Bulletin of the American Physical Society
2006 Division of Nuclear Physics Annual Meeting
Wednesday–Saturday, October 25–28, 2006; Nashville, Tennessee
Session CH: Mini-symposium on From Crust to Core: QCD in Neutron Stars |
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Sponsoring Units: DNP Chair: George Fuller, University of California, San Diego Room: Gaylord Opryland Cheekwood F |
Friday, October 27, 2006 9:00AM - 9:36AM |
CH.00001: Neutron Star Structure From Observations Invited Speaker: Neutron stars are laboratories for dense matter physics. Observations of neutron stars, in the form of radio pulsars, X-ray binaries, X-ray bursters, and thermally-emitting isolated stars, are rapidly accumulating. Especially interesting are the radio pulsars PSR J0751+1807, Terzan 5 I and Terzan 5 J (with suprisingly large measured masses of $2.1\pm0.2, 1.69\pm0.1$ and $1.85\pm0.05$ solar masses, respectively), the pulsar PSR J1748-2446ad with the most rapid spin rate of 716 Hz, and the radio pulsar binary PSR J0737-3039 for which a moment of inertia of one of the neutron stars might be measured within a few years. Extremely massive neutron stars are important because they set limits to the maximum mass and upper limits to the maximum density found in cold, static, objects, and might limit the appearance of exotic matter such as hyperons, Bose condensates or deconfined quarks in a star's interior. The spin rate sets an upper limit to the radius of a star of a given mass, and the moment of inertia, being roughly proportional to $M R^2$, is a sensitive measure of neutron star radius. While the maximum mass speaks to the relative stiffness of the high-density equation of state at several times nuclear matter density, the radius is a measure of the relative stiffness of the low-density equation of state in the vicinity of the nuclear saturation density. For the nearly pure neutron matter found in neutron stars, it is a direct measure of the density dependence of the nuclear symmetry energy. Other promising observational constraints might be obtained from neutron star seismology (which limits the relative crustal thickness) and Eddington limited fluxes observed from bursting sources, and from thermal emissions from cooling neutron stars. The latter have the potential of constraining $R_\infty=R/\sqrt{1-2GM/Rc^2}$ if the source's distance can be accurately assessed. The distances of two nearby isolated sources, RX J1856-3754 and Geminga, have been determined by parallax. However, there are major difficulties in accounting for atmospheres of unknown composition and uncertain magnetic field strenghs for these stars. The distances to several distant X-ray emitting neutron stars have also been estimated with some precision because they are members of globular clusters, These sources have advantages because, having undergone recent accretion, they should have relatively weak surface magnetic fields and hydrogen-dominated atmospheres. Preliminary results from the interpretation of thermal emissions indicate consistency with a radius in the range of 10-15 km, but only a restricted subset of possible equations of state can account for the $(M, R)$ constraints of all the sources. [Preview Abstract] |
Friday, October 27, 2006 9:36AM - 9:48AM |
CH.00002: QCD in Neutron Stars and Strange Stars Fridolin Weber Neutron stars contain matter in one of the densest forms found in the Universe. This feature, together with the unprecedented progress in observational astrophysics, makes such stars superb astrophysical laboratories for a broad range of exciting physical studies, several of which are intimately connected to QCD. This talk summarizes the role of QCD for neutron stars and strange stars. Particular emphasis will be put on the role of strangeness. Strangeness is carried by hyperons, mesons, H-dibaryons, and color superconducting strange quark matter, and may leave its mark in the masses, radii, cooling behavior, surface composition and the spin evolution of neutron stars. I also discuss the effects of a net electric charge distribution on the bulk properties of strange quark stars. Depending on the amount of electric charge distributed over the surface of such objects, the mass-radius relationship of strange quark stars may deviate substantially from the standard mass-radius relationship of electrically uncharged stars. This finding is of key importance for the properties of hypothetical strange quark stars made of color superconducting quark matter, since these objects could possess electric surface fields strong enough to alter the mass-radius relationship significantly. [Preview Abstract] |
Friday, October 27, 2006 9:48AM - 10:00AM |
CH.00003: Evidence for White Dwarfs with Strange-Matter Cores Grant Mathews, InSaeng Suh, Nguyen Lan, William Zech, Kaori Otsuki, Friedolin Weber We summarize masses and radii for a number of white dwarfs as deduced from a combination of proper motion studies, Hipparcos parallax distances, effective temperatures, and binary or spectroscopic masses. A puzzling feature of these data, however, is that some stars appear to have radii which are significantly smaller than that expected for a standard electron-degenerate white-dwarf equations of state. We construct a projection of white-dwarf radii for fixed effective mass and conclude that there is at least marginal evidence for bimodality in the radius distribution for white dwarfs. We argue that if such compact white dwarfs exist it is unlikely that they contain an iron core. We propose an alternative of strange-quark matter within the white-dwarf core. We also discuss the impact of the so-called color-flavor locked (CFL) state in strange-matter core associated with color superconductivity. We show that the data exhibit several features consistent with the expected mass-radius relation of strange dwarfs. We identify eight nearby white dwarfs which are possible candidates for strange matter cores and suggest observational tests of this hypothesis. [Preview Abstract] |
Friday, October 27, 2006 10:00AM - 10:12AM |
CH.00004: Nuclear Equation of State at Supernuclear Densities. Jirina Stone The density and temperature dependence of the energy per particle of a system (the Equation of State (EoS)) is a fundamental ingredient of all models of nuclear matter and stars. As nucleons and leptons form the main components of all stars, the best possible description of the strong and weak interactions amongst these particles is essential for a correct understanding of birth, life and death of stars. At supernuclear densities, the presence of strange baryons and/or partialy deconfined quarks is energetically favorable. We review recent development in EoS models in the high density region, based on selected relativisitic mean field and quark models. These EoS shed new light on the density dependence of the phase transition between pure nuclear uniform matter and matter containing strange baryons and deconfined quarks. Application of these EoS to neutron star models (assuming general beta-equilibrium) and to core-collapse supernova models (non-equilibrium matter) will be discussed. [Preview Abstract] |
Friday, October 27, 2006 10:12AM - 10:24AM |
CH.00005: What is the crust composition of accreting neutron stars? Jacob Fisker, Mary Beard, Edward Brown The nuclear reaction flow of an X-ray burst (XRB) on an accreting neutron star (NS) determines the composition of the burst ashes. These ashes subsequently descend down into the crust and influence many crustal properties of the NS such as the electric conductivity, the amount of heat, $Q_c$, deposited in the crust, and the competing neutrino loss rate. These factors determine the equilibrium core temperature relevant for probing the equation of state of the interior neutron star. Reciprocally, the crustal properties determine the heat flux through the atmosphere and thus influence the reaction flow of the XRBs. The crustal heat flux, $\dot{Q}_c$ has previously been calculated by assuming a composition of ashes given by pure ${}^{56}\textrm{Fe}$ and the ashes of the one-zone model of respectively. However, as the thermonuclear burning of the XRB may depend on $\dot{Q}_c$, we expand on these studies by using a self-consistent spherically symmetric XRB model to calculate the composition of the resulting XRB ashes for different values of the crustal heating rates. We report the results of this study for different accretion rates. [Preview Abstract] |
Friday, October 27, 2006 10:24AM - 10:36AM |
CH.00006: Heating from Electron Captures in the Crusts of Accreting Neutron Stars Sanjib Gupta, Edward Brown, Hendrik Schatz, Peter M\"oller, Karl-Ludwig Kratz We present new calculations of nuclear reactions in the outer crust (densities less than neutron drip) of an accreting neutron star. Our crust model improves on previous work by starting with a realistic distribution of nuclei and by allowing for electron captures into excited states, rather than just transitions into the ground state. We find that the heat deposited in the outer crust is substantially larger (factor of 4) than previous estimates and that the amount of heat deposited depends strongly on the composition of matter synthesized during rp-process burning of accreted hydrogen and helium. This increased heating raises the temperature in the crust and makes the unstable ignition of carbon---which is thought to power superbursts observed from some accreting neutron stars---occur at lower density. This alleviates some of the discrepancy between the ignition depth inferred from observations and theoretical superburst models. [Preview Abstract] |
Friday, October 27, 2006 10:36AM - 10:48AM |
CH.00007: Constraints on dense matter physics from deep heating of accreting neutron stars Edward Brown Accretion of matter from a stellar companion compresses the crust of a neutron star and induces reactions that heat the interior. The temperature in the crust is set by balancing this heating with thermal radiation from the surface and neutrino emission from the crust and core. Many neutron stars accrete intermittently; when the accretion halts, the crust cools. Recent observations have now observed evidence of crustal cooling. In this talk, I present theoretical models of heating and cooling in the neutron star crust, and compare them with observations. I assess how these observations constrain the neutrino emissivity of the neutron star core. These new crust models improve on previous ones by incorporating electron captures into excited states, which increases the heat deposited into the crust. In addition, our models allow us to compute the heating in the outer crust for a wide range of possible crust compositions. [Preview Abstract] |
Friday, October 27, 2006 10:48AM - 11:00AM |
CH.00008: Probing the interiors of accreting neutron stars Andrew Cumming Neutron stars in low mass X-ray binaries accrete hydrogen and helium from a low mass companion. The neutron star can be observed directly in X-rays during periods of quiescence, when accretion switches off, or during thermonuclear X-ray bursts which result from unstable thermonuclear burning of the accreted matter. Recent long term monitoring observations of these systems have revealed new types of long duration X-ray bursts resulting from unstable burning of thick helium or carbon layers. The properties of these long bursts are sensitive to the heat flux emerging from deep in the star, and therefore give a new way to probe neutron star cooling. I discuss the current constraints on the neutrino emissivity of the stellar core, and the dense matter interior, including the possibility that these stars are in fact strange stars. [Preview Abstract] |
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