801 research outputs found
alphaPDE: A New Multivariate Technique for Parameter Estimation
We present alphaPDE, a new multivariate analysis technique for parameter
estimation. The method is based on a direct construction of joint probability
densities of known variables and the parameters to be estimated. We show how
posterior densities and best-value estimates are then obtained for the
parameters of interest by a straightforward manipulation of these densities.
The method is essentially non-parametric and allows for an intuitive graphical
interpretation. We illustrate the method by outlining how it can be used to
estimate the mass of the top quark, and we explain how the method is applied to
an ensemble of events containing background.Comment: 11 pages, published versio
Clinical management of 52 consecutive retro-rectal tumours treated at a tertiary referral centre
Abstract Aim The aim of this single institution study was to analyse the diagnostic methods, pre-operative work-up and outcomes of 52 retrorectal tumours. Method All patients treated for retro-rectal tumours from 2012 to 2017 were included. Results Out of 52 patients, 40 (77%) were women. The patients? median age at the time of surgery was 43 (19-76) years, and 30 (58%) of the patients were asymptomatic at the time of diagnosis. All tumours were visible on magnetic resonance imaging (MRI) prior to surgery. The sensitivity and specificity for predicting malignancy on pre-operative imaging for retro-rectal tumours were 25% and 98%, respectively. Forty-four procedures (85%) were performed using the perineal approach. The median hospital stay was three (1-18) days. There was no 30-day post-operative mortality. Eleven (21%) patients developed post-operative complications, mostly surgical site infections. Twenty-nine tumours (56%) were benign tailgut cysts. Four (8%) tumours were malignant and were considered to be removed with a tumour-free resection margin. Local recurrent disease was detected on MRI in 14 (27%) patients at a median of 1.05 (range: 0.78-1.77) years after primary surgery. Only the multi-lobular shape of the tumour was found to be an independent risk factor for recurrence (p=0.030). Conclusion A pre-operative MRI is mandatory in order to plan the surgical strategy for retro-rectal tumours. Symptomatic, solid, large tumours should be removed because of the risk of malignancy. Minor cystic lesions with thin walls as well as asymptomatic recurrences of benign tumours are suitable to be followed conservatively.Peer reviewe
Stellar wind interaction and pick-up ion escape of the Kepler-11 "super-Earths"
We study the interactions between stellar wind and the extended
hydrogen-dominated upper atmospheres of planets and the resulting escape of
planetary pick-up ions from the 5 "super-Earths" in the compact Kepler-11
system and compare the escape rates with the efficiency of the thermal escape
of neutral hydrogen atoms. Assuming the stellar wind of Kepler-11 is similar to
the solar wind, we use a polytropic 1D hydrodynamic wind model to estimate the
wind properties at the planetary orbits. We apply a Direct Simulation Monte
Carlo Model to model the hydrogen coronae and the stellar wind plasma
interaction around Kepler-11b-f within a realistic expected heating efficiency
range of 15-40%. The same model is used to estimate the ion pick-up escape from
the XUV heated and hydrodynamically extended upper atmospheres of Kepler-11b-f.
From the interaction model we study the influence of possible magnetic moments,
calculate the charge exchange and photoionization production rates of planetary
ions and estimate the loss rates of pick-up H+ ions for all five planets. We
compare the results between the five "super-Earths" and in a more general sense
also with the thermal escape rates of the neutral planetary hydrogen atoms. Our
results show that for all Kepler-11b-f exoplanets, a huge neutral hydrogen
corona is formed around the planet. The non-symmetric form of the corona
changes from planet to planet and is defined mostly by radiation pressure and
gravitational effects. Non-thermal escape rates of pick-up ionized hydrogen
atoms for Kepler-11 "super-Earths" vary between approximately 6.4e30 1/s and
4.1e31 1/s depending on the planet's orbital location and assumed heating
efficiency. These values correspond to non-thermal mass loss rates of
approximately 1.07e7 g/s and 6.8e7 g/s respectively, which is a few percent of
the thermal escape rates.Comment: 8 pages, 3 figures, accepted to A&
Transit Ly- signatures of terrestrial planets in the habitable zones of M dwarfs
We modeled the transit signatures in the Lya line of a putative Earth-sized
planet orbiting in the HZ of the M dwarf GJ436. We estimated the transit depth
in the Lya line for an exo-Earth with three types of atmospheres: a
hydrogen-dominated atmosphere, a nitrogen-dominated atmosphere, and a
nitrogen-dominated atmosphere with an amount of hydrogen equal to that of the
Earth. We calculated the in-transit absorption they would produce in the Lya
line. We applied it to the out-of-transit Lya observations of GJ 436 obtained
by the HST and compared the calculated in-transit absorption with observational
uncertainties to determine if it would be detectable. To validate the model, we
also used our method to simulate the deep absorption signature observed during
the transit of GJ 436b and showed that our model is capable of reproducing the
observations. We used a DSMC code to model the planetary exospheres. The code
includes several species and traces neutral particles and ions. At the lower
boundary of the DSMC model we assumed an atmosphere density, temperature, and
velocity obtained with a hydrodynamic model for the lower atmosphere. We showed
that for a small rocky Earth-like planet orbiting in the HZ of GJ436 only the
hydrogen-dominated atmosphere is marginally detectable with the STIS/HST.
Neither a pure nitrogen atmosphere nor a nitrogen-dominated atmosphere with an
Earth-like hydrogen concentration in the upper atmosphere are detectable. We
also showed that the Lya observations of GJ436b can be reproduced reasonably
well assuming a hydrogen-dominated atmosphere, both in the blue and red wings
of the Lya line, which indicates that warm Neptune-like planets are a suitable
target for Lya observations. Terrestrial planets can be observed in the Lya
line if they orbit very nearby stars, or if several observational visits are
available.Comment: 17 pages, 12 figures, accepted for publication in Astronomy &
Astrophysic
The top-down crystallisation of Mercury's core
The regime governing the growth of Mercury's core is unknown, but the dynamics of core growth are vital to understanding the origin and properties of the planet's weak magnetic field. Here, we use advanced first-principles methods, which include a magnetic entropy contribution, to investigate the magnetic and thermo-elastic properties of liquid Fe-S-Si and of pure liquid iron at the conditions of Mercury's core. Our results support a ‘top-down’ evolution of the core, whereby solid iron-rich material crystallises at shallow depths and sinks. This process would likely result in a compositionally driven dynamo within a stably stratified uppermost liquid layer, providing an explanation for the observed properties of the weak magnetic field of Mercury
The Fermi Surface Effect on Magnetic Interlayer Coupling
The oscillating magnetic interlayer coupling of Fe over spacer layers
consisting of CuPd alloys is investigated by first principles
density functional theory. The amplitude, period and phase of the coupling, as
well as the disorder-induced decay, are analyzed in detail and the consistency
to the Ruderman-Kittel-Kasuya-Yoshida (RKKY) theory is discussed. For the first
time an effect of the Fermi surface nesting strength on the amplitude is
established from first principles calculations. An unexpected variation of the
phase and disorder-induced decay is obtained and the results are discussed in
terms of asymptotics
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