267 research outputs found
Effects of Boson Dispersion in Fermion-Boson Coupled Systems
We study the nonlinear feedback in a fermion-boson system using an extension
of dynamical mean-field theory and the quantum Monte Carlo method. In the
perturbative regimes (weak-coupling and atomic limits) the effective
interaction among fermions increases as the width of the boson dispersion
increases. In the strong coupling regime away from the anti-adiabatic limit,
the effective interaction decreases as we increase the width of the boson
dispersion. This behavior is closely related with complete softening of the
boson field. We elucidate the parameters that control this nonperturbative
region where fluctuations of the dispersive bosons enhance the delocalization
of fermions.Comment: 14 pages RevTeX including 12 PS figure
CS Cha B: A disc-obscured M-type star mimicking a polarised planetary companion
Context. Direct imaging provides a steady flow of newly discovered giant
planets and brown dwarf companions. These multi-object systems can provide
information about the formation of low-mass companions in wide orbits and/or
help us to speculate about possible migration scenarios. Accurate
classification of companions is crucial for testing formation pathways. Aims.
In this work we further characterise the recently discovered candidate for a
planetary-mass companion CS Cha b and determine if it is still accreting.
Methods. MUSE is a four-laser-adaptive-optics-assisted medium-resolution
integral-field spectrograph in the optical part of the spectrum. We observed
the CS Cha system to obtain the first spectrum of CS Cha b. The companion is
characterised by modelling both the spectrum from 6300 \unicode{x212B} to
9300 \unicode{x212B} and the photometry using archival data from the visible
to the near-infrared (NIR). Results. We find evidence of accretion and outflow
signatures in H and OI emission. The atmospheric models with
the highest likelihood indicate an effective temperature of K with
a of dex. Based on evolutionary models, we find that the
majority of the object is obscured. We determine the mass of the faint
companion with several methods to be between 0.07 and 0.71
with an accretion rate of
Myr. Conclusions. Our results show that CS Cha B is most likely a
mid-M-type star that is obscured by a highly inclined disc, which has led to
its previous classification using broadband NIR photometry as a planetary-mass
companion. This shows that it is important and necessary to observe over a
broad spectral range to constrain the nature of faint companionsComment: 9 pages, 6 figures, accepted for publication by A&
Polarization aberrations in next-generation giant segmented mirror telescopes (GSMTs) I. Effect on the coronagraphic performance
Next-generation large segmented mirror telescopes are expected to perform
direct imaging and characterization of Earth-like rocky planets, which requires
contrast limits of to at wavelengths from I to J band. One
critical aspect affecting the raw on-sky contrast are polarization aberrations
arising from the reflection from the telescope's mirror surfaces and instrument
optics. We simulate the polarization aberrations and estimate their effect on
the achievable contrast for three next-generation ground-based large segmented
mirror telescopes. We performed ray-tracing in Zemax and computed the
polarization aberrations and Jones pupil maps using the polarization
ray-tracing algorithm. The impact of these aberrations on the contrast is
estimated by propagating the Jones pupil maps through a set of idealized
coronagraphs using hcipy, a physical optics-based simulation framework. The
optical modeling of the giant segmented mirror telescopes (GSMTs) shows that
polarization aberrations create significant leakage through a coronagraphic
system. The dominant aberration is retardance defocus, which originates from
the steep angles on the primary and secondary mirrors. The retardance defocus
limits the contrast to to at 1 at visible
wavelengths, and to at infrared wavelengths. The
simulations also show that the coating plays a major role in determining the
strength of the aberrations. Polarization aberrations will need to be
considered during the design of high-contrast imaging instruments for the next
generation of extremely large telescopes. This can be achieved either through
compensation optics, robust coronagraphs, specialized coatings, calibration,
and data analysis approaches or by incorporating polarimetry with high-contrast
imaging to measure these effects.Comment: 18 pages, 12 figures, Accepted in Astronomy & Astrophysics manuscript
no. aa45651-2
Functional performance after complex endovascular aortic repair: a single-center retrospective cohort study
Purpose Complex endovascular aortic repair (EVAR) procedures provide a treatment option for patients with aortic aneurysms involving visceral branches. Good technical results and short-term outcomes have been reported. Whether complex EVAR provides acceptable functional outcomes is not clear. The current study aims to describe postoperative functional outcomes in complex EVAR patients-an older and relatively frail patient group. Materials and Methods A single-center retrospective cohort study was performed, using data from a computerized database of consecutive patients who underwent complex EVAR in the Leiden University Medical Center (LUMC, The Netherlands) between July 2013 and September 2020. As of May 2017, patients scheduled for complex EVAR were referred to a geriatric care pathway to determine (Instrumental) Activities of Daily Living ((I)ADL) scores at baseline and, if informed consent was given, after 12 months. For the total patient group, adverse functional performance outcomes were: discharge to a nursing home and 12-month mortality. For the patients included in geriatric follow-up, the additional outcome was the incidence of functional decline (defined by a >= 2 point increase in (I)ADL-score) at 12-month follow-up Results Eighty-two patients underwent complex EVAR, of which 68 (82.9%) were male. Mean age was 73.3 years (SD=6.3). Within 30 days postsurgery, 6 patients (7.3%) died. Mortality within 12 months for the total patient group was 14.6% (n=12). After surgery, no patients had to be discharged to a nursing home. Fifteen patients (18.3%) were discharged to a rehabilitation center. Twenty-three patients gave informed consent and were included in geriatric follow-up. Five patients (21.7%) presented functional decline 12 months postsurgery and 4 patients had died (17.4%) by that time. This means that 39.1% of the patients in the care pathway suffered an adverse outcome. Conclusion To our knowledge, this is the only study that examined functional performance after complex EVAR, using a prospectively maintained database. No patients were newly discharged to a nursing home and functional performance results at 12 months are promising. Future multidisciplinary research should focus on determining which patients are most prone to deterioration of function, so that efforts can be directed toward preventing postoperative functional decline.Cardiovascular Aspects of Radiolog
Spontaneous emission and level shifts in absorbing disordered dielectrics and dense atomic gases: A Green's function approach
Spontaneous emission and Lamb shift of atoms in absorbing dielectrics are
discussed. A Green's-function approach is used based on the multipolar
interaction Hamiltonian of a collection of atomic dipoles with the quantised
radiation field. The rate of decay and level shifts are determined by the
retarded Green's-function of the interacting electric displacement field, which
is calculated from a Dyson equation describing multiple scattering. The
positions of the atomic dipoles forming the dielectrics are assumed to be
uncorrelated and a continuum approximation is used. The associated unphysical
interactions between different atoms at the same location is eliminated by
removing the point-interaction term from the free-space Green's-function (local
field correction). For the case of an atom in a purely dispersive medium the
spontaneous emission rate is altered by the well-known Lorentz local-field
factor. In the presence of absorption a result different from previously
suggested expressions is found and nearest-neighbour interactions are shown to
be important.Comment: 6 pages no figure
The angular distribution of the reaction
The reaction is very important for low-energy
( MeV) antineutrino experiments. In this paper we calculate
the positron angular distribution, which at low energies is slightly backward.
We show that weak magnetism and recoil corrections have a large effect on the
angular distribution, making it isotropic at about 15 MeV and slightly forward
at higher energies. We also show that the behavior of the cross section and the
angular distribution can be well-understood analytically for MeV by calculating to , where is the nucleon mass. The
correct angular distribution is useful for separating events from other reactions and detector backgrounds, as well as for
possible localization of the source (e.g., a supernova) direction. We comment
on how similar corrections appear for the lepton angular distributions in the
deuteron breakup reactions and . Finally, in the reaction , the
angular distribution of the outgoing neutrons is strongly forward-peaked,
leading to a measurable separation in positron and neutron detection points,
also potentially useful for rejecting backgrounds or locating the source
direction.Comment: 10 pages, including 5 figure
Variable-Range Hopping of Spin Polarons: Magnetoresistance in a Modified Mott Regime
We analize electrical conductivity controlled by hopping of bound spin
polarons in disordered solids with wide distributions of electron energies and
polaron shifts (barriers). By means of percolation theory and Monte Carlo
simulations we have shown that in such materials at low temperatures, when
hopping occurs in the vicinity of the Fermi level, a hard polaron gap does not
manifest itself in the transport properties. This happens because as
temperature decreases the hopping polaron trades the decreasing electron and
polaron barriers for increasing hopping distance. As a result, in the absence
of the Coulomb correlation effects, in this variable-range variable-barrier
hopping regime, the electrical resistivity as a function of temperature obeys a
non-activation law, which differs from the standard Mott law
Fluctuations and Dissipation of Coherent Magnetization
A quantum mechanical model is used to derive a generalized Landau-Lifshitz
equation for a magnetic moment, including fluctuations and dissipation. The
model reproduces the Gilbert-Brown form of the equation in the classical limit.
The magnetic moment is linearly coupled to a reservoir of bosonic degrees of
freedom. Use of generalized coherent states makes the semiclassical limit more
transparent within a path-integral formulation. A general
fluctuation-dissipation theorem is derived. The magnitude of the magnetic
moment also fluctuates beyond the Gaussian approximation. We discuss how the
approximate stochastic description of the thermal field follows from our
result. As an example, we go beyond the linear-response method and show how the
thermal fluctuations become anisotropy-dependent even in the uniaxial case.Comment: 22 page
Frailty and treatment decisions in older patients with vulvar cancer: a single-center cohort study
Introduction: Vulvar cancer is a disease that mainly affects older women. Frailty is an important predictor of outcomes and geriatric assessment can help tailor treatment decisions and improve outcomes. This study aims to assess the prevalence of frailty in older women with vulvar cancer, and how it relates to integrated geriatric care and treatment according to the oncological guidelines. Materials and Methods: A single-center cohort study was performed, among patients 70 years and older, who were diagnosed with vulvar cancer at Leiden University Medical Center, between January 2012 and May 2020. Data on geriatric assessment, treatment decision-making and treatment-related outcomes were collected. Results: Our study included 114 patients. Mean age was 79.7 years, and 52 patients (45.6%) were frail. Of the frail patients, 42.0% were referred to a geriatrician. In eight of these cases, the geriatrician was actively involved in weighing the benefit and harm of standard oncological treatment versus de-escalated treatment. Frailty, higher age, impairment in the somatic domain, cognitive impairment, and functional dependency were associated with referral to a geriatrician and with active involvement of a geriatrician in decision making. In 26 of frail patients (50.0%) oncological treatment was de-escalated. Frailty, higher age, impairment in the somatic domain, cognitive impairment, and functional dependency were associated with de-escalation of treatment. De-escalated treatment did not compromise survival. Discussion: Frailty is prevalent among older women with vulvar cancer and is associated with referral to a geriatrician and de-escalation of oncological treatment. While this reflects that it is deemed important to tailor treatment decision for frail patients, most frail patients are not routinely evaluated by a geriatrician. Further multidisciplinary collaboration and research is necessary to optimize tailored treatment decisions for this patient group.Metabolic health: pathophysiological trajectories and therap
Disk Evolution Study Through Imaging of Nearby Young Stars (DESTINYS): Diverse outcomes of binary-disk interactions
CONTEXT: Circumstellar disks play an essential role in the outcomes of planet formation. Disks do not evolve in isolation, as about half of solar-type stars were born in binary or multiple systems. The presence of stellar companions modifies the morphology and evolution of disks, potentially resulting in a different planet population. Resolving disks in binary systems provides the opportunity to examine the influence of stellar companions on the outcomes of planet formation. AIMS: We aim to investigate and compare disks in stellar multiple systems with near-infrared scattered-light imaging as part of the Disk Evolution Study Through Imaging of Nearby Young Stars (DESTINYS) large program. In particular, we present observations of circumstellar disks in three systems, namely, CHX 22, S CrA, and HP Cha. METHODS: We used polarimetric differential imaging with SPHERE/IRDIS at the VLT to search for scattered light from the circum-stellar disks in these multiple systems. We performed astrometric and orbit analyses for the stellar companions using archival HST, VLT/NACO, and SPHERE data to better understand the interplay between disks and companions. RESULTS: Combined with the age and orbital constraints, the observed disk structures in scattered light provide insights into the evolutionary history and the impact of the stellar companions. The small grains in CHX 22 form a tail-like structure surrounding the close binary, which likely results from a close encounter and capture of a cloudlet. S CrA shows intricate structures (tentative ringed and spiral features) in the circumprimary disk as a possible consequence of perturbations by companions. The circumsecondary disk is truncated and connected to the primary disk via a streamer, suggesting tidal interactions. In HP Cha, the primary disk is less disturbed and features a tenuous streamer, through which the material flows toward the companions. CONCLUSIONS: The comparison of the three systems spans a wide range of binary separation (50â500 au) and illustrates the decreasing influence on disk structures with the distance of companions. This agrees with the statistical analysis of the exoplanet population in binaries, that planet formation is likely obstructed around close binary systems, while it is not suppressed in wide binaries
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