Showing papers in "The Astrophysical Journal in 1973"

Perturbations of a rotating black hole. I. Fundamental equations for gravitational, electromagnetic, and neutrino-field perturbations

Abstract: Decoupled, separable equations describing perturbations of a Kerr black hole are derived. These equations can be used to study black-hole processes involving scalar, electromagnetic, neutrino or gravitational fields. A number of astrophysical applications are made: Misner's idea that gravitational synchrotron radiation might explain Weber's observations is shown to be untenable; rotating black holes are shown to be stable against small perturbations; energy amplification by "superradiant scattering" of waves off a rotating black hole is computed; the "spin down" (loss of angular momentum) of a rotating black hole caused by a stationary non-axisymmetric perturbation is calculated.

The formation of planetesimals.

Abstract: Four stages in the accretion of planetesimals are described. The initial stage is the condensation of dust particles from the gaseous solar nebula as it cools. These dust particles settle into a thin disk which is gravitationally unstable. A first generation of planetesimals, whose radii range up to about 0.1 km, form from the dust disk by direct gravitational collapse to solid densities on a time scale of the order of 1 year. The resulting disk, composed of first-generation planetesimals, is still gravitationally unstable, and the planetesimals are grouped into clusters containing approximately 10,000 members. The contraction of these clusters is controlled by the rate at which gas drag damps their internal rotational and random kinetic energies. On a time scale of a few thousand years, the clusters contract to form a second generation of planetesimals having radii of the order of 5 km. Further coalescence of planetesimals proceeds by direct collisions which seem capable of producing growth at a rate of the order of 15 cm per year at 1 AU.

Observations of Gamma-Ray Bursts of Cosmic Origin

Abstract: Sixteen short bursts of photons in the energy range 0.2–1.5 MeV have been observed between 1969 July and 1972 July using widely separated spacecraft. Burst durations ranged from less than 0.1 s to ∼30 s, and time‐integrated flux densities from ∼10−5 ergs cm−2 to ∼2 × 10−4 ergs cm−2 in the energy range given. Significant time structure within bursts was observed. Directional information eliminates the Earth and Sun as sources.

Neutron-star accretion in a stellar wind - Model for a pulsed X-ray source.

Abstract: Description of a relatively simple stellar-wind-driven accretion model that appears to satisfactorily account for many of the observed features of the pulsing X-ray sources Cen X-3 and Her X-1 A rotating magnetized neutron star orbits a more massive slightly evolved star and accretes mass from the stellar wind emanating from that star Some of the implications of the model are discussed

A Model for Compact X-Ray Sources: Accretion by Rotating Magnetic Stars

Abstract: The physics of accretion onto compact stars is considered, taking into account both the effects of stellar rotation and a stellar magnetic field. We show that far from the star the stellar magnetic field is screened by currents flowing in the accreting plasma, while close to the star the stellar field forces matter to corotate with the star. The location of the Alfven surface, where the transition between the two regimes occurs, depends on the flow pattern of the accreting matter beyond the Alfvea surface and the rotation period of the compact star, as well as the mass accretion rate and the strength of the stellar magnetic field. Three types of flow pattern are considered: radial inflow toward the compact object, orbital motion about it, and streaming motion past it. For the accretion rates of interest the radius of the Alfven surface r/sub A/, is found to be ~10/sup 7/-10/sup 8/ cm for a typical neutron star and ~ 10/sup 8/ -10/sup 10/cm for a typical magnetic degenerate dwarf: For neutron stars it is shown that inside the Alfvea surface there is comparatively little flow of matter across field lines, so that accreting matter is channeled toward the magnetic polesmore » of the star where it forms hot spots. The resulting radiation is shown to emerge from the neighborhood of the stellar surface in a strongly anisotropic angular pattern with a spectrum which depends on the details of the accretion process; for an oblique rotator, one has a natural mechanism for the production of pulsed radiation, which is not generally expected to he blackbody in character, will be in excess of 6 keV. Comparison of the above model with the observations suggests strongly that the X-ray stars in the pulsating binary X-ray sources Cen X-3 and Her X-1 are accreting neutron stars with their magnetic axes inclined at substantial angles to their axes of rotation. The implications of this interpretation for the observed X-ray spectra and pulse wave-forms are discussed. The change in the X-ray pulsation period due to accretion of matter is calculated; the calculated time scales for the spin-up which occurs when there is orbital inflow toward a star rotating ia the same sense as the orbital motion, is in excellent agreement with these observed in Her X-1 and Gn X-3. The possibility of significant optical pulsations and the likely existence of a minimum pulsation period for accreting X-ray stars are discussed. (auth)« less

Structure of the Solar Chromosphere. Basic Computations and Summary of the Results

Abstract: A procedure is demonstrated that is used to compute a one-component model of the solar atmosphere, including in that model the photosphere, chromosphere, and chromosphere-corona transition zone. In particular, the equations are described that are solved in order to determine this theoretical model.

Screening Factors for Nuclear Reactions. 11. Intermediate Screen-Ing and Astrophysical Applications

Abstract: The theory of intermediate screening is developed from the cluster expansion for the screening function. This systematic perturabation technique is used to extend weak-screening theory analytically to higher order in the scrcening parameters, and is solved numerically to yield screening-function data for intermediate and strong screening regions. for both the one-component system, equal charge reactions, and the two-component system in the form of 10 mixtures of low-Z elements, the resultant intermediate-screening function is shown to obey a simple power-law dependence on the screening parameter, H12(0) xf(z/sub i/) LAMBDA /sub 12/ sup b/, where b=0.860. This result is found to be equivalent to the general screening function presented in Paper I. The new screening thceory is studied in relation to various astrophysical situations. Three areas are found where screen~ng effects may produce quantitative changes in evolutionary star models: minimum mass limits for H, D, He, C, and 0 main sequences; the structure and extent of flash events in red-giant interiors; and the strength of detonations in degenerate carbon and oxygen cores. This latter case may require screening-factor increases of two orders of magnitude. (auth)

Gravitational-Radiation Recoil and Runaway Black Holes

Abstract: In the nonspherical gavitational collapse of a stellar core to a black hole, linear momentum will in general be radiated with gravitational waves. As a result the black hole will recoil. Here we make a rough estimate of the possible recoil velecities by means of the linearized theory of gravitational waves extended to ectopole order. Velocities ranging from small values for nearly up- down symmetrical collapse up to a few hundred kilometers per second for highly asymmetrical collapse may be possible. We also explore some likely consequences of black-hole recoil: breakup of a binary upon collapse of one of its components, runaway binaries with a black-hole component, detachment of the stellar envelope from the black hole formed by the collapsed stellar core, and escape of black holes from globular clusters and the Galaxy. (auth)

8 to 13 micron spectra of NGC 7027, BD + 30$sup 0$3639, and NGC 6572

Abstract: Observations of the 8 to 13, mu spectra of the planetary nebulae NGC 7027, BD + 30 deg 3639, and NGC 6572 are presented. The spectra consist of both fine-structure line radiation and continuum radiation. The results are discussed in terms of abundances, predicted line strengths, and composition of radiating material. (auth)

ALFVtN Waves in a Two-Fluid Model of the Solar Wind

Abstract: A two-fluid model for the solar wind is presented that includes the presence of Alfven waves which originate at the sun. The effective pressure of these Alfven waves is considered, as well as a model representation for proton heating through nonlinear Alfven wave damping. The effects of rotation in the solar equatorial plane are allowed for. The implications of the computational results obtained are discussed.