New Class of Compact Stars at High Density

We discuss the equation of state for cold, dense quark matter in perturbation theory, and how it might match onto that of hadronic matter. Certain choices of the renormalization scale correspond to a strongly first order chiral transition, and may generate a new class of small and very dense quark stars. The results for the mass-radius relation are compatible with the recent determination of the mass and the radius of an isolated neutron star by Pons et al.Comment: Latex, 7 pages, 4 figures. Presented at the International Conference on Statistical QCD, Bielefeld, Germany, 26-30 August 2001. Requires espcrc1.st

The sdA problem - II. Photometric and Spectroscopic Follow-up

Subdwarf A star (sdA) is a spectral classification given to objects showing H-rich spectra and sub-main sequence surface gravities, but effective temperature lower than the zero-age horizontal branch. Their evolutionary origin is an enigma. In this work, we discuss the results of follow-up observations of selected sdAs. We obtained time resolved spectroscopy for 24 objects, and time-series photometry for another 19 objects. For two targets, we report both spectroscopy and photometry observations. We confirm seven objects to be new extremely-low mass white dwarfs (ELMs), one of which is a known eclipsing star. We also find the eighth member of the pulsating ELM class.Comment: Accepted for publication in MNRAS. 19 pages, 30 figures, 6 table

Nucleation of quark matter in protoneutron star matter

The phase transition from hadronic to quark matter may take place already during the early post-bounce stage of core collapse supernovae when matter is still hot and lepton rich. If the phase transition is of first order and exhibits a barrier, the formation of the new phase occurs via the nucleation of droplets. We investigate the thermal nucleation of a quark phase in supernova matter and calculate its rate for a wide range of physical parameters. We show that the formation of the first droplet of a quark phase might be very fast and therefore the phase transition to quark matter could play an important role in the mechanism and dynamics of supernova explosions.Comment: v3: fits version published in Physical Review

Remarks on transient photon production in heavy ion collisions

In this note, we discuss the derivation of a formula that has been used in the literature in order to compute the number of photons emitted by a hot or dense system during a finite time. Our derivation is based on a variation of the standard operator-based $S$-matrix approach. The shortcomings of this formula are then emphasized, which leads to a negative conclusion concerning the possibility of using it to predict transient effects for the photon rate.Comment: 13 page

Asteroseismological study of massive ZZ Ceti stars with fully evolutionary models

We present the first asteroseismological study for 42 massive ZZ Ceti stars based on a large set of fully evolutionary carbon$-$oxygen core DA white dwarf models characterized by a detailed and consistent chemical inner profile for the core and the envelope. Our sample comprise all the ZZ Ceti stars with spectroscopic stellar masses between 0.72 and $1.05M_{\odot}$ known to date. The asteroseismological analysis of a set of 42 stars gives the possibility to study the ensemble properties of the massive pulsating white dwarf stars with carbon$-$oxygen cores, in particular the thickness of the hydrogen envelope and the stellar mass. A significant fraction of stars in our sample have stellar mass high enough as to crystallize at the effective temperatures of the ZZ Ceti instability strip, which enables us to study the effects of crystallization on the pulsation properties of these stars. Our results show that the phase diagram presented in Horowitz et al. (2010) seems to be a good representation of the crystallization process inside white dwarf stars, in agreement with the results from white dwarf luminosity function in globular clusters.Comment: 58 pages, 11 figures, accepted in Ap

Mathematical modelling of intensified extraction for spent nuclear fuel reprocessing

Small scale extractors seem to be a promising intensified alternative to the conventional solvent extraction technologies, because of the well described hydrodynamics, enhanced mass transfer, and good phase separation at the end. One of the most interesting applications of intensified extractions is the reprocessing of spent nuclear fuel. Operating in small channels can reduce the volumes of involved hazardous materials and the residence times, thus minimising the degradation of the solvent and its regeneration cost. Finally, nuclear criticality safety may be easily achieved. In this paper, the application of small channels on spent nuclear fuel reprocessing has been investigated. A mathematical model of a multi-component liquid-liquid extraction has been developed. The multi-component system consists of U, Pu, HNO 3 , HNO 2 , Zr, Ru, Tc, Np(IV), Np(V) and Np(VI), the organic solvent is a mixture of 30% (v/v) Tri-Butyl Phosphate (TBP) and a paraffinic diluent. A segmented flow pattern, with the aqueous phase dispersed in a continuous organic phase, has been assumed. Calculations for the estimation of mass transfer, redox reactions, pressure drop, nuclear criticality and TBP hydrolysis have been included in the model. To increase the flow rates, the number of small channels was increased (scale out) and a comb-like manifold was considered to ensure good flow distribution in each channel. The problem is formulated as a mixed integer nonlinear programming problem and is implemented in the General Algebraic Modeling System (GAMS). The results show that this alternative technology for liquid-liquid extraction offers advantages, especially in terms of solvent degradation and low holdup volume

Process intensification applied to spent nuclear fuel reprocessing: An alternative flowsheet using small channels

Commercial plants for spent nuclear fuel reprocessing rely on the Plutonium Uranium Extraction (PUREX) process, based on traditional liquid–liquid extraction technologies. In this paper, an alternative flowsheet for spent nuclear fuel reprocessing is proposed, based on small-scale extractors to overcome some of the issues related to the conventional technologies, such as solvent degradation, size and nuclear criticality control. The main goal of the process is to preclude the risk of nuclear proliferation, hence a mixed uranium/plutonium oxide is produced instead of pure plutonium. A superstructure optimisation based framework has been used to identify a process with several benefits over the conventional process. Novel flow configurations and organic solvent composition have been investigated. A large number of components and chemical reactions are included in the framework. The resulting model is a mixed integer nonlinear optimisation problem, implemented in the General Algebraic Modeling System (GAMS). The most promising flowsheet identified is more cost effective than the conventional one. Furthermore, advantages in terms of safety and separation efficiency have been achieved. It was found that increasing the inner diameter of the small channels up to 2.5 mm, as well as increasing the tributyl phosphate fraction in the organic solvent, are advantageous