Radius

About: Radius is a research topic. Over the lifetime, 20599 publications have been published within this topic receiving 413557 citations.

Neutron Star Structure and the Equation of State

Abstract: The structure of neutron stars is considered from theoretical and observational perspectives We demonstrate an important aspect of neutron star structure: the neutron star radius is primarily determined by the behavior of the pressure of matter in the vicinity of nuclear matter equilibrium density In the event that extreme softening does not occur at these densities, the radius is virtually independent of the mass and is determined by the magnitude of the pressure For equations of state with extreme softening or those that are self-bound, the radius is more sensitive to the mass Our results show that in the absence of extreme softening, a measurement of the radius of a neutron star more accurate than about 1 km will usefully constrain the equation of state We also show that the pressure near nuclear matter density is primarily a function of the density dependence of the nuclear symmetry energy, while the nuclear incompressibility and skewness parameters play secondary roles In addition, we show that the moment of inertia and the binding energy of neutron stars, for a large class of equations of state, are nearly universal functions of the star's compactness These features can be understood by considering two analytic, yet realistic, solutions of Einstein's equations, by, respectively, Buchdahl and Tolman We deduce useful approximations for the fraction of the moment of inertia residing in the crust, which is a function of the stellar compactness and, in addition, the pressure at the core-crust interface

Chandra sample of nearby relaxed galaxy clusters: mass, gas fraction, and mass-temperature relation

Abstract: We present gas and total mass profiles for 13 low-redshift, relaxed clusters spanning a temperature range 0.7-9 keV, derived from all available Chandra data of sufficient quality. In all clusters, gas temperature profiles are measured to large radii (Vikhlinin et al.) so that direct hydrostatic mass estimates are possible to nearly r_500 or beyond. The gas density was accurately traced to larger radii; its profile is not described well by a beta-model, showing continuous steepening with radius. The derived rho_tot profiles and their scaling with mass generally follow the Navarro-Frenk-White model with concentration expected for dark matter halos in LambdaCDM cosmology. In the inner region (r<0.1r_500), the gas density and temperature profiles exhibit significant scatter and trends with mass, but they become nearly self-similar at larger radii. Correspondingly, we find that the slope of the mass-temperature relation for these relaxed clusters is in good agreement with the simple self-similar behavior, M_500 ~ T^alpha, where alpha=(1.5-1.6)+-0.1, if the gas temperatures are measured excluding the central cool cores. The normalization of this M-T relation is significantly, by =~ 30%, higher than most previous X-ray determinations. We derive accurate gas mass fraction profiles, which show increase both with radius and cluster mass. The enclosed f_gas profiles within r_2500 =~ 0.4 r_500 have not yet reached any asymptotic value and are still far (by a factor of 1.5-2) from the Universal baryon fraction according to the CMB observations. The f_gas trends become weaker and its values closer to Universal at larger radii, in particular, in spherical shells r_2500

Liquids on solid surfaces: static and dynamic contact lines

Abstract: A contact line is formed at the intersection of two immiscible fluids and a solid. That the mutual interaction between the three materials in the immediate vicinity of a contact line can significantly affect the statics as well as the dynamics of an entire flow field is demonstrated by the behavior of two immiscible fluids in a capillary. It is well known that the height to which a column of liquid will rise in a vertical circular capillary with small radius, a, whose lower end is placed into a bath, is given by (2(j/apg) cos (), where (j is the surface tension of the air/liquid interface, f) is the static contact angle as measured from the liquid side of the contact line, p is the density, and g is the magnitude of the accelera­ tion due to gravity.! Thus, depending on the value of the contact angle, e, which is a direct consequence of the molecular interactions among the three materials at the contact line, the height can take on any value within the interval [ 2(J/apg, 2(J/apg]. In a sense, the influence of the contact angle is indirect: the contact angle, in capillaries with small radii, controls the radius of curvature of the meniscus which, in turn, regulates the pressure in the liquid under the meniscus. It is this pressure that determines the height of the column. In a similar manner, the dynamic contact angle can influence the rate of displacement of tbe meniscus through the capillary. The pressure drop

Simulations of X-ray clusters

Abstract: We present simulations of the formation and evolution of galaxy clusters in the Cold Dark Matter cosmogony. Clusters with a wide range of mass were selected from previous N-body models, and were resimulated at higher resolution using a combined N-body/Smooth Particle Hydrodynamics code. The effects of radiative cooling on the gas are neglected. While many present-day clusters are predicted to be undergoing mergers, the density profiles of those that are approximately in equilibrium are all very similar, both for the gas and for the dark matter. These profiles show no sign of a uniform density core and steepen gradually from the centre outwards. The standard $\beta$-model is a reasonable fit over most of the radius range observable in real clusters. However, the value obtained for the slope parameter $\beta_f$ increases with the outermost radius of the fit. Temperature profiles of different simulated clusters are also similar. Typically the temperature is almost uniform in the regions which emit most of the X-ray flux but drops at larger radii. The gas temperature and dark matter velocity dispersion in equilibrium clusters give values of $\beta_T\equiv \mu m_p\sigma_{DM}^2/kT$ which are consistent with unity provided an X-ray emission-weighted temperature is used. Larger values of $\beta_T$ are found in merging objects where there is a transient boost in the velocity dispersion of the system. Thus $\beta_T >1$ may be an observational indicator of merging in real clusters. The similar structure of clusters of differing mass results in scaling relations between the X-ray and dynamical properties of clusters identified at

The Universal Rotation Curve of Spiral Galaxies: I. the Dark Matter Connection

Abstract: We use a homogeneous sample of about 1100 optical and radio rotation curves (RCs) and relative surface photometry to investigate the main mass structure properties of spirals, over a range of 6 magnitudes and out to � 1.5 and 2 optical radii (for the optical and radio data, respectively). We definitely confirm the strong dependence on luminosity for both the profile and the amplitude of RCs claimed by Persic & Salucci (1991). Spiral RCs show the striking feature that a single global parameter, e.g. luminosity, dictates the rotation velocity at any radius for any object, so unveiling the existence of a Universal RC. At high luminosities, there is a slight discrepancy between the profiles of RCs and those predicted from the luminous matter (LM) distributions: this implies a small, yet detectable, amount of dark matter (DM). At low luminosities, the failure of the LM prediction is much more severe, and the DM is the only relevant mass component. We show that the Universal RC implies a number of scaling properties between dark and luminous galactic structure parameters: (a) the DM/LM mass ratio scales inversely with luminosity; (b) the central halo density scales as L 0.7 ; (c) the halo core radius is comparable to the optical radius, but shrinks for low luminosities; (d) the total halo mass scales as L 0.5 . Such scaling properties can be represented as a curve in the (luminosity)-(DM/LM mass ratio)-(DM core radius)-(DM central density) space, which provides a geometrical description of the tight coupling between the dark and the luminous matter in spiral galaxies.