Bulletin of the American Physical Society
2005 2nd Joint Meeting of the Nuclear Physics Divisions of the APS and The Physical Society of Japan
Sunday–Thursday, September 18–22, 2005; Maui, Hawaii
Session 2WH: Workshop 8B: Neutron-Rich Nuclei in Nuclear Astrophysics |
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Sponsoring Units: DNP JPS Chair: Hiroyuki Sagawa, University of Aizu Room: Ritz-Carlton Hotel Plantation 1 |
Sunday, September 18, 2005 2:00PM - 2:30PM |
2WH.00001: Nuclear equation of state from neutron star structure and cooling Invited Speaker: Neutron stars represent the ultimate laboratory for the study of dense matter, especially neutron-rich dense matter. Such matter may exhibit phenomena and conditions not observed anywhere else in the universe, such as hyperon-dominated matter, deconfined strange quark matter, superfluidity and superconductivity, opaqueness to neutrinos, and extreme magnetic fields. To date, the two most important properties of neutron stars, their typical radii and their maximum mass, remain elusive. The determination of each would yield important information about two different aspects of dense matter, the radius being primarily a function of the isospin dependence of the nucleon-nucleon force near the nuclear saturation density, and the maximum mass depending upon the composition and stiffness of supranuclear matter. This talk will focus on how the structure of neutron stars ({\it i.e.}, the maximum mass, radii, moments of inertia, crustal thicknesses, and central densities) depends upon the equation of state and the composition of dense matter. In addition, it will summarize how recent observations are constraining these structural properties. These observations include radio and X-ray studies of binary pulsars, radio studies of pulsar glitches, X-ray and optical studies of the thermal emission from isolated neutron stars and pulsars, and observations of burst sources believed to be associated with the neutron star surface. Radio binary pulsars already yield several accurate mass measurements, and several more estimated masses, some of which challenge conventional wisdom concerning the maximum neutron star mass. In addition, the potential exists to measure the moment of inertia of at least one neutron star (PSR J0737-3039) in a radio binary which could provide a radius determination of unprecedented accuracy. Glitches from pulsars can help determine the thickness of neutron star crusts, which depends upon the stellar mass and radius, as well as the unknown pressure at the core-crust interface at approximately one half of the nuclear saturation density. Thermally emitting sources yield valuable data about the redshifted area, redshifted temperatures, and ages of the emitting sources, which in turn proffer information about the cooling histories of neutron stars. Neutron star cooling indirectly informs us about the internal composition and the superfluid properties of dense matter. Burst sources, including quasi-periodic oscillators, may convey surface redshift data, which together with radiation radius information, will yield neutron star masses and radii. Parallel constraints from laboratory data, such as nuclear binding energies, dipole resonance energies, and neutron skin thickness determinations are also discussed for comparison. [Preview Abstract] |
Sunday, September 18, 2005 2:30PM - 3:00PM |
2WH.00002: Pycnonuclear burning on accreting neutron stars and constraints on the nuclear equation of state Invited Speaker: Many neutron stars accrete H- and He-rich matter from a stellar companion. Over the lifetime of the binary, enough matter can be transferred to replace the crust of neutron star. As the material is compressed, the rising electron Fermi energy induces electron captures. When the ionic charge becomes sufficiently small, the zero-point motion of the ions induces the pycnonuclear fusion of nuclei. These reactions release approximately 1~MeV per accreted nucleon deep in the crust, where the thermal diffusion time is years to decades. 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 surface is detectable with X-ray telescopes such as \emph{Chandra} and \emph{XMM}. Observations of transiently accreting neutron stars thus provide a means to infer the neutrino emissivity of the core, complimentary to observations of isolated neutron stars. In this talk, I compare theory and observations of transiently accreting neutron stars. Observations can detect the thermal relaxation of the neutron star crust following the end of rapid accretion, and appear consistent with an enhanced neutrino emissivity, such as direct Urca, from the core. The temperature of the crust also sets, in part, the depth at which carbon unstably ignites. Unlike the transients, the recurrence time and energetics of unstable carbon burning are more consistent with a hotter neutron star crust, as if the neutrino emissivity were suppressed. I discuss ongoing work to improve the nuclear physics input for models of the neutron star crust. [Preview Abstract] |
Sunday, September 18, 2005 3:00PM - 3:30PM |
2WH.00003: Measuring Neutron Star Radii and the Dense Matter Equation of State Invited Speaker: Normal stars -- like our sun, which is $\approx 10^{6}$ km in radius -- have their size governed by the gas equation of state, which is well understood. White dwarfs -- $\approx$ 5000 km -- have their size governed by electron degeneracy pressure, also well understood. Neutron stars -- $\approx 10$ km -- have their size governed by the dense matter equation of state (dEOS), which is not well understood. Measuring neutron star radii can place important constraints on the dEOS. Precise measurements of neutron star radii have only recently been made possible, using X-ray spectroscopy from modern observatories: NASA's Chandra X-ray Observatory, and ESA's XMM-Newton Observatory. I will review the theoretical background which makes these measurements possible, the observational results to date, the resulting constraints on the dEOS, and future prospects for improved constraints. [Preview Abstract] |
Sunday, September 18, 2005 3:30PM - 4:00PM |
2WH.00004: Phases of Dense Quark Matter and Neutron Star Structure Invited Speaker: We review recent work on the phase structure of dense quark matter. At densities of relevance to compact objects the strange quark mass is not negligible compared to quark chemical potential. We discuss the role of the strange quark mass in determining the phase structure. The color-flavor-locked (CFL) phase, the 2SC (two-color superconducting) phase, gapless phases, and the LOFF phase as well as Bose condensates may all play a role. We discuss how these quark matter phases may affect observable aspects of neutron star evolution and the neutrino signal in core-collapse supernovae. We highlight a recent calculation which includes the six-fermion interactions in the quark-quark channel for the first time. [Preview Abstract] |
Sunday, September 18, 2005 4:00PM - 4:30PM |
2WH.00005: Origin of r-process elements Invited Speaker: Astrophysical sites for the r-process are still unknown. Recent observations indicate that there is a scatter of the r-process elements to iron ratios in metal-poor stars. In addition, there is a scatter of light r-process elements (Sr, Y, Zr) to heavy r-process elements (heavier than Ba) ratios. Those results imply that there are at least two different r-processes, main r-process and weak r-process. As the candidates of main r-process site, Type II supernovae and neutron star mergers have been discussed by several authors. I will show new nucleosynthesis calculations in high entropy environments (Type II SNe) and low entropy environments (neutron star merger). Our results suggest that it is difficult to reproduce observed abundance distribution with r-process in low entropy environments. On the other hand, the mass range of progenitor of r-process supernovae is also a point of issue. Two different authors support main r-process in light mass progenitor supernovae based on their Galactic chemical evolution models. However, there are uncertainties related to models. I will discuss the possibilities of main r-process in massive Type II supernovae. [Preview Abstract] |
Sunday, September 18, 2005 4:30PM - 5:00PM |
2WH.00006: The future of neutron rich matter in heaven and earth Invited Speaker: Neutron stars and other compact astrophysical objects are made of neutron rich matter. We describe a variety of laboratory experiments that indirectly probe neutron rich matter using beams of electrons, heavy ions, and radioactive isotopes. The Parity Radius Experiment (PREX) aims to measure the neutron radius of $^{208}$Pb using parity violating electron scattering. Precisely measuring the neutron rich skin of a heavy nucleus determines the density dependence of the symmetry energy, or how the energy rises for systems with excess neutrons. This has many implications for the crust, composition, radius, and cooling rate of neutron stars. PREX is a precision experiment on a stable nucleus and yields complimentary information to experiments with neutron rich radioactive beams. Next, we present semiclassical molecular dynamics simulations of the nonuniform neutron rich matter in the inner crust of a neutron star. This complex matter is called nuclear pasta and results from competition between nuclear attraction and coulomb repulsion. Pasta phases are closely related to the multiple large fragments formed in some heavy ion collisions. The properties of pasta may be important for the electromagnetic, neutrino, and gravitational radiations of neutron stars. Finally, we present the model independent equation of state of low density nuclear matter based on the virial expansion using nn, n-alpha, and alpha-alpha elastic scattering phase shifts. [Preview Abstract] |
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