265 research outputs found
Memory Systems, the Epistemic Arrow of Time, and the Second Law
The epistemic arrow of time is the fact that our knowledge of the past seems
to be both of a different kind and more detailed than our knowledge of the
future. Just like with the other arrows of time, it has often been speculated
that the epistemic arrow arises due to the second law of thermodynamics.
In this paper we investigate the epistemic arrow of time, using a fully
formal framework. We begin by defining a memory system as any physical system
whose present state can provide information about the state of the external
world at some time other than the present. We then identify two types of memory
systems in our universe, along with an important special case of the first
type, which we distinguish as a third type of memory system.
We show that two of these types of memory system are time-symmetric, able to
provide knowledge about both the past and the future. However, the third type
of memory systems exploits the second law of thermodynamics in all of its
instances we find in our universe. The result is that in our universe, this
type of memory system only ever provides information about the past. Finally,
we argue that human memory is of this third type, completing the argument. Our
analysis is indebted to prior work in Wolpert 1992, but expands and improves
upon this work in several respects.Comment: 24 page
Recovering 3D structural properties of galaxies from SDSS-like photometry
Because of the 3D nature of galaxies, an algorithm for constructing spatial
density distribution models of galaxies on the basis of galaxy images has many
advantages over surface density distribution approximations. We present a
method for deriving spatial structure and overall parameters of galaxies from
images and estimate its accuracy and derived parameter degeneracies on a sample
of idealised model galaxies. The test galaxies consist of a disc-like component
and a spheroidal component with varying proportions and properties. Both
components are assumed to be axially symmetric and coplanar. We simulate these
test galaxies as if observed in the SDSS project through ugriz filters, thus
gaining a set of realistically imperfect images of galaxies with known
intrinsic properties. These artificial SDSS galaxies were thereafter remodelled
by approximating the surface brightness distribution with a 2D projection of a
bulge+disc spatial distribution model and the restored parameters were compared
to the initial ones. Down to the r-band limiting magnitude 18, errors of the
restored integral luminosities and colour indices remain within 0.05 mag and
errors of the luminosities of individual components within 0.2 mag. Accuracy of
the restored bulge-to-disc ratios (B/D) is within 40% in most cases, and
becomes worse for galaxies with low B/D, but the general balance between bulges
and discs is not shifted systematically. Assuming that the intrinsic disc axial
ratio is < 0.3, the inclination angles can be estimated with errors < 5deg for
most of the galaxies with B/D < 2 and with errors < 15deg up to B/D = 6. Errors
of the recovered sizes of the galactic components are below 10% in most cases.
In general, models of disc components are more accurate than models of
spheroidal components for geometrical reasons.Comment: 15 pages, 13 figures, accepted for publication in RA
Pharmacokinetics of penicillin G in preterm and term neonates.
Group B streptococci are common causative agents of early-onset neonatal sepsis (EOS). Pharmacokinetic (PK) data for penicillin G have been described for extremely preterm neonates but poorly for late-preterm and term neonates. Thus, evidence-based dosing recommendations are lacking. We described PK of penicillin G in neonates with gestational age (GA) ≥32 weeks and postnatal age 90% for MICs ≤2 mg/L with doses of 25,000 IU/kg/q12h. In neonates, regardless of GA, PK parameters of penicillin G are similar. The dose of 25,000 IU/kg/q12h is suggested for treatment of group B streptococcal EOS diagnosed within the first 72 hours of life
Scaling beta-lactam antimicrobial pharmacokinetics from early life to old age
AIMS
Beta-lactam dose optimization in critical care is a current priority. We aimed to review the pharmacokinetics (PK) of three commonly used beta-lactams (amoxicillin ± clavulanate, piperacillin-tazobactam and meropenem) to compare PK parameters reported in critically and noncritically ill neonates, children and adults, and to investigate whether allometric and maturation scaling principles could be applied to describe changes in PK parameters through life.
METHODS
A systematic review of PK studies of the three drugs was undertaken using MEDLINE and EMBASE. PK parameters and summary statistics were extracted and scaled using allometric principles to 70 kg individual for comparison. Pooled data were used to model clearance maturation and decline using a sigmoidal (Hill) function.
RESULTS
A total of 130 papers were identified. Age ranged from 29 weeks to 82 years and weight from 0.9-200 kg. PK parameters from critically ill populations were reported with wider confidence intervals than those in healthy volunteers, indicating greater PK variability in critical illness. The standard allometric size and sigmoidal maturation model adequately described increasing clearance in neonates, and a sigmoidal model was also used to describe decline in older age. Adult weight-adjusted clearance was achieved at approximately 2 years postmenstrual age. Changes in volume of distribution were well described by the standard allometric model, although amoxicillin data suggested a relatively higher volume of distribution in neonates.
CONCLUSIONS
Critical illness is associated with greater PK variability than in healthy volunteers. The maturation models presented will be useful for optimizing beta-lactam dosing, although a prospective, age-inclusive study is warranted for external validation
Sakurai's Object: characterizing the near-infrared CO ejecta between 2003 and 2007
We present observations of Sakurai's Object obtained at 1–5 μm between 2003 and 2007. By fitting a radiative transfer model to an echelle spectrum of CO fundamental absorption features around 4.7 μm, we determine the excitation conditions in the line-forming region. We find 12C/13C = 3.5+2.0−1.5, consistent with CO originating in ejecta processed by the very late thermal pulse, rather than in the pre-existing planetary nebula. We demonstrate the existence of 2.2 × 10−6≤MCO≤ 2.7 × 10−6 M⊙ of CO ejecta outside the dust, forming a high-velocity wind of 500 ± 80 km s−1. We find evidence for significant weakening of the CO band and cooling of the dust around the central star between 2003 and 2005. The gas and dust temperatures are implausibly high for stellar radiation to be the sole contributor
Convective-reactive proton-C12 combustion in Sakurai's object (V4334 Sagittarii) and implications for the evolution and yields from the first generations of stars
Depending on mass and metallicity as well as evolutionary phase, stars
occasionally experience convective-reactive nucleosynthesis episodes. We
specifically investigate the situation when nucleosynthetically unprocessed,
H-rich material is convectively mixed with a He-burning zone, for example in
convectively unstable shell on top of electron-degenerate cores in AGB stars,
young white dwarfs or X-ray bursting neutron stars. Such episodes are
frequently encountered in stellar evolution models of stars of extremely low or
zero metal content [...] We focus on the convective-reactive episode in the
very-late thermal pulse star Sakurai's object (V4334 Sagittarii). Asplund etal.
(1999) determined the abundances of 28 elements, many of which are highly
non-solar, ranging from H, He and Li all the way to Ba and La, plus the C
isotopic ratio. Our simulations show that the mixing evolution according to
standard, one-dimensional stellar evolution models implies neutron densities in
the He that are too low to obtain a significant neutron capture nucleosynthesis
on the heavy elements. We have carried out 3D hydrodynamic He-shell flash
convection [...] we assume that the ingestion process of H into the He-shell
convection zone leads only after some delay time to a sufficient entropy
barrier that splits the convection zone [...] we obtain significantly higher
neutron densities (~few 10^15 1/cm^3) and reproduce the key observed abundance
trends found in Sakurai's object. These include an overproduction of Rb, Sr and
Y by about 2 orders of magnitude higher than the overproduction of Ba and La.
Such a peculiar nucleosynthesis signature is impossible to obtain with the
mixing predictions in our one-dimensional stellar evolution models. [...] We
determine how our results depend on uncertainties of nuclear reaction rates,
for example for the C13(\alpha, n)O16 reaction.Comment: ApJ in press, this revision contains several changes that improve
clarity of presentation reflecting the suggestions made by the referee; this
version represents no change in substance compared to version 1; some
technical material has been moved to an appendix; an additional appendix
deals in more detail with the combustion time scales; this version is
practically identical to the ApJ versio
What Therapists Learn from Psychotherapy Clients: Effects on Personal and Professional Lives
While considerable research has examined how clients learn from psychotherapists, there is only sparse literature on what therapists learn from their therapy clients. In a qualitative, exploratory study, nine researchers interviewed 61 psychologists from across North America in order to see what psychotherapists may have learned and how they have been affected by their clients both personally and professionally. Participants responded to nine open-ended questions on learning about life-lessons, relationships, ethical decision-making, coping, courage, wisdom, psychopathology, personality, cultural differences, lifespan development and more. Participants’ richly elaborated responses were coded thematically and narrative data illustrates the most frequent themes. Therapists reported learning a great deal across each of the questions, consistently expressing respect for their clients\u27 resilience, courage and moral sensibilities
Mimicking in-vivo exposures to drug combinations in-vitro : anti-tuberculosis drugs in lung lesions and the hollow fiber model of infection
This research was supported by a British Society of Antimicrobial Chemotherapy Grant (GA2015-172R). FK has conducted the research as part of a Medical Research Council fellowship (MR/P014534/1) and consumables to develop the ultra-high-performance liquid chromatographic-tandem mass spectrometric detection method and analyse the samples were paid to Analytical Services International Ltd. from a Medical Research Council fellowship (MR/P014534/1).Here, we evaluate protocol requirements to mimic therapeutically relevant drug concentrations at the site of infection (i.e. lung lesion) in an in-vitro hollow fibre model of infection using pulmonary tuberculosis as a paradigm. Steady-state pharmacokinetic profiles in plasma, lung tissue and lung lesion homogenate were simulated for isoniazid, rifampicin and pyrazinamide and moxifloxacin. An R-shiny User Interface was developed to support conversion of in-vivo pharmacokinetic CMAX, TMAX and T1/2 estimates into pump settings. A monotherapy protocol mimicking isoniazid in lung lesion homogenate (isoniazid CMAX = 1,200 ng/ml, TMAX = 2.2 hr and T1/2 = 4.7 hr), and two combination therapy protocols including drugs with similar (isoniazid and rifampicin (CMAX = 400 ng/ml)) and different half-lives (isoniazid and pyrazinamide (CMAX = 28,900 ng/ml and T1/2 = 8.0 hr)) were implemented in a hollow-fiber system. Drug levels in the perfusate were analysed using ultra-high-performance liquid chromatographic-tandem mass spectrometric detection. Steady state pharmacokinetic profiles measured in the hollow fiber model were similar to the predicted in-vivo steady-state lung lesion homogenate pharmacokinetic profiles. The presented approach offers the possibility to use pharmacological data to study the effect of target tissue exposure for drug combinations. Integration with pharmacokinetics modelling principles through a web interface will provide access to a wider community interested in the evaluation of efficacy of anti-tubercular drugs.Publisher PDFPeer reviewe
Modeling the spectrum of V4334 Sgr (Sakurai's Object)
Theoretical spectral energy distributions were computed for a grid of
hydrogen-deficient and carbon-rich model atmospheres of T(eff) in the range of
5000-6250 K and log g = 1.0 - 0.0 by the technique of opacity sampling, taking
into account continuous, molecular band and atomic line absorption. These
energy distributions were compared with the spectrum of V4334 Sgr (Sakurai's
object) of April, 1997 in the wavelength interval 300-1000 nm. We show that (1)
the shape of the theoretical spectra depends strongly on T(eff) but only very
weakly on the hydrogen abundance; (2) the comparison of the observed and
computed spectra permits to estimate T(eff) approximately 5500 K for V4334 Sgr
in April, 1997, and its interstellar reddening (plus a possible circumstellar
contribution) E(B-V) approximately 0.70.Comment: 7 pages, 8 figures, LaTeX, accepted by Astronomy and Astrophysic
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