 180 kG, and the quadrupolar
strength is about 230 kG. These data suggest a fossil origin of the low-field
component. In contrast, the strong-field component exhibits a peculiar,
localized structure (``magnetic spot'') that confirms the conclusions of Maxted
and co-workers. The mean field modulus of the spot (
kG) together with its variable longitudinal magnetic field having a maximum of
about +400 kG make it difficult to describe it naturally as a high-order
component of the star's global poloidal field. Instead, we suggest that the
observed strong-field region has a geometry similar to a magnetic flux tube.Comment: 11 figure
Italian intersociety consensus statement on antithrombotic prophylaxis in hip and knee replacement and in femoral neck fracture surgery
Anticoagulant prophylaxis for preventing venous thromboembolism (VTE) is a worldwide established procedure in hip and knee replacement surgery, as well as in the treatment of femoral neck fractures (FNF). Different guidelines are available in the literature, with quite different recommendations. None of them is a multidisciplinary effort as the one presented. The Italian Society for Studies on Haemostasis and Thrombosis (SISET), the Italian Society of Orthopaedics and Traumatology (SIOT), the association of Orthopaedists and Traumatologists of Italian Hospitals (OTODI), together with the Italian Society of Anesthesia, Analgesia, Resuscitation, and Intensive Care (SIAARTI) have set down easy and quick suggestions for VTE prophylaxis in hip and knee surgery as well as in FNF treatment. This inter-society consensus statement aims at simplifying the grading system reported in the literature, and its goal is to benefit its clinical application. Special focus is given to fragile patients, those with high bleeding risk, and those receiving chronic antiplatelet (APT) and vitamin K antagonists treatment. A special chapter is dedicated to regional anaesthesia and VTE prophylaxis
The treatment of polycythaemia vera: an update in the JAK2 era
The clinical course of polycythaemia vera is marked by a high incidence of thrombotic complications, which represent the main cause of morbidity and mortality. Major predictors of vascular events are increasing age and previous thrombosis. Myelosuppressive drugs can reduce the rate of thrombosis, but there is concern that their use raises the risk of transformation into acute leukaemia. To tackle this dilemma, a risk-oriented management strategy is recommended. Low-risk patients should be treated with phlebotomy and low-dose aspirin. Cytotoxic therapy is indicated in high-risk patients, with the drug of choice being hydroxyurea because its leukaemogenicity is low. The recent discovery of JAK2 V617F mutation in the vast majority of polycythaemia vera patients opens new avenues for the treatment of this disease. Novel therapeutic options theoretically devoid of leukaemic risk, such as alpha-interferon and imatinib, affect JAK2 expression in some patients. Nevertheless, these drugs require further clinical experience and, for the time being, should be reserved for selected cases
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