arXiv subjects

F. A. Rasio

Publications and source records attributed to F. A. Rasio.

43 records · Page 3Linked to original sources

arXiv1994

Secular Evolution of the Eccentricity in the PSR 1620-26 Triple System

A simple analytic calculation is presented for the secular evolution of the eccentricities in a hierarchical triple system such as the one containing the millisecond pulsar PSR 1620-26 in M4. If the second companion of PSR 1620-26 is of stellar mass ($m_2\ga0.1\,M_\odot$), an eccentricity as large as that observed today for the inner binary ($e_1\simeq0.03$) could very well have been induced by secular perturbations. In contrast, such a large eccentricity cannot be induced by a second companion of planetary mass.

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arXiv1994

Ejection of Fragments in Supernova Explosions

Recent observations by the ROSAT X-ray satellite of the Vela supernova remnant have revealed, in addition to the previously identified compact nebula, a nearly circular emitting region with a radius of about 4 degrees. The Vela pulsar is slightly off the center of this circular region, consistent with its measured proper motion of about 100km/s and an age of about 10^4 yr. The emitting region is bounded by the main supernova shock. Just outside the shock, the X-ray image reveals several well-defined V-shaped features extending radially outwards. These features are most likely wakes produced by objects moving supersonically through the outside medium. The shapes and orientations of the wakes suggest that these objects have been ejected from the center of the supernova explosion. Their present positions indicate that they have been moving with a mean velocity of a few thousand km/s. We show that pre-existing objects, such as planets in orbit around the progenitor star, could not have been accelerated to sufficiently high velocities or would have been destroyed. Instead, we propose that the observed objects are fragments ejected during the formation of the neutron star. Fragmentation during gravitational collapse is a natural consequence of both convective and rotational instabilities.

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arXiv1994

Is There a Planet in the PSR 1620-26 Triple System?

The unusually large eccentricity ($e_1=0.025$) of the low-mass binary millisecond pulsar PSR B1620-26 can be explained naturally as arising from the secular perturbation of a second, more distant companion. Such a triple configuration has been proposed recently as the most likely cause of the anomalous second period derivative of the pulsar. The current timing data are consistent with a second companion mass $m_2$ as low as $\sim10^{-3}\,M_\odot$, i.e., comparable to that of Jupiter. However, {\em if\/} the eccentricity is indeed produced by secular perturbations, then the second companion must be another star, most likely of mass $m_2\lo1M_\odot$ and in a very eccentric ($e_2\go0.5$) orbit of period $P_2\sim10^2$--$10^3\,$yr. A second companion of planetary mass cannot induce the observed eccentricity. Independent of the mass of the second companion, small changes in the binary pulsar's orbit should become detectable with just a few additional years of timing data. This detection would provide direct confirmation of the triple nature of the system, and an accurate measurement of the effects would place important new constraints on the orbital parameters.

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arXiv1994

Collapse of Primordial Gas Clouds and the Formation of Quasar Black Holes

The formation of quasar black holes during the hydrodynamic collapse of protogalactic gas clouds is discussed. The dissipational collapse and long- term dynamical evolution of these systems is analysed using three-dimensional numerical simulations. The calculations focus on the final collapse stages of the inner baryonic component and therefore ignore the presence of dark matter. Two types of initial conditions are considered: uniformly rotating spherical clouds, and irrotational ellipsoidal clouds. In both cases the clouds are initially cold, homogeneous, and not far from rotational support ($T/|W| \approx 0.1$). Although the details of the dynamical evolution depend sensitively on the initial conditions, the qualitative features of the final configurations do not. Most of the gas is found to fragment into small dense clumps, that eventually make up a spheroidal component resembling a galactic bulge. About 5\% of the initial mass remains in the form of a smooth disk of gas supported by rotation in the gravitational potential well of the outer spheroid. If a central seed black hole of mass $\gsim10^6M_\odot$ forms, it can grow by steady accretion from the disk and reach a typical quasar black hole mass $\sim10^8M_\odot$ in less than $5\times 10^8\,$yr. In the absence of a sufficiently massive seed, dynamical instabilities in a strongly self- gravitating inner region of the disk will inhibit steady accretion of gas and may prevent the immediate formation of a quasar.

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arXiv1994

Hydrodynamics of Binary Coalescence.I. Polytropes with Stiff Equations of State

We have performed a series of three-dimensional hydrodynamic calculations of binary coalescence using the smoothed particle hydrodynamics (SPH) method. The initial conditions are exact polytropic equilibrium configurations with $\gam > 5/3$, on the verge of dynamical instability. We calculate the emission of gravitational radiation in the quadrupole approximation. The fully nonlinear development of the instability is followed until a new equilibrium configuration is reached. We find that the properties of this final configuration depend sensitively on both the compressibility and mass ratio. An {\em axisymmetric} merged configuration is always produced when $\gam\lo2.3$. As a consequence, the emission of gravitational radiation shuts off abruptly right after the onset of dynamical instability. In contrast, {\em triaxial\/} merged configurations are obtained when $\gam\go2.3$, and the system continues to emit gravitational waves after the final coalescence. Systems with mass ratios $q\ne1$ typically become dynamically unstable before the onset of mass transfer. Stable mass transfer from one neutron star to another in a close binary is therefore probably ruled out. The maximum amplitude $h_{max}$ and peak luminosity $L_{max}$ of the gravitational waves emitted during the final coalescence are nearly independent of $\gam$, but depend very sensitively on the mass ratio $q$.

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arXiv1993

Equilibrium, Stability and Orbital Evolution of Close Binary Systems

We present a new analytic study of the equilibrium and stability properties of close binary systems containing polytropic components. Our method is based on the use of ellipsoidal trial functions in an energy variational principle. We consider both synchronized and nonsynchronized systems, constructing the compressible generalizations of the classical Darwin and Darwin-Riemann configurations. Our method can be applied to a wide variety of binary models where the stellar masses, radii, spins, entropies, and polytropic indices are all allowed to vary over wide ranges and independently for each component. We find that both secular and dynamical instabilities can develop before a Roche limit or contact is reached along a sequence of models with decreasing binary separation. High incompressibility always makes a given binary system more susceptible to these instabilities, but the dependence on the mass ratio is more complicated. As simple applications, we construct models of double degenerate systems and of low-mass main-sequence-star binaries. We also discuss the orbital evolution of close binary systems under the combined influence of fluid viscosity and secular angular momentum losses from processes like gravitational radiation. We show that the existence of global fluid instabilities can have a profound effect on the terminal evolution of coalescing binaries. The validity of our analytic solutions is examined by means of detailed comparisons with the results of recent numerical fluid calculations in three dimensions.

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arXiv1993

Hydrodynamic Instability and Coalescence of Binary Neutron Stars

We study the importance of hydrodynamic effects on the evolution of coalescing binary neutron stars. Using an approximate energy functional constructed from equilibrium solutions for polytropic binary configurations, we incorporate hydrodynamic effects into the calculation of the orbital decay driven by gravitational wave emission. In particular, we follow the transition between the quasi-static, secular decay of the orbit at large separation and the rapid dynamical evolution of configurations approaching contact. We show that a purely Newtonian hydrodynamic instability can significantly accelerate the coalescence at small separation. Such an instability occurs in all close binary configurations containing sufficiently incompressible stars. Calculations are performed for various neutron star masses, radii, spins, and effective polytropic indices. Typically, we find that the radial infall velocity just prior to contact is about 10\% of the tangential orbital velocity. Post-Newtonian effects can move the stability limit to a larger binary separation, and may induce an even larger radial velocity. We also consider the possibility of mass transfer from one neutron star to the other. We show that stable mass transfer is impossible except when the mass of one of the components is very small (less than about 0.4 solar mass) and the viscosity is high enough to maintain corotation.

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