Search

Systematic calculations of asymmetric nuclear matter have been performed in the framework of the Brueckner-Bethe-Goldstone approach in a wide range of both density and asymmetry parameter. The empirical parabolic law fulfilled by the binding energy per nucleon is confirmed by the present results in all the range of the asymmetry parameter values. The predominant role of the $^{3}$${\mathit{S}}_{1\mathrm{\ensuremath{-}}}^{3}$${\mathit{D}}_{1}$ component of the NN interaction is elucidated. A linear variation of the proton and neutron single-particle potentials is found as increasing the neutron excess; a deviation from the phenomenological potentials occurs for highly asymmetric matter as an effect of the self-consistency. The present calculations of the incompressibility predict a strong softening of the equation of state going from symmetric to asymmetric nuclear matter. The proton fraction in equilibrium with neutron matter has been determined from the beta-stability condition and its relevance to the superfluidity of neutron stars has been investigated.

The saturation curve of symmetric nuclear matter is calculated at the Brueckner-Hartree-Fock level of approximation within the continuous choice for the single-particle potential. The realistic local Argonne ${\mathit{v}}_{14}$ potential is used and the results are compared with similar calculations presented in the literature. The binding energies per nucleon around saturation agree closely with previous results obtained with separable versions of the same potential as well as of the Paris potential.

Pure hadronic compact stars, above a threshold value of their gravitational mass (central pressure), are metastable to the conversion to quark stars (hybrid or strange stars). In this paper, we present a systematic study of the metastability of pure hadronic compact stars using different relativistic models for the equation of state (EoS). In particular, we compare results for the quark-meson coupling (QMC) model with those for the Glendenning--Moszkowski parametrization of the non-linear Walecka model (NLWM). For QMC model, we find large values ($M_{cr} = 1.6$ -- $1.9 M_\odot$) for the critical mass of the hadronic star sequence and we find that the formation of a quark star is only possible with a soft quark matter EoS. For the Glendenning--Moszkowski parametrization of the NLWM, we explore the effect of different hyperon couplings on the critical mass and on the stellar conversion energy. We find that increasing the value of the hyperon coupling constants shifts the bulk transition point for quark deconfinement to higher densities, increases the stellar metastability threshold mass and the value of the critical mass, and thus makes the formation of quark stars less likely. For the largest values of the hyperon couplings we find a critical mass which may be as high as 1.9 - 2.1 $M_\odot$. These stellar configurations, which contain a large central hyperon fraction ($f_{Y,cr} \sim 30 %$), would be able to describe highly-massive compact stars, such as the one associated to the millisecond pulsars PSR B1516+02B with a mass $M = 1.94^{+ 0.17}_{- 0.19} M_{\odot}$., 15 pages, 7 figures, 3 Tables. Accepted for publication in Phys. Rev. D

After a brief non technical introduction of the basic properties of strange quark matter (SQM) in compact stars, we consider some of the late important advances in the field, and discuss some recent astrophysical observational data that could shed new light on the possible presence of SQM in compact stars. We show that above a threshold value of the gravitational mass a neutron star (pure hadronic star) is metastable to the decay (conversion) to an hybrid neutron star or to a strange star. We explore the consequences of the metastability of "massive" neutron stars and of the existence of stable compact "quark" stars (hybrid neutron stars or strange stars) on the concept of limiting mass of compact stars, and we give an extension of this concept with respect to the "classical" one given in 1939 by Oppenheimer and Volkoff., Comment: Invited talk at "the Eleventh Marcel Grossman Meeting on General Relativity", Berlin 2006

We study the nucleation of quark matter drops at the center of cold deleptonized neutron stars. These drops can be made up of unpaired quark matter or color-superconducting quark matter, depending on the details of the equation of state for quark and hadronic matter. The nature of the nucleated phase is relevant in determining the critical mass Mcr of hadronic stars above which a transition to a quark star (hybrid or strange) is possible.

The binding energy of nuclear matter is calculated within the Brueckner-Bethe-Goldstone theory using as realistic N-N interaction a separable version of the Argonne v14 potential. The authors examine the influence of the choice for the self-consistent auxiliary potential on the rate of convergence of the perturbation series. The results show that using the continuous choice gives a faster convergence than the standard choice. The same conclusion can be drawn from the results on the depletion parameter. The lowest-order potential insertions are evaluated in the perspective to study the three-body cluster contributions.

A separable representation of the Paris interaction is used as input for the investigation of various nuclear matter properties. The faithfulness of the separable representation is checked by comparison with results previously obtained from the original Paris interaction. Calculations are performed for four different values of the Fermi momentum, namely ${\mathit{k}}_{\mathit{F}}$=1.10, 1.36, 1.55, and 1.75 ${\mathrm{fm}}^{\mathrm{\ensuremath{-}}1}$. One evaluates the contributions to the quasiparticle potential energy that are of first, second, and third order in the reaction matrix. The momentum distribution n(k) in the correlated ground state is calculated up to second order in the reaction matrix. For 0k2 ${\mathrm{fm}}^{\mathrm{\ensuremath{-}}1}$, it mainly depends upon the ratio k/${\mathit{k}}_{\mathit{F}}$; in the domain 2k4.5 ${\mathrm{fm}}^{\mathrm{\ensuremath{-}}1}$, it is accurately reproduced by the expression 1/7${\mathit{k}}_{\mathit{F}}^{5}$${\mathit{e}}^{\mathrm{\ensuremath{-}}1.6\mathit{k}}$, with k and ${\mathit{k}}_{\mathit{F}}$ in units of ${\mathrm{fm}}^{\mathrm{\ensuremath{-}}1}$. The quasiparticle strength at the Fermi surface is calculated, as well as the mean-square deviation of the one-body density matrix from that of the unperturbed Fermi sea: This quantity gives an estimate of the minimum value of the norm of the difference between the one-body density matrix of a correlated nucleus and that associated with any Slater determinant. The average kinetic energy per nucleon is evaluated. Various contributions to the average binding energy per nucleon are investigated in the framework of Brueckner's expansion; particular attention is paid to the dependence of the calculated binding energy upon the choice of the ``auxiliary'' potential which is added to and subtracted from the Hamiltonian before performing the expansion. One also evaluates diagrams that are characteristic of the difference between the Green's function and the Brueckner hole-line expansions. The fulfillment of the Hugenholtz--Van Hove theorem is studied.

We examine the present status of the theoretical calculations for the internal structure of neutron stars, and the connection with the microscopic properties of ultradense hadronic matter. We discuss the possibility to have quark deconfinement phase transition in the core of neutron stars, and we explore some of its astrophysical implications as the quark-deconfinement nova model for gamma-ray bursts.

We discuss a model for the energy source of gamma-ray burst which is based on the "delayed conversion" of a metastable pure hadronic compact star ("neutron star") to a quark star (hybrid star or strange star).

We study the deconfinement transition of hadronic matter into quark matter in neutron star conditions in the light of color superconductivity. Deconfinement is considered to be a first order phase transition that conserves color and flavor. It gives a short-lived {($\tau \sim \tau_{weak}$)} transitory colorless-quark-phase that is {\it not} in $\beta$-equilibrium. We deduce the equations governing deconfinement when quark pairing is allowed and find the regions of the parameter space (pairing gap $\Delta$ versus bag constant $B$) where deconfinement is possible inside cold neutron stars. We show that for a wide region of ($B,\Delta$) a pairing pattern is reachable within a strong interaction timescale, and the resulting ``2SC-like'' phase is preferred energetically to the unpaired phase. We also show that although $\beta$-stable hybrid star configurations are known to be possible for a wide region of the ($B,\Delta$)-space, many of these configurations could not form in practice because deconfinement is forbidden, i.e. the here studied non-$\beta$-stable \emph{intermediate} state cannot be reached., Comment: Version to appear in Phys. Rev. D (10 pages, 6 figures)

We discuss the role of strangeness on the internal constitution and structural properties of neutron stars. In particular, we report on recent calculations of hyperon star properties derived from microscopic equations of state for hyperonic matter. Next, we discuss the possibility of having a strange quark matter core in a neutron star, or the possible existence of strange quark matter stars, the so-called strange stars., Invited review talk at the VIII International Conference on Hypernuclear and Strange Particle Physics, Jefferson Lab, Newport News, Virginia, October 14-18, 2003. 10 pages, 5 figures

We report on a new model which is able to explain how a gamma‐ray burst (GRB) can take place days or years after a supernova explosion. We show that above a threshold value of the gravitational mass a pure hadronic star (“neutron star”) is metastable to the conversion into a quark star (hybrid star or strange star), i.e. a star made at least in part of deconfined quark matter. The stellar conversion process can be delayed if finite size effects at the interface between hadronic and deconfined quark matter phases are taken into account. A huge amount of energy, on the order of 1052 – 1053 ergs, is released during the conversion process and can produce a powerful gamma‐ray burst. The delay between the supernova explosion generating the metastable neutron star and the new collapse can explain the delay inferred in GRB 990705 and in GRB 011211.

We study the role played by hyperons on the hadron-star to quark-star conversion mechanism proposed recently by Berezhiani and collaborators. We find that the quantum nucleation of a quark matter drop inside hadron matter is faster when the presence of hyperons is taken into account in the hadron phase. As a consequence the conversion mechanism is then favored.

We study the role played by neutrino trapping on the hadron star (HS) to quark star (QS) conversion mechanism proposed recently by Berezhiani and collaborators. We find that the nucleation of quark matter drops inside hadron matter, and therefore the conversion of a HS into a QS, is strongly inhibit by the presence of neutrinos., 3 pages, 3 figures. Talk given at the VIII International Conference on Strangeness in Quark Matter. Cape Town, South Africa, Septembre 2004

We propose a model to explain how a Gamma Rays Burst can take place days or years after a supernova explosion. Our model is based on the conversion of a pure hadronic star (neutron star) into a star made at least in part of deconfined quark matter. The conversion process can be delayed if the surface tension at the interface between hadronic and deconfined-quark-matter phases is taken into account. The nucleation time (i.e. the time to form a critical-size drop of quark matter) can be extremely long if the mass of the star is small. Via mass accretion the nucleation time can be dramaticaly reduced and the star is finally converted into the stable configuration. A huge amount of energy, of the order of 10$^{52}$--10$^{53}$ erg, is released during the conversion process and can produce a powerful Gamma Ray Burst. The delay between the supernova explosion generating the metastable neutron star and the new collapse can explain the delay proposed in GRB990705 and in GRB011211., Comment: AAS LaTeX, 12 pages, 1 figure, 2 tables Added references and comments

We propose that the bimodal nature of the kick velocity distribution of radio pulsars is connected with the dichotomy between hadronic stars ({\it i.e.} neutron stars with no quark matter content) and quark stars. Bimodality can appear due to different mechanisms of explosion which leads to the formation of two types of compact stars or due to two different sets of parameters mastering a particular kick mechanism. The low velocity maximum (at $\sim 100$ km s$^{-1}$) is connected with hadronic star formation, whereas the second peak corresponds to quark stars. In the model of delayed collapse of hadronic stars to quark stars (Berezhiani et al. 2003\nocite{bbd2003}) quark deconfinement leads to a second energy release, and to a second kick, in addition to the kick imparted to the newly formed hadronic star during the supernova explosion. If the electromagnetic rocket mechanism can give a significant contribution to pulsar kicks, then the high velocity peak can be connected with the shorter initial spin periods of quark stars with respect to hadronic stars. We discuss {\it pro et contra} of these scenarios., Comment: 8 pages with 2 figures; accepted to A&A

International audience; We calculate the neutrino mean free path and the Equation of State of pure neutron matter at finite temperature within a selfconsistent scheme based on the Brueckner--Hartree--Fock approximation. We employ the nucleon-nucleon part of the recent realistic baryon-baryon interaction (model NSC97e) constructed by the Nijmegen group. The temperatures considered range from 10 to 80 MeV. We report on the calculation of the mean field, the residual interaction and the neutrino mean free path including short and long range correlations given by the Brueckner--Hartree--Fock plus Random Phase Approximation (BHF+RPA) framework. This is the first fully consistent calculation in hot neutron matter dedicated to neutrino mean free path. We compare systematically our results to those obtain with the D1P Gogny effective interaction, which is independent of the temperature. The main differences between the present calculation and those with nuclear effective interactions come from the RPA corrections to BHF (a factor of about 8) while the temperature lack of consistency accounts for a factor of about 2.

Properties of spin polarized isospin asymmetric nuclear matter are studied within the framework of the Brueckner--Hartree--Fock formalism. The single-particle potentials of neutrons and protons with spin up and down are determined for several values of the neutron and proton spin polarizations and the asymmetry parameter. It is found an almost linear and symmetric variation of the single-particle potentials as increasing these parameters. An analytic parametrization of the total energy per particle as a function of the asymmetry and spin polarizations is constructed. This parametrization is employed to compute the magnetic susceptibility of nuclear matter for several values of the asymmetry from neutron to symmetric matter. The results show no indication of a ferromagnetic transition at any density for any asymmetry of nuclear matter., 23 pages, 8 figures, 2 tables (submitted to Phys. Rev. C)

Employing the most recent parametrization of the baryon-baryon interaction of the Nijmegen group, we investigate, in the framework of the Brueckner--Bethe--Goldstone many-body theory at zero temperature, the influence of neutrino trapping on the composition, equation of state, and structure of neutron stars, relevant to describe the physical conditions of a neutron star immediately after birth (protoneutron star). We find that the presence of neutrinos changes significantly the composition of matter delaying the appearance of hyperons and making the equation of state stiffer. We explore the consequences of neutrino trapping on the early evolution of a neutron star and on the nature of the final compact remnant left by the supernova explosion., Comment: Astronomy & Astrophysics, 399, 687-693 (2003)

We propose a model to explain how a Gamma Rays Burst can take place days or years after a supernova explosion. Our model is based on the conversion of a pure hadronic star (neutron star) into a star made at least in part of deconfined quark matter. The conversion process can be delayed if the surface tension at the interface between hadronic and deconfined‐quark‐matter phases is taken into account. The nucleation time (i.e. the time to form a critical‐size drop of quark matter) can be extremely long if the mass of the star is small. Via mass accretion the nucleation time can be dramaticaly reduced and the star is finally converted into the stable configuration. A huge amount of energy, of the order of 1052–1053 erg, is released during the stellar conversion and can produce a powerful Gamma Ray Burst. The delay between the supernova explosion generating the metastable neutron star and the new collapse can explain the delay proposed in GRB990705 [1] and in GRB011211 [2].

We compute the temperature profiles of accretion discs around rapidly rotating strange stars, using constant gravitational mass equilibrium sequences of these objects, considering the full effect of general relativity. Beyond a certain critical value of stellar angular momentum ($J$), we observe the radius ($r_{\rm orb}$) of the innermost stable circular orbit (ISCO) to increase with J (a property seen neither in rotating black holes nor in rotating neutron stars). The reason for this is traced to the crucial dependence of $dr_{\rm orb}/dJ$ on the rate of change of the radial gradient of the Keplerian angular velocity at $r_{\rm orb}$ with respect to $J$. The structure parameters and temperature profiles obtained are compared with those of neutron stars, as an attempt to provide signatures for distinguishing between the two. We show that when the full gamut of strange star equation of state models, with varying degrees of stiffness are considered, there exists a substantial overlap in properties of both neutron stars and strange stars. However, applying accretion disc model constraints to rule out stiff strange star equation of state models, we notice that neutron stars and strange stars exclusively occupy certain parameter spaces. This result implies the possibility of distinguishing these objects from each other by sensitive observations through future X-ray detectors., Comment: Contains 10 figures, uses psbox.tex. Accepted for publication in A&A main journal

The properties of isospin-asymmetric nuclear matter have been investigated in the framework of the extended Brueckner-Hartree-Fock approximation at zero temperature. Self-consistent calculations using the Argonne $V_{14}$ interaction are reported for several asymmetry parameters $\beta = \frac{N - Z}{A}$ ranging from symmetric nuclear matter to pure neutron matter. The binding energy per nucleon fulfills the $\beta^2$ law in the whole asymmetry range. The symmetry energy is calculated for different densities and discussed in comparison with other predictions. At the saturation point it is in fairly good agreement with the empirical value. The present approximation, based on the Landau definition of quasiparticle energy, is investigated in terms of the Hugenholtz-Van Hove theorem, which is proved to be fulfilled with a good accuracy at various asymmetries. The isospin dependence of the single-particle properties is discussed, including mean field, effective mass, and mean free path of neutrons and protons. The isospin effects in nuclear physics and nuclear astrophysics are briefly discussed., Comment: 26 pages, 11 figures

We give a brief introduction to the physics of strange quark matter, and explore the possibility this novel deconfined phase of matter might be absolutely stable. Strange quark stars represent one of the most intriguing consequences of such a possibility. We study the structural properties of this hypothetical new class of stellar compact objects, both for non-rotating and rapidly rotating configurations in general relativity. Next, using recent observational data for the X-ray burster 4U 1820-30, the newly discovered millisecond X-ray pulsar SAX J1808.4-3658, and for the atoll source 4U 1728-34, we argue that the compact stars in these X-ray sources are likely strange star candidates. Finally, we study the conversion of a neutron star to a strange quark star. We show that the total amount of energy liberated in the conversion process is in the range (1 - 4) x 1053 erg, in agreement with the energy required to power gamma-ray bursts at cosmological distances.

There are about 2000 gamma ray burst (GRB) events known to us with data pouring in at the rate of one per day. While the afterglows of GRBs in radio, optical and X-ray bands are successfully explained by the fireball model, a significant difficulty with the proposed mechanisms for GRBs is that a small amount ($\le 10^{-6} M_{\odot}$) of baryons in the ejecta can be involved. There are very few models that fulfill this criteria together with other observational features, among which are the differentially rotating collapsed object model and the "supernova" model. These models generally invoke rapidly rotating neutron stars, and may be subject to uncertainties in the formation mechanisms and the equations of state of neutron stars. According to Spruit, the problem of making a GRB from an X-ray binary is reduced to finding a plausible way to make the star rotate differentially. We suggest that a model of strange star (SS) can naturally explain many of these bursts with not only their low baryon content, but the differential rotation which leads to an enhanced magnetic field that surfaces up and is responsible for GRBs., Comment: 7 pages, 2 Postscript figures, uses psfig.sty

Using a recent microscopic equation of state (EOS) for neutron star matter, we construct equilibrium sequences of rapidly rotating neutron stars in general relativity. The sequences are the normal and supramassive evolutionary sequences of constant rest mass. Also calculated is the radius of the marginally stable orbit and its dependence on the rotation rate Ω, relevant for modeling of kilo-Hertz quasi-periodic oscillations in X-ray binaries.

For a new equation of state of strange quark matter, we construct equilibrium sequences of rapidly rotating strange stars in general relativity. The sequences are the normal and supramassive evolutionary sequences of constant rest mass. We also calculate equilibrium sequences for a constant value of $\Omega$ corresponding to the most rapidly rotating pulsar PSR 1937 + 21. In addition to this, we calculate the radius of the marginally stable orbit and its dependence on $\Omega$, relevant for modeling of kilo-Hertz quasi-periodic oscillations in X-ray binaries., Comment: Two figures, uses psbox.tex and emulateapj5.sty

Definitely, an affirmative answer to this question would have implications of fundamental importance for astrophysics (a new class of compact stars), and for the physics of strong interactions (deconfined phase of quark matter, and strange matter hypothesis). In the present work, we use observational data for the newly discovered millisecond X-ray pulsar SAX J1808.4-3658 and for the atoll source 4U 1728-34 to constrain the radius of the underlying compact stars. Comparing the mass-radius relation of these two compact stars with theoretical models for both neutron stars and strange stars, we argue that a strange star model is more consistent with SAX J1808.4-3658 and 4U 1728-34, and suggest that they are likely strange star candidates., Comment: In memory of Bhaskar Datta. -- Invited talk at the Pacific Rim Conference on Stellar Astrophysics (Hong Kong, aug. 1999)

Strange stars could represent a new class of stellar compact objects, where a new phase of dense matter (deconfined strange quark matter) comes into play. Here, we discuss some possible observational evidence for their existence.

The discovery of kilohertz quasi-periodic oscillations (kHz QPOs) in low-mass X-ray binaries (LMXBs) with the Rossi X-ray Timing Explorer has stimulated extensive studies of these sources. Recently, Osherovich & Titarchuk suggest a new model for kHz QPOs and the related correlations between kHz QPOs and low frequency features in LMXBs. Here we use their results to study the mass - radius relation for the atoll source 4U 1728-34. We find that, if this model is correct, 4U 1728-34 is possibly a strange star rather a neutron star., 7 pages, 1 figure

One of the most important questions in the study of compact objects is the nature of pulsars, including whether they are composed of $\beta$-stable nuclear matter or strange quark matter. Observations of the newly discovered millisecond X-ray pulsar \sax with the Rossi X-Ray Timing Explorer place firm constraint on the radius of the compact star. Comparing the mass - radius relation of \sax with the theoretical mass - radius relation for neutron stars and for strange stars, we find that a strange star model is more consistent with SAX J1808.4-3658, and suggest that it is a likely strange star candidate., Comment: 5 pages, Latex, 1 figure, corrected for some typos

We investigate the structure of neutron stars shortly after they are born, when the entropy per baryon is of order 1 or 2 and neutrinos are trapped on dynamical timescales. In all cases, the thermal effects for an entropy per baryon of order 2 or less are small when considering the maximum neutron star mass. Neutrino trapping, however, significantly changes the maximum mass due to the abundance of electrons. When matter is allowed to contain only nucleons and leptons, trapping decreases the maximum mass by an amount comparable to, but somewhat larger than, the increase due to finite entropy. When matter is allowed to contain strongly interacting negatively charged particles, in the form of strange baryons, a kaon condensate, or quarks, trapping instead results in an increase in the maximum mass of order $0.2M_\odot$, which adds to the effects of finite entropy. The presence of negatively-charged particles has two major implications. First, the value of the maximum mass will decrease during the early evolution of a neutron star as it loses trapped neutrinos, so that if a black hole forms, it either does so immediately after the bounce or it is delayed for a neutrino diffusion timescale of $\sim 10$ s. The latter case is most likely if the maximum mass of the hot star with trapped neutrinos is near $1.5M_\odot$. In the absence of negatively-charged hadrons, black hole formation would be due to accretion and therefore is likely to occur only immediately after bounce. Second, the appearance of hadronic negative charges results in a general softening of the equation of state that may be observable in the neutrino luminosities and average energies. Further, these additional negative charges decrease the electron fraction and may be observed in the relative excess of electron neutrinos compared to other neutrinos., Comment: 83 pages, 30 postscript figures, Physics Reports, to be published

We investigate the consequences of the quark deconfinement phase transition on the evolution of proto-neutron stars. Assuming a first order phase transition, we calculate the nucleation time due to thermal and quantum nucleation mechanisms. We introduce the concepts of critical mass Mcr and limiting conversion temperature for proto-hadronic stars. We show that proto-hadronic stars with a mass M < Mcr could survive the early stages of their evolution without decaying to quark stars.

Some aspects of the superfluidity in nuclear matter and neutron matter are numerically investigated by using realistic interactions. It is shown that the gap equation with bare pairing interaction contains the particle-particle correlations corresponding to the ladder diagrams. The effects of the short range part of the interaction on the gap function are discussed in momentum space in connection with the weak coupling approximation. Extending the BCS theory to coupled channels, it is found that the pairing between neutron and proton in the 3 S 1 - 3 D 1 channel is surprisingly large, of the order of about 10 MeV.

A phase of strong interacting matter with deconfined quarks is expected in the core of massive neutron stars. We study the quark deconfinement phase transition in cold (T = 0) and hot β-stable hadronic matter. Assuming a first order phase transition, we calculate and compare the nucleation rate and the nucleation time due to thermal and quantum nucleation mechanisms. We show that above a threshold value of the central pressure a pure hadronic star (HS) is metastable to the conversion to a quark star (QS) (i.e. hybrid star or strange star). We introduce the concept of critical mass Mcr for cold HSs and proto-hadronic stars (PHSs), and the concept of limiting conversion temperature for PHSs. We show that PHSs with a mass M < Mcr could survive the early stages of their evolution without decaying to QSs. Finally, we discuss the possible evolutionary paths of proto-hadronic stars.

(Abridged) A phase of strong interacting matter with deconfined quarks is expected in the core of massive neutron stars. If this deconfinement phase transition is of the first order then it will be triggered by the nucleation of a critical size drop of the stable quark phase in the metastable hadronic phase. Within these circumstances it has been shown that cold pure hadronic compact stars above a threshold value of their gravitational mass are metastable with respect to the "decay" to quark stars (compact stars made at least in part of quark matter). This stellar conversion process liberates a huge amount of energy, and it could be the energy source of some of the long GRBs. The main goal of the present work is to establish whether a newborn hadronic star (proto-hadronic star) could survive the early stages of its evolution without "decaying" to a quark star. To this aim, we study the nucleation process of quark matter in hot beta-stable hadronic matter, with and without trapped neutrinos. We calculate and compare the nucleation rate and the nucleation time due to thermal and quantum nucleation mechanisms. We compute the crossover temperature above which thermal nucleation dominates the finite temperature quantum nucleation mechanism. We next discuss the consequences of quark matter nucleation for the physics and the evolution of proto-neutron stars. We introduce the new concept of limiting conversion temperature and critical mass M_cr for proto-hadronic stars, and we show that proto-hadronic stars with a mass M < M_cr could survive the early stages of their evolution without decaying to a quark star. We extend the concept of maximum mass of a "neutron star" with respect to the classical one introduced by Oppenheimer & Volkoff to account for the existence of two distinct families of compact stars (hadronic stars and quark stars) as predicted by the present scenario., Accepted for publication in Astronomy and Astrophysics

The temperature dependence of the nuclear incompressibility is investigated in connection with the problem of supernova explosions. The present calculations have been carried out within an extended Brueckner-Bethe-Goldstone approach and using the Paris nucleon-nucleon interaction. A strong reduction of the nuclear incompressibility with increasing temperature is found, in agreement with previous results obtained with phenomenological nuclear interactions. The temperature and pressure of asymmetric nuclear matter in the collapsing presupernova core are calculated along isentropic collapse paths.

We investigate the off-the-energy-shell properties of the mass operator M(k; ω) = V(k; ω) + iW(k; ω), i.e., its dependence upon the nucleon momentum k and upon the nucleon frequency ω, separately. Particular attention is paid to the dispersion relation which connects its real and imaginary parts. We limit ourselves to the first two terms of the hole-line expansion of the mass operator, namely to the Brueckner-Hartree-Fock field M1(k; ω) and to the second-order “rearrangement” term M2(k; ω). Most previous works only dealt with “on-shell values” obtained by setting ω equal to the root e(k) of an energy-momentum relation, or equivalently by setting k equal to the root k(e) of this energy-momentum relation. We use as input a finite-rank representation of the realistic Argonne v14 nucleon-nucleon interaction. The Fermi momentum is set equal to 1.36 fm−1. For momenta larger than the Fermi momentum, the calculated k-dependence of the on-shell depth V1(k; e(k)) can be approximated by a gaussian. The corresponding nonlocality range is close to that assumed by Perey and Buck in their phenomenological analysis of scattering cross sections; it is somewhat smaller than that associated with the k-dependence of the off-shell potential V1(k; ω) for fixed ω. The calculated ω-dependence of V2(k; ω) is in excellent agreement with the dispersion relation which connects V2(k; ω) to the values of W2(k; ω′) for all ω′

These Proceedings contain the invited and contributed papers presented at the 12th Conference on Theoretical Nuclear Physics in Italy held in Cortona, Italy, from 8–10 October 2008. As usual, the meeting was held at il Palazzone, a 16th century castle owned by the Scuola Normale Superiore di Pisa. The aim of this biennal conference is to bring together Italian theorists working in various fields of Nuclear Physics to discuss their latest results and confront their points of view in a lively and informal way. This offers the opportunity to promote collaborations between different groups. There were about 50 participants at the conference, coming from 14 Italian Universities (Cagliari, Catania, Ferrara, Firenze, Genova, Lecce, Milano, Napoli, Padova, Pavia, Pisa, Roma, Trento, Trieste). The program of the conference, prepared by the Organizing Committee (Ignazio Bombaci, Aldo Covello, Laura Elisa Marcucci and Sergio Rosati) focused on six main topics: Few-Nucleon Systems, Nuclear Matter and Nuclear Dynamics, Nuclear Astrophysics, Structure of Hadrons and Hadronic Matter, Nuclear Structure, Nuclear Physics with Electroweak Probes. Winfried Leidemann, Maria Colonna, Marcello Lissia, Elena Santopinto, Silvia Lenzi and Omar Benhar took the burden of giving general talks on these topics and reviewing the research activities of the various Italian groups. In addition, 19 contributed papers were presented, most of them by young participants. In the last session of the Conference there were two invited talks related to experimental activities of great current interest. Gianfranco Prete from the Laboratori Nazionali di Legnaro spoke about the Italian radioactive ion beam facility SPES and the status of the European project EURISOL, while Nicola Colonna from the INFN, Bari, gave an overview of the perspectives of development of fourth-generation nuclear reactors. We would like to thank the authors of the general reports for their hard work in reviewing the main achievements in the various fields as well as our experimental colleagues for having kindly agreed to talk to an audience consisting only of theoretical physicists. I Bombaci, A Covello, L E Marcucci and S Rosati

The momentum distribution and the quasi-hole strength in symmetric nuclear matter have been calculated for two densities in the framework of the Brueckner-Bethe-Goldstone theory. A separable version of the Argonne v14 has been used as realistic NN interaction in the self-consistent procedure to determine the g -matrix and the corresponding mean field. The results for quasi-hole strength have been compared with other calculations and also with the hole-state spectroscopic factor extracted from recent (e,e′p) knock-out reactions on 90 Zr and 208 Pb. On the basis of the hole-line expansion higher-order contributions to the momentum distribution have been included in the calculations resulting in a better fulfillment of the Migdal-Luttinger theorem.

The dependence on the momentum of the nuclear mean field is studied in the framework of the self-consistent Bethe-Brueckner theory. It is pointed out that the rearrangement term, coming from the variation of the {ital G} matrix, gives a substantial contribution at the lowest momenta. The resulting single particle potential exhibits a good rate of convergence. Its momentum dependence appears to be negligible up to 2 fm{sup {minus}1}, in contrast with potentials used in calculations of heavy-ion collisions at intermediate energies.

We study the quark deconfinement phase transition in hot β-stable hadronic matter. Assuming a first order phase transition, we calculate the enthalpy per baryon of the hadron–quark phase transition. We calculate and compare the nucleation rate and the nucleation time due to thermal and quantum nucleation mechanisms. We compute the crossover temperature above which thermal nucleation dominates the finite temperature quantum nucleation mechanism. We next discuss the consequences for the physics of proto-neutron stars. We introduce the concept of limiting conversion temperature and critical mass M cr for proto-hadronic stars, and we show that proto-hadronic stars with a mass M M cr could survive the early stages of their evolution without decaying to a quark star.

Symmetrical nuclear matter is studied in the vicinity of the saturation point in the framework of an approximation scheme which includes single-particle correlations. The Hugenholtz-Van Hove theorem is well satisfied, in contrast with standard Brueckner calculations. The incompressibility and the saturation points are only slightly affected by correlations, while the effective mass shows an enhancement at the Fermi momentum.

Symmetrical nuclear matter at finite temperature is studied in the framework of the Brueckner-Hartree-Fock approximation extended to include single-particle correlations. A liquid-vapor phase transition is observed, wtih a critical temperature of about 20 MeV, in close similarity with Skyrme force calculations. The inclusion of single-particle correlations introduces a significant temperature dependence in the single-particle potential as well as in the nucleon effective mass. In this scheme the Hughenholtz-Van Hove theorem is well satisfied throughout the range of density and temperature considered.