Showing papers in "The Astrophysical Journal in 1979"

Parameter estimation in astronomy through application of the likelihood ratio

Abstract: Many problems in the experimental estimation of parameters for models can be solved through use of the likelihood ratio test. Applications of the likelihood ratio, with particular attention to photon counting experiments, are discussed. The procedures presented solve a greater range of problems than those currently in use, yet are no more difficult to apply. The procedures are proved analytically, and examples from current problems in astronomy are discussed.

Relativistic jets as compact radio sources

Abstract: Variable extragalactic radio sources, associated with the nuclei of galaxies and quasars, are interpreted in terms of a supersonic relativistic jet. It is proposed that radio emission originates both from the quasi-steady jet itself and from behind strong shock waves which either propagate in the jet, or which are formed behind dense condensations (clouds) that are accelerated to relativistic speeds by the flow. In this way the source could display apparent superluminal expansion in which the moving component (associated with a shock) and the stationary component (associated with the optically-thick core of the jet) would have comparable, Doppler-boosted fluxes. Specific models for the dynamical and radiative properties of the jet and of individual shocks are presented. Kinematical consequences of the relativistic motion are described for flux and polarization measurements, as well as for VLBI observations of superluminal sources. It is argued that the majority of bright compact sources are observed along lines of sight making small (< or approx. =10/sup 0/) angles to the jet velocity. This hypothesis has important consequences for the interpretation of low-frequency variable sources, optically-violent variable quasars, Lacertids, and extended double sources. These are briefly outlined, and some specific observational tests are proposed.

Accretion by rotating magnetic neutron stars. III. Accretion torques and period changes in pulsating X-ray sources.

Abstract: The solutions of the two-dimensional hydromagnetic equations are used to calculate the torque on a magnetic neutron star accreting from a Keplerian disk. It is found that the magnetic coupling between the star and the plasma in the outer transition zone is appreciable; that as a result, the spin-up torque on fast rotators is substantially less than that on slow rotators, and that for sufficiently high stellar angular velocities or sufficiently low mass accretion rates, the rotation of the star can be braked while accretion continues. These results are applied to pulsating X-ray sources, revealing that at high luminosities a star of given spin period rotating in the same direction as the disk can experience either spin-up or spin-down, depending on its luminosity. Also considered are the general problem of interpreting period changes in pulsating X-ray sources, and the dipole magnetic moments of nine pulsating X-ray sources are estimated by fitting the theoretical spin-up equation to estimates of the average luminosity and spin-up rate of each source.

A theory of the terrestrial kilometric radiation

Abstract: During magnetospheric substorms, electrons with energies of about 1 keV are injected from the plasma-sheet region into the auroral region. A fraction of these energetic electrons can precipitate into the upper atmosphere, and the rest are reflected because of the mirror effect of the convergent geomagnetic field. It is found that these reflected electrons can result in the amplification of electromagnetic waves via a relativistic normal cyclotron resonance. This process may explain the recently discovered terrestrial kilometric radiation.

Eccentric orbit generalization and simultaneous solution of binary star light and velocity curves

Abstract: The computation of binary star light and radial velocity curves are discussed, including the effects of eccentric orbits and nonsynchronous rotation. Logical relations needed to impose physical constraints (semidetached condition, etc.) are specified. Some of the more important mathematical relations are also given. One can now model semidetached, detached, double-contact, and X-ray binaries for arbitrary rotation rates and orbital eccentricity. Contact binaries can also be modeled, but are restricted to the synchronous, circular orbit case. In all cases the figures of the components and the surface gravity fields are described by surfaces of constant potential energy.

The excitation of density waves at the Lindblad and corotation resonances by an external potential.

Abstract: We calculate the linear response of a differentially rotating two-dimensional gas disk to a rigidly rotating external potential. The main assumptions are that the sound speed is much smaller than the orbital velocity and that the external potential varies on the scale of the disk radius. We investigate disks both with and without self-gravity. The external potential exerts torques on the disk only at the Lindblad and corotation resonances. The torque is positive at the outer Lindblad resonance and negative at the inner Lindblad resonance; at corotation the torque has the sign of the radial gradient of vorticity per unit surface density. The torques are of the same order of magnitude at both types of resonance and are independent of the sound speed in the disk. The external potential also excites density waves in the vicinity of the Lindblad and corotation resonances. The long trailing wave is excited at a Lindblad resonance. It transports away from the resonance all of the angular momentum which is deposited there by the external torque. Short trailing waves are excited at the corotation resonance. The amplitudes of the excited waves are the same on both sides of the resonance and are small unless the disk is almost gravitationally unstable. No net angular momentum is transported away from the corotation region by the waves. Thus the angular momentum deposited there by the external torque accumulates in the gas. We briefly discuss the behavior of particle disks and prove that the external torques on particle disks are identical to those on gas disks.

On the physics of dust grains in hot gas.

Abstract: Charging of dust grains in hot (10/sup 4/--10/sup 9/ K) plasma is studied, including photoelectron and secondary electron emission, field emission, and transmission of electrons and ions through the grain; resulting grain potentials are (for T > or approx. = 10/sup 5/ K) considerably smaller in magnitude than found by Burke and Silk. Even so, large electrostatic stresses can cause ion field emission and rapid destruction of small grains in very hot gas. Rapid rotation can also disrupt small grains, but damping (by microwave emission) usually limits the centrifugal stress to acceptable values for plasma densities n/sub H/< or approx. =1 cm/sup -3/. Sputtering rates are estimated for grains in hot gas, based upon a semiempirical fit to experimental data. Predicted sputtering rates for possible grain constituents are similar to estimates by Barlow, but in some cases differ significantly. Useful approximation formulae are given for the drag forces acting on a grain with arbitrary Mach number.

Structured coronae of accretion disks.

Abstract: A model for the fluctuating hard component of intense cosmic X-ray sources (such as Cyg X-1) is developed, based upon the amplification of magnetic fields by convective motions and differential rotation within a hot (T> or approx. =10/sup 6/K) accretion disk. Field reconnection within the inner portion of the disk is shown to be ineffective in limiting field amplification; magnetic fields may therefore attain strengths comparable to the equipartition value, leading to their emergence via buoyancy in the form of looplike structures and resulting in a very hot (T>10/sup 8/K) magnetically confined, structured corona analogous to the observed structure of the solar corona. The energy balance of these loop structures is examined, and it is shown that the disk soft X-ray luminosity determines the predominant energy loss mechanism in loops: at low disk luminosities, thermal bremstrahlung from these loops dominates and contributes a steady, shot-noise--like hard X-ray component. At high disk luminosities the emerging loops are Compton-cooled; the soft X-ray flux from the disk is Comptonized by the emerged loops, forming a transient, flarelike hard X-ray component.

Accretion by rotating magnetic neutron stars. II. Radial and vertical structure of the transition zone in disk accretion.

Abstract: The radial and vertical structure of the transition zone at the magnetospheric boundary of an aligned rotating neutron star accreting matter from a Keplerian disk are calculated. The results obtained indicate that: (1) the inner edge of the disk is located where the integrated magnetic stress acting on the disk plasma becomes comparable to the integrated material stress associated with its inward radial drift and orbital motion; (2) the stellar magnetic field threads the disk near its inner edge via the Kelvin-Helmholtz instability, turbulent diffusion, and reconnection, producing a broad transition zone between the unperturbed disk flow and corotating magnetosphere; (3) the transition zone consists of two qualitatively different regions, viz., a broad outer transition zone where the motion is Keplerian and a narrow inner zone, or boundary layer, where the departure from Keplerian motion is substantial; (4) the stellar magnetic field is largely but not entirely screened by currents flowing in the boundary layer; and (5) there are no steady-flow solutions for sufficiently fast stellar rotation.

Pair formation above pulsar polar caps: Structure of the low altitude acceleration zone

Abstract: Solutions are presented for the space-charge-limited flow of electrons above the polar caps of a pulsar whose magnetic moment has a component parallel to its spin angular momentum, when the accelerating electric field is shorted out by the formation of electron-positron pairs. Only the oblique rotator is considered. Some predictions of the soft X-ray luminosity of the polar caps of the Vela pulsar are made, based on the heating of the star's polar caps by the back flux of positrons., with results in accord with the known maximum periods for pulsars, if the surface field has sufficient complexity. Estimates of the final density and momentum distribution function of the pairs are given. The power generated by each polar acceleration zone is shown to be L/sub pole/approx.5 x 10/sup 30/P/sup -19/8/ ergs s/sup -1/ if the surface magnetic field has strength approx.10/sup 12/ gauss, sufficient to explain most radio pulsars but insufficient to account for the X-ray and ..gamma..-ray emission from the Crab pulsar. A mechanism for further energization of the pair plasma at high altitude is proposed, based on the interaction between the plasma and the partial vacuum zone surrounding the plasma stream. It is pointed out that the modelmore » is consistent with the phenomenological hollow cone model for pulsar wave forms and with the single-pole interpretation of interpulses, since the flow is asymmetric with respect to the magnetic axis. Some predictions of the soft X-ray luminosity of the polar caps of the Vela pulsar are made, based on the heating of the star's polar caps by the back flux of positrons.« less

Pulsar timing measurements and the search for gravitational waves

Abstract: Pulse arrival time measurements of pulsars may be used to search for gravitational waves with periods on the order of 1 to 10 years and dimensionless amplitudes approx.10/sup -11/. The analysis of published data on pulsar regularity sets an upper limit to the energy density of a stochastic background of gravitational waves, with periods approx.1 year, which is comparable to the closure density of the universe.

Stellar lifetimes and abundance ratios in chemical evolution.

Abstract: Models for chemical evolution and stellar abundances are used to test assumptions about the nucleosynthesis of various elements. For primary elements, the emphasis is on empirical evidence that carbon and iron, but not oxygen, are ejected in significant quantities by longer-lived stars than those proposed by Arnett for their entire nucleosynthesis, which all have lifetimes <2 x 10/sup 7/ years.

Viking relativity experiment - Verification of signal retardation by solar gravity

Abstract: Analysis of 14 months of data obtained from radio ranging to the Viking spacecraft verified, to an estimated accuracy of 0.1%, the prediction of the general theory of relativity that the round-trip times of light signals traveling between the earth and Mars are increased by the direct effect of solar gravity. The corresponding value for the metric parameter gamma is 1.000 plus or minus 0.002, where the quoted uncertainty, twice the formal standard deviation, allows for possible systematic errors.

Destruction mechanisms for interstellar dust

Abstract: Destruction rates are estimated for interstellar dust grains as a function of size and composition of the grains, and the type of region in which the grain is located. Several idealized models of the interstellar medium are considered. The destruction mechanisms examined include sputtering and grain-grain collisions in interstellar shocks, sputtering and sublimation in H II regions, photodesorption by UV, sputtering by cosmic rays, and sublimation during a supernova radiation pulse. Sublimation by the supernova radiation pulse is found to be very important for the more volatile mantle materials (binding energy U/sub o/ or =150 km s/sup -1/) shocks and grain-grain collisions in low-velocity (upsilon/sub s/< or =50 km s/sup -1/) shocks account for most refractory grain destruction. Polymerized ''HCO-material'' is almost as resistant to sputtering as refractory grains. Water icemantles may be marginally able to survive in low-density H II regions (n/sub H/< or approx. =10 cm/sup -3/), especially if the gas temperature is T< or approx. =10/sup 3.8/ K. The implicationsmore » of these destruction rates for the survival of interstellar grains are briefly discussed.« less

Theoretical models of interstellar shocks. I - Radiative transfer and UV precursors

Abstract: Theoretical models of interstellar radiative shocks are constructed, with special attention to the transfer of ionizing radiation. These models are 'self-consistent' in the sense that the emergent ionizing radiation (the UV precursor) is coupled with the ionization state of H, He, and the metals in the preshock gas. For shock velocities of at least 110 km/s the shocks generate sufficient UV radiation for complete preionization of H and He, the latter to He(+). At lower velocities the preionization can be much smaller, with important consequences for the cooling function, the shock structure, and the emission. For models with shock velocities of 40 to 130 km/s the intensities of the strongest emission lines in the UV, optical, and infrared are tabulated, as well as postshock column densities of metal ions potentially observable by UV absorption spectroscopy. Possible applications to supernova remnants and high-velocity interstellar gas are assessed.

X-ray studies of quasars with the Einstein Observatory. II

Abstract: Results of an investigation of the X-ray properties of quasars conducted using the Einstein Observatory (HEAO 2) are reported. The positions, fluxes and luminosities of 35 known quasars were observed by the Einstein high-resolution imaging detector and the imaging proportional counter. Assuming optical redshifts as valid distance indicators, 0.5-4.5 keV X-ray luminosities ranging from 10 to the 43rd to 10 to the 47 ergs/sec are obtained, with evidence of very little cold gas absorption. Flux variability on a time scale of less than 10,000 sec is observed for the quasar OX 169, which implies a mass between 8 x 10 to the 5th and 2 x 10 to the 8th solar masses for the black hole assumed to be responsible for the emission. Preliminary results of the quasar survey also indicate that quasars contribute significantly to the diffuse X-ray background.

The Einstein /HEAO 2/ X-ray Observatory

Abstract: The Einstein (HEAO 2) X-ray Observatory, launched in 1978, includes a fully imaging focusing X-ray telescope with an angular resolution of a few arc sec, a field of view of up to one deg, and a sensitivity several hundred times greater than previously available in any X-ray astronomy experiment. A high-resolution imager, an imaging proportional counter, a focal plane crystal spectrometer, and a monitor proportional counter are among the principal instruments on board the Einstein X-ray Observatory. About 20% of the total effective observing time in the first year of the X-ray astronomy experiment has been reserved for guest observers.

Studies of luminous stars in nearby galaxies. III. Comments on the evolution of the most massive stars in the Milky Way and the large magellanic cloud

Abstract: An empirical comparison of the observed H-R diagrams for the supergiants in our region of the Galaxy and the Large Magellanic Cloud reveals comparable distributions of spectral types and luminosities in the two galaxies. Supergiants of similar spectral types have the same luminosities, except for the A-type stars, where selection effects may be important. These results suggest that the same basic physical processes govern the evolution of the most massive stars in the two galaxies.Variations in the blue-to-red supergiant ratio with galactocentric distance and with luminosity involve chemical composition gradients and varying rates of mass loss. Since the relative numbers of the most luminous stars are more sensitive to mass loss, the B/R ratio from the less luminous supergiants may be a better indicator of galactic abundance gradients.The upper luminosity boundary for both the galactic and the LMC supergiants is characterized by (1) decreasing luminosity with decreasing temperature for the hottest stars and (2) an upper limit to the luminosity near M/sub bol/approx. =-9.5 to -10 mag for stars cooler than 15,000 K. We suggest that the observed luminosity limits are due primarily to the effects of large mass loss on the evolution of the most massive stars. The examplesmore » of eta Car and P Cyg suggest that mass-loss rates can be very rapid and unsteady--higher on the average than presently observed for most of the hot supergiants. The evolution of stars greater than 60 M/sub sun/ to cooler temperatures is consequently limited by instabilities and the accompanying high mass loss. An initial mass near 50--60 M/sub sun/ may be an empirical upper limit to the mass at which a star can evolve to the region of the M supergiants and probably accounts for the observed upper bound to the luminosities of the cooler supergiants.« less

H I shells and supershells

Abstract: The entire sky has now been surveyed in the 21-cm line with an angular resolution of about 05 degree In the north, above declination-20° or so, the “galactic plane” |b|<10° has been completely sampled Weaver and Williams (1973; 1974) Above declination -30° or so, the sky outside the galactic plane has been almost completely sampled (Heiles and Habing, 1974; Heiles, 1975)

Meridional circulation and CNO anomalies in red giant stars

Abstract: The possibility is investigated that meridional circulation driven by internal rotation might lead to the mixing of CNO-processed material from the vicinity of the hydrogen shell into the envelope of a red giant star. This theory of meridional mixing is found to be generally consistent with available data and to be capable of explaining a number of observational results without invoking a radical departure from the standard physics of stellar interiors. It is suggested that meridional circulation must be a normal characteristic of a rotating star and that meridional mixing provides a reasonable framework for understanding many of the CNO anomalies exhibited by weak-G-band and CN-strong stars as well as the low C-12/C-13 ratios measured among field red giants.

Neutrino emissivities of neutron stars.

Abstract: Neutrino emissivities from the modified Urca process, considered previously by Bahcall and Wolf, and neutrino pair bremsstrahlung from nucleon-nucleon scattering, considered previously by Flowers et al., are recalculated using a nucleon-nucleon interaction that consists of a long-range, one-pion-exchange tensor part and a short-range part parametrized with nuclear Fermi liquid (Landau) parameters. Effects of short-range correlations on the tensor part are approximated through insertion of a cutoff in the one-pion exchange potential. The resulting emissivities differ dramatically from previous calculations: the emissivities from the Urca process and the neutron-neutron neutrino pair bremsstrahlung exceed the results of Bahcall and Wolf and Flowers et al., respectively, by an order of magnitude. It is argued that these results are due to the one-pion-exchange tensor force in the nucleon-nucleon interaction used here. The neutrino mean free path arising from a reaction related to the Urca process is also calculated, and the influence of finite neutrino mean free path and nucleon superfluidity on neutrino emissivities is briefly discussed.

A morphological survey of bar, lens, and ring components in galaxies Secular evolution in galaxy structure

Abstract: A morphological survey of barred galaxies is made to investigate the frequency of occurrence, nature, and size distributions of bars, lenses, inner and outer rings, and global spiral structure. The 121 brightest available barred galaxies are examined on Sky Survey copy plates, and on deeper and larger-scale plates, with the following main results.1. Lenses and inner rings are components of major importance in barred galaxies, occurring, respectively, in 54% of SBO--SBa, and 76% of SBab--SBc galaxies. Few early-type galaxies have rings; almost no late-type ones have lenses.2. There is an intimate connection between bars and lenses: in 17 of 20 galaxies with both components, the bar exactly fills the lens in one dimension.3. We suggest that lenses originate as bars, through an unknown process which makes some bars evolve away to a nearly axisymmetric state. Several properties of the proposed process are deduced. We emphasize the possible importance of internal processes of secular evolution in galaxy structure.4. Several galaxies, notably NGC 3945, seem to have strongly triaxial bulge components.5. Inner rings are round. Lenses tend to be slightly triaxial, flattened ellipsoids, with a preferred equatorial axis ratio of approx.0.9 +- 0.05. Most outer rings are prolate, the shortest dimension beingmore » the one filled by the bar.6. The sizes of bars, rings, and lenses are well correlated with the absolute magnitude of the galaxy, such that the mean surface brightness is constant for each morphological type. The form of the correlation M/sub B/+5 log D= constant is such that these diameters cannot be used as distance indicators. We show that the galaxy mass determines the bar size uniquely.7. Spiral structure in SB galaxies is distorted to resemble inner and outer rings, showing that the arms feel the potential of the bar. Also, of 61 survey galaxies with spiral structure, 55 have global patterns usually interpreted as density waves.« less

Diffuse fluxes of cosmic high-energy neutrinos

Abstract: Production spectra of high-energy neutrinos from galactic cosmic-ray interactions with interstellar gas and extragalactic ultrahigh-energy cosmic-ray interactions with microwave blackbody photons are presented and discussed. These production processes involve the decay of charged pions and are thus related to the production of cosmic gamma rays from the decay of neutral pions. Estimates of the neutrino fluxes from various diffuse cosmic sources are then made, and the reasons for significant differences with previous estimates are discussed. Small predicted event rates for a DUMAND (deep underwater muon and neutrino detector) type detection system, combined with a possible significant flux of prompt neutrinos from the atmosphere above 50 TeV, may make the study of diffuse extraterrestrial neutrinos more difficult than previously thought.

Sunspots and the physics of magnetic flux tubes. I - The general nature of the sunspot. II - Aerodynamic drag

Abstract: Analysis of the dynamical stability of a large flux tube suggests that the field of a sunspot must divide into many separate tubes within the first 1000 km below the surface. Buoyancy of the Wilson depression at the visible surface and probably also a downdraft beneath the sunspot hold the separate tubes in a loose cluster. Convective generation of Alfven waves, which are emitted preferentially downward, cools the tubes. Aerodynamic drag on a slender flux tube stretched vertically across a convective cell is also studied. Since the drag is approximately proportional to the local kinetic energy density, the density stratification weights the drag in favor of the upper layers. Horizontal motions concentrated in the bottom of the convective cell may reverse this density effect. A downdraft of about two km/sec through the flux tubes beneath the sunspot is hypothesized.

Fragmentation in a rotating protostar - A comparison of two three-dimensional computer codes

Abstract: The collapse of an isothermal protostellar cloud with pressure, gravity, and rotation included is followed with two independent computer codes. For the initial condition, a nonaxisymmetric perturbation of mode m = 2 and 50% amplitude is introduced into a cloud of 1 solar mass with a mean density of 1.44 x 10 to the -17g/cu cm and a uniform angular velocity of 1.6 x 10 to the -12 rad/sec. The collapse is followed through an increase in density of over four orders of magnitude to the point where a binary protostar forms. The agreement between the results of the two calculations is good.