A synoptic comparison of the MHD and the OPAL equations of state

A detailed comparison is carried out between two popular equations of state (EOS), the Mihalas-Hummer-Dappen (MHD) and the OPAL equations of state, which have found widespread use in solar and stellar modeling during the past two decades. They are parts of two independent efforts to recalculate stellar opacities; the international Opacity Project (OP) and the Livermore-based OPAL project. We examine the difference between the two equations of state in a broad sense, over the whole applicable rho-T range, and for three different chemical mixtures. Such a global comparison highlights both their differences and their similarities. We find that omitting a questionable hard-sphere correction, tau, to the Coulomb interaction in the MHD formulation, greatly improves the agreement between the MHD and OPAL EOS. We also find signs of differences that could stem from quantum effects not yet included in the MHD EOS, and differences in the ionization zones that are probably caused by differences in the mechanisms for pressure ionization. Our analysis do not only give a clearer perception of the limitations of each equation of state for astrophysical applications, but also serve as guidance for future work on the physical issues behind the differences. The outcome should be an improvement of both equations of state.Comment: 33 pages, 26 figures. Corrected discussion of Basu & Antia, 2004, ApJ, 606, L85-L8

Kardiale Niedrigdosis-Computertomographie

Cardiac CT has evolved to a robust and accurate imaging modality in the cardiac diagnostic armamentarium. However, technical developments had been accompanied with an overall increase in radiation exposure. In the last years, several technical developments and algorithms aimed at the reduction of radiation exposure in cardiac CT. The most relevant dose reduction strategies will be highlighted in this article including appropriate indications for cardiac CT, different ECG synchronization techniques, reduction of tube voltage, and high-pitch CT studies

Metallic artefact reduction with monoenergetic dual-energy CT: systematic ex vivo evaluation of posterior spinal fusion implants from various vendors and different spine levels

Objectives: To evaluate optimal monoenergetic dual-energy computed tomography (DECT) settings for artefact reduction of posterior spinal fusion implants of various vendors and spine levels. Methods: Posterior spinal fusion implants of five vendors for cervical, thoracic and lumbar spine were examined ex vivo with single-energy (SE) CT (120 kVp) and DECT (140/100 kVp). Extrapolated monoenergetic DECT images at 64, 69, 88, 105keV and individually adjusted monoenergy for optimised image quality (OPTkeV) were generated. Two independent radiologists assessed quantitative and qualitative image parameters for each device and spine level. Results: Inter-reader agreements of quantitative and qualitative parameters were high (ICC = 0.81-1.00, κ = 0.54-0.77). HU values of spinal fusion implants were significantly different among vendors (P < 0.001), spine levels (P < 0.01) and among SECT, monoenergetic DECT of 64, 69, 88, 105keV and OPTkeV (P < 0.01). Image quality was significantly (P < 0.001) different between datasets and improved with higher monoenergies of DECT compared with SECT (V = 0.58, P < 0.001). Artefacts decreased significantly (V = 0.51, P < 0.001) at higher monoenergies. OPTkeV values ranged from 123-141keV. OPTkeV according to vendor and spine level are presented herein. Conclusions: Monoenergetic DECT provides significantly better image quality and less metallic artefacts from implants than SECT. Use of individual keV values for vendor and spine level is recommended. Key Points: • Artefacts pose problems for CT following posterior spinal fusion implants. • CT images are interpreted better with monoenergetic extrapolation using dual-energy (DE) CT. • DECT extrapolation improves image quality and reduces metallic artefacts over SECT. • There were considerable differences in monoenergy values among vendors and spine levels. • Use of individualised monoenergy values is indicated for different metallic hardware device

Correlation energy of an electron gas in strong magnetic fields at high densities

The high-density electron gas in a strong magnetic field B and at zero temperature is investigated. The quantum strong-field limit is considered in which only the lowest Landau level is occupied. It is shown that the perturbation series of the ground-state energy can be represented in analogy to the Gell-Mann Brueckner expression of the ground-state energy of the field-free electron gas. The role of the expansion parameter is taken by r_B= (2/3 \pi^2) (B/m^2) (\hbar r_s /e)^3 instead of the field-free Gell-Mann Brueckner parameter r_s. The perturbation series is given exactly up to o(r_B) for the case of a small filling factor for the lowest Landau level.Comment: 10 pages, Accepted for publication in Phys.Rev.

Equation of state of fully ionized electron-ion plasmas

Thermodynamic quantities of Coulomb plasmas consisting of point-like ions immersed in a compressible, polarizable electron background are calculated for ion charges Z=1 to 26 and for a wide domain of plasma parameters ranging from the Debye-Hueckel limit to the crystallization point and from the region of nondegenerate to fully degenerate nonrelativistic or relativistic electrons. The calculations are based on the linear-response theory for the electron-ion interaction, including the local-field corrections in the electronic dielectric function. The thermodynamic quantities are calculated in the framework of the N-body hypernetted-chain equations and fitted by analytic expressions. We present also accurate analytic approximations for the free energy of the ideal electron gas at arbitrary degeneracy and relativity and for the excess free energy of the one-component plasma of ions (OCP) derived from Monte Carlo simulations. The extension to multi-ionic mixtures is discussed within the framework of the linear mixing rule. These formulae provide a completely analytic, accurate description of the thermodynamic quantities of fully ionized electron-ion Coulomb plasmas, a useful tool for various applications from liquid state theory to dense stellar matter.Comment: 13 pages, 2 tables, 7 figures, REVTeX using epsf.sty. To be published in Phys. Rev. E, vol. 58 (1998

A helium spread among the main sequence stars in NGC 2808

We studied the color distribution of the main sequence of the Globular Cluster NGC 2808, based on new deep HST-WFPC2 photometry of a field in the uncrowded outskirts of the cluster. The color distribution of main sequence stars is wider than expected for a single stellar population, given our (carefully determined) measurement errors. About 20% of the sample stars are much bluer than expected and are most plausibly explained as a population having a much larger helium abundance than the bulk of the main sequence. We estimate that the helium mass fraction of these stars is Y ~ 0.4. NGC 2808 may have suffered self-enrichment, with different stellar populations born from the ejecta of the intermediate mass asymptotic giant branch (AGB) stars of the first generation. In addition to the Y=0.40 stars, roughly 30% of the stars should have Y distributed between 0.26-0.29 while 50% have primordial Y, to explain also the peculiar horizontal branch morphology. Three main stages of star formation are identified, the first with primordial helium content Y ~ 0.24, the second one born from the winds of the most massive AGBs of the first stellar generation (6-7msun), having Y ~ 0.4, and a third one born from the matter ejected from less massive AGBs (~ 3.5-4.5msun) with Y ~ 0.26-0.29. For a long hiatus of time (several 10^7yr) between the second and third generation, star formation might have been inhibited by the explosion of late Supernovae II deriving from binary evolution.Comment: accepted for publication on "The Astrophysical Journal

Technical principles of computed tomography in patients with congenital heart disease

Cardiac magnetic resonance imaging and echocardiography are often the primary imaging techniques for many patients with congenital heart disease (CHD). However, with modern generations of CT systems and recent advances in temporal and spatial resolution, cardiac CT has been gaining an increasing reputation in the field of cardiac imaging and in the evaluation of patients with congenital heart disease. The CT imaging protocol depends on the suspected cardiac defect, the type of previous surgical repair, and the patient’s age and level of cooperation. Various strategies are available for reducing radiation exposure, which is of utmost importance particularly in paediatric patients. A sequential segmental analysis is a commonly used approach to analysing congenital heart defects. Familiarity of the performing radiologist with dedicated CT protocols, the complex anatomy, morphology and terminology of CHD, as well as with the surgical procedures used to correct congenital abnormalities is a prerequisite for correct diagnosis

A detailed study of the main sequence of the Globular Cluster NGC 6397: can we derive constraints on the existence of multiple population?

If NGC 6397 contains a large fraction of "second generation" stars (>70% according to recent analysis), the helium abundance of its stars might also be affected, show some star-to-star variation, and be larger than the standard Big Bang abundance Y~0.24. Can we derive constraints on this issue from the analysis of the main sequence width and from its luminosity function? We build up new models for the turnoff masses and the main sequence down to the hydrogen burning minimum mass, adopting two versions of an updated equation of state (EOS) including the OPAL EOS. Models consider different initial helium and CNO abundances to cover the range of possible variations between the first and second generation stars. We compare the models with the observational main sequence. We also make simulations of the theoretical luminosity functions, for different choices of the mass function and of the mixture of first and second generation stars, and compare them with the observed luminosity function, by means of the Kolmogorov Smirnov --KS-- test. The study of the width of the main sequence at different interval of magnitude is consistent with the hypothesis that both generations are present in the cluster. If the CNO increase suggested by spectroscopic observation is taken into account the small helium spread of the main sequence in NGC 6397 implies a substantial helium uniformity (DY~0.02) between first and second generation stars. The possible spread in helium doubles if an even larger increase of CNO is considered. The luminosity function is in any case well consistent with the observed data.Comment: 13 pages, 13 figures, accepted for publication to A&