Maximum Central Density Research Articles

The Observed Properties of Dark Matter on Small Spatial Scales

We present a synthesis of recent photometric and kinematic data for several of the most dark matter dominated galaxies, the dwarf spheroidal Galactic satellites, and compare them to star clusters. There is a bimodal distribution in half-light radii, with stable star clusters always being smaller than ~30 pc, while stable galaxies are always larger than ~120 pc. We extend the previously known observational relationships and interpret them in terms of a more fundamental pair of intrinsic properties of dark matter itself: dark matter forms cored mass distributions, with a core scale length of greater than about 100 pc, and always has a maximum central mass density within a narrow range. The dark matter in dSph galaxies appears to be clustered such that there is a mean volume mass density within the stellar distribution which has the very low value of less than about 0.1 M☉ pc-3 (about 5 GeV/c2 cm-3). All dSph's have velocity dispersions at the edge of their light distributions equivalent to circular velocities of ~15 km s-1. The maximum central dark matter density derived is model dependent but is likely to have a characteristic value (averaged over a volume of radius 10 pc) of ~0.1 M☉ pc-3 for the favored cored dark mass distributions (where it is similar to the mean value), or ~60 M☉ pc-3 (about 2 TeV/c2 cm-3) if the dark matter density distribution is cusped. Galaxies are embedded in dark matter halos with these properties; smaller systems containing dark matter are not observed. These values provide new information about the nature of the dominant form of dark matter.

Nucleon superfluidity vs. observations of cooling neutron stars

Cooling simulations of neutron stars (NSs) are performed assuming that stellar cores consist of neutrons, protons and electrons and using realistic density profiles of superfluid critical temperatures $T_{cn}(\rho)$ and $T_{cp}(\rho)$ of neutrons and protons. Taking a suitable profile of $T_{cp}(\rho)$ with maximum $\sim 5 \times 10^9$ K one can obtain smooth transition from slow to rapid cooling with increasing stellar mass. Adopting the same profile one can explain the majority of observations of thermal emission from isolated middle--aged NSs by cooling of NSs with different masses either with no neutron superfluidity in the cores or with a weak superfluidity, $T_{cn} < 10^8$ K. The required masses range from $\sim 1.2 M_\odot$ for (young and hot) RX J0822-43 and (old and warm) PSR 1055-52 and RX J1856-3754 to $\approx 1.45 M_\odot$ for the (colder) Geminga and Vela pulsars. Observations constrain the $T_{cn}(\rho)$ and $T_{cp}(\rho)$ profiles with respect to the threshold density of direct Urca process and maximum central density of NSs.

New dark matter physics: Clues from halo structure

We examine the effect of primordial dark matter velocity dispersion and/or particle self-interactions on the structure and stability of galaxy halos, especially with respect to the formation of substructure and central density cusps. Primordial velocity dispersion is characterized by a ``phase density'' $Q\ensuremath{\equiv}\ensuremath{\rho}/〈{v}^{2}{〉}^{3/2},$ which for relativistically decoupled relics is determined by particle mass and spin and is insensitive to cosmological parameters. Finite Q leads to small-scale filtering of the primordial power spectrum, which reduces substructure, and limits the maximum central density of halos, which eliminates central cusps. The relationship between Q and halo observables is estimated. The primordial Q may be preserved in the cores of halos and if so leads to a predicted relation, closely analogous to that in degenerate dwarf stars, between the central density and velocity dispersion. Classical polytrope solutions are used to model the structure of halos of collisional dark matter, and to show that self-interactions in halos today are probably not significant because they destabilize halo cores via heat conduction. Constraints on masses and self-interactions of dark matter particles are estimated from halo stability and other considerations.

Tidal Stablization of Neutron Stars and White Dwarfs.

Theoretical Astrophysics, 130-33, California Institute of TechnologyPasadena, CA 91125E-mail: dong@tapir.caltech.edu(Accepted for publication in Physical Review Letters, 1996)What happens to a neutron star or white dwarf near its maximum mass limit when it is broughtinto a close binary orbit with a companion? Such situation may occur in the progenitors of Type Iasupernovae and in coalescing neutron star binaries. Using an energy variational principle, we showthat tidal field reduces the central density of the compact object, making it more stable against radialcollapse. For a cold white dwarf, the tidal field increases the maximum stable mass only slightly, butcan actually lower the maximum central density by as much as 30%. Thus a white dwarf in a closebinary may be more susceptible to general relativistic instability than the instability associated withelectron capture and pycronuclear reaction (depending on the white dwarf composition). We analysethe radial stability of neutron star using post-Newtonian approximation with an ideal degenerateneutron gas equation of state. The tidal stablization effect implies that the neutron star in coalescingneutron star-neutron star or neutron star-black hole binaries does not collapse prior to merger ortidal disruption.PACS Numbers: 97.80.Fk, 04.25.Dm, 04.40.Dg, 97.60.Jd

Klinische Bedeutung der altersabhängigen, regional unterschiedlichen Verteilung des menschlichen Hornhautendothels

In 30 donor corneas (age of donors 19-91 years), a total of 1,560,812 cells were counted following supravital endothelial staining in order to map cell distribution. This statistical material was analyzed in the present study in order to determine the following parameters for each cornea: total cell count, mean cell density, central cell density and maximum cell density. These parameters were plotted against the ages of the donors and a regression analysis was performed. A significant decrease (p = 0.001), of an average of 1582 cells per cornea and year was found. Mean cell density and central cell density decreased on average by 16 cells per mm2 and year, while maximum peripheral cell density only decreased by 14 cells per mm2 and year. There was a positive difference, practically unrelated to age, between maximum peripheral cell density minus central cell density, averaging 1374 cells per mm2. This finding makes it appear improbable that there is any recompensation of central age-related cell loss from the peripheral region of high cell density. Irrespective of age, a mean difference of 140 cells per mm2 was found between mean cell density minus central cell density. In regard to clinical specular endothelial microscopy this means that the approximate average cell density of the entire cornea can be deduced from the cell density determined at its center.

Quark matter in the Hartree-Fock approximation

A calculation is made of the critical density ${\ensuremath{\rho}}_{t}$ at which it is more favorable for matter to be in the quark phase than in the nuclear phase. The equation of state of the quark phase is obtained assuming that the quantumchromodynamic effective Lagrangian satisfactorily describes the interaction at all energies, and by making a relativistic Hartree-Fock approximation. The main result we get is that ${\ensuremath{\rho}}_{t}&gt;{\ensuremath{\rho}}_{m}$, the maximum central density of stable neutron stars.

Phase transition to quark matter in neutron stars

A calculation is made of the critical density ϱ t at which it is more favorable for the matter to be in the quark phase than in the nuclear one. The equation of state of the quark phase is obtained assuming that the QCD effective lagrangian satisfactorily describes the interaction at all energies, and by making a relativistic Hartree-Fock approximation. The main result we get is that ϱ t > ϱ m, the maximum central density of stable neutron stars.

PULSARS AND COMPACT X-RAY SOURCES : COSMIC LABORATORIES FOR THE STUDY OF NEUTRON STARS AND HADRON MATTER

PULSARS AND COMPACT X-RAY SOURCES : COSMIC LABORATORIES FOR THE STUDY OF NEUTRON STARS AND HADRON MATTER

Asymptotic freedom and the baryon-quark phase transition

We have calculated the ground-state properties of a quark gas to second order in the quark-gluon coupling constant. Asymptotic freedom has been taken into account by using the renormalized coupling constant of Politzer and Gross and Wilczek. We find that this asymptotically free perturbation theory leads to an equation of state for a quark gas which for pressure P > 0 is very similar to the equation of state obtained from the MIT bag model of hadrons. In particular, we can identify a ''bag pressure'' term in the perturbation theory expression for the pressure as a function of density. We obtain estimates for the baryon-quark transition pressure by comparing the perturbation theory results for the Gibb's energy per baryon of baryonic matter. Our calculations show that the baryon-quark transition takes place at densities on the order of 10--20 times that in ordinary nuclei. These transition densities are higher than the maximum central density calculated for a neutron star.

On the maximum gravitational redshift of white dwarfs

view Abstract Citations (10) References (9) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the maximum gravitational redshift of white dwarfs. Shapiro, S. L. ; Teukolsky, S. A. Abstract The stability of rigidly rotating cold white dwarfs of uniform composition is analyzed by the energy method in the framework of the parameterized post-Newtonian (PPN) formalism of Will and Nordtvedt (1973). The maximum central density and corresponding maximum gravitational redshift of a white dwarf are determined in terms of a dimensionless constant that is a function of five of the nine PPN parameters, and the effects of uniform rotation on the stability limit are considered. The results show that general relativity predicts a maximum observable redshift of 571 km/s for a stable white dwarf composed of carbon or helium and a lower value for stars composed of heavier nuclei (in which case neutronization determines the maximum value). It is found that uniform rotation can increase the maximum redshift to 647 km/s in the case of carbon and to 893 km/s in the case of helium. It is concluded that a systematic survey of white dwarf redshifts may help to determine the composition of their cores and to confirm theoretical calculations of the late stages of stellar evolution for stars that are smaller than about 4 solar masses. Publication: The Astrophysical Journal Pub Date: February 1976 DOI: 10.1086/154130 Bibcode: 1976ApJ...203..697S Keywords: Gravitation Theory; Red Shift; Relativistic Theory; White Dwarf Stars; Gravitational Fields; Heavy Nuclei; Stellar Rotation; Astrophysics full text sources ADS |

The structure of degenerate stars and their minimal properties

In this paper in section I the structure of massive fluid spheres (stars) which are composed of completely degenerate matters is investigated. Equations of equilibrium govering such configurations have been put in convenient forms. In section II a few minimal properties of the stars, such as the maximum (or minimum) central energy density and temperature, etc., have been studied. It has further been shown that for nondegenerate matters the reductions in maximum central energy density and temperature are proportional to 1/K33c−2 and 1/K3 respectively (c=2.998×1010 cm/sec andK3 is assumed to be greater thanK1(=3.17×1012).

The density distribution near the base of the mantle and near the earth's center

A detailed study of times and amplitudes of P waves in the range 95° < Δ < 120° from two intermediate focus earthquakes has provided new information on the physical state of the transition shell D″ just above the core. P waves of period near 2 s are clearly observed out to 118° (Uinta Basin Observatory). For 105° < Δ < 115° the mean wave slowness is 4.55 ± 0.05 s/°. This value and empirical PcP times then yield, relative to the 1968 P travel-times, a least squares estimate for a core radius of 3475 ± 2 km; from 13.63 km/s at the top of D″, the P velocity drops to an average of 13.33 ± 0.15 km/s towards the mantle-core boundary. Short-period P waves in the core shadow do not attenuate like pure diffracted waves; the notation Pc rather than dif PcP is suggested. The P and S velocity decrease in D″ entails an upper limit in density of about 5.9 g/cm 3 at the mantle-core boundary. Reflections of steeply incident PKiKP short-period waves from the inner core boundary provide an estimate of the density contrast there. They also entail that there must be a jump in incompressibility and/or density (and not just in rigidity). Three measurements of the amplitude ratio PKiKP/PcP give a weighted mean value for the minimum density ratio across the inner core boundary of 0.87 ± 0.04. The maximum central density is then about 14.10 g/cm 3. A recent, well-constrained, P velocity model KOR5 for the central core is now available as a basis for inferences on physical properties. Computation of many central core models compatible with KOR5 then demonstrates that the central density probably lies between 13.0 and 14.0 g/cm 3. A value near the lower limit is more compatible with shock wave experiments on iron. A model CAL 1G for the central core with g9 0 = 13.0 g/cm 3 and which satisfies all available constraints has k 0 = 15.05 × 10 12 dyne/cm 2, μ 0 = 1.25 × 10 12 dyne/cm 2. The corresponding seismic velocities at the center are α 0 = 11.35 and β 0 = 3.10 km/s, respectively. A formula is derived which gives the central density directly from the gradient of the empirical P and S velocity distributions in the inner core.

A Monte Carlo simulation of air shower cores

The electromagnetic and strongly interacting particle distributions falling on the Sydney 64 scintillator array for a variety of primary particles (from protons to iron) of a given incident total energy have been simulated using Monte Carlo and analytic methods. For a given primary particle, various fundamental nucleon–nucleon and pion–nucleon interactions have been tried. The results show that a number of characteristics–e.g. the average type of core structure (multiple or single), the mean maximum central electron density, and the mean maximum energy of a strongly interacting particle–are all strongly dependent on the nature of the primary. They are only weakly dependent on, or are independent of the nature of the fundamental interaction within the range covered.The results are compared with experimental results from the Sydney 64 scintillator array.

SOME EFFECTS OF NUCLEAR FORCES ON NEUTRON-STAR MODELS

Two composite equations of state have been used in the investigation of the structure of neutron (or hyperon or baryon) stars. These have been based upon two forms of the neutron–neutron potential suggested by Levinger and Simmons. In one form, repulsive forces come in quickly at greater than nuclear densities, while, in the other form, the repulsive forces come in slowly. In the former case the maximum stable mass of a neutron star is about two solar masses, whereas in the latter case it is only about one solar mass. This probably represents a measure of the basic uncertainty in the properties of neutron-star models due to our lack of knowledge of nuclear forces. The maximum central density of a stable configuration is similarly uncertain; this density probably lies in the range 1015 to 1016 g/cm3. Details of many of the neutron-star models calculated are summarized and discussed.

Possibility of Explosive He3 (e-,ν) T3 and T3 (p,γ)He4 Reactions in Low Mass Stars.

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Possibility of Explosive He3 (e-,ν) T3 and T3 (p,γ)He4 Reactions in Low Mass Stars. Iben, Icko, Jr. Abstract At sufficiently high densities (P > Pm2 x t0~ g/cm1) the p-p chain may be closed by the reactions He1 + e- + t 8 keV T'+v and T1+p~He4+~+ t9.8 MeV. Since the first of these reactions is extremely sensitive to changes in density (Bahcall, J. N., private communication) and the second reaction is very rapid in any case, it is of interest to determine whether or not conditions in a star become favorable for this method of closure to occur explosively. On the basis of models that appear in the literature, regions that attain densities in excess of Pm in stars of one solar mass or more are depleted of H1 and He1. If no explosive nuclear reactions occur, the evolution of stars of mass less than 0.5 M0 may be followed fairly simply to the completely degenerate stage. The maximum central density for a starof mass 0.0t ~M/M0 ~0.50 followsthe low (Pc)rns1~4.22Xt06(M/M0)2/(t+X)1. The method of closure in question is therefore limited to stars of mass in excess of 0.07 M0. Whether stars in the range 0.07 ~ M/M0 ~0.5 will attain central densities in excess of Pm before burning up theirH1andH~, and whether the nuclear fuel remaining is sufficient to disrupt the star can be answered by constructing evolutionary sequences. Preliminary results indicate that regions attaining P > Pm are devoid of H1 and He1. This work was supported by the Office of Naval Research and The National Aeronautics and Space Administration. Publication: The Astronomical Journal Pub Date: September 1963 DOI: 10.1086/109065 Bibcode: 1963AJ.....68Q.538I full text sources ADS |