Showing papers in "Astronomy and Astrophysics in 1995"

Formation of MACHO primordial black holes in inflationary cosmology

Abstract: As a nonbaryonic explanation of massive compact halo objects, a phenomenological model is presented which predicts formation of primordial black holes at a desired mass scale. The required feature of initial density fluctuation is realized making use of the primordially isocurvature fluctuation generated in an inflationary universe model with multiple scalar fields.

Amplitudes of stellar oscillations: The Implications for asteroseismology

Abstract: There are no good predictions for the amplitudes expected from solar-like oscillations in other stars. In the absence of a definitive model for convection, which is thought to be the mechanism that excites these oscillations, the amplitudes for both velocity and luminosity measurements must be estimated by scaling from the Sun. In the case of luminosity measurements, even this is difficult because of disagreement over the solar amplitude. This last point has lead us to investigate whether the luminosity amplitude of oscillations (dL/L) can be derived from the velocity amplitude v_osc. Using linear theory and observational data, we show that p-mode oscillations in a large sample of pulsating stars satisfy (dL/L)_bol proportional to v_osc/T_eff. Using this relationship, together with the best estimate of v_osc(Sun) = (23.4 +/- 1.4) cm/s, we estimate the luminosity amplitude of solar oscillations at 550 nm to be dL/L = (4.7 +/- 0.3) ppm. Next we discuss how to scale the amplitude of solar-like (i.e., convectively-powered) oscillations from the Sun to other stars. The only predictions come from model calculations by (Christensen-Dalsgaard & Frandsen, Sol. Phys. 82, 469). However, their grid of stellar models is not dense enough to allow amplitude predictions for an arbitrary star. Nevertheless, although convective theory is complicated, we might expect that the general properties of convection -- including oscillation amplitudes -- should change smoothly through the colour-magnitude diagram. Indeed, we find that the velocity amplitudes predicted by the model

Coalescing neutron stars: A Step towards physical models. 1: Hydrodynamic evolution and gravitational wave emission

Abstract: We investigate the dynamics and evolution of coalescing neutron stars. Although the code (Piecewise Parabolic Method) is purely Newtonian, we do include the emission of gravitational waves and their backreaction on the hydrodynamic flow. The properties of neutron star matter are described by the physical equation of state of Lattimer \\& Swesty (1991). Energy loss by all types of neutrinos and changes of the electron fraction due to the emission of electron neutrinos and antineutrinos are taken into account by an elaborate ``neutrino leakage scheme''. We simulate the coalescence of two identical, cool neutron stars with a baryonic mass of $\\approx\\!1.6\\,M_\\odot$ and a radius of $\\approx\\!15$~km and with an initial center-to-center distance of 42~km. The initial distributions of density and electron concentration are given from a model of a cold neutron star in hydrostatic equilibrium (central temperature about $8\\,{\\rm MeV}$). We investigate three cases which differ by the initial velocity distribution in the neutron stars, representing different cases of the neutron star spins relative to the direction of the orbital angular momentum vector. Within about 1~ms the neutron stars merge into a rapidly spinning ($P_{\\rm spin}\\approx 1$~ms), high-density body ($\\rho\\approx 10^{14}$~g/cm$^3$) with a surrounding thick disk of material with densities $\\rho\\approx 10^{10}-10^{12}$~g/cm$^3$ and orbital velocities of~0.3--0.5~c. In this work we evaluate the models in detail with respect to the gravitational wave emission using the quadrupole approximation. In a forthcoming paper we will concentrate on the neutrino emission and implications for gamma-ray bursters. A maximum luminosity in excess of $10^{55}$~erg/s is reached for about 1~ms.

Gravitational waves from pulsars: Emission by the magnetic field induced distortion

Abstract: The gravitational wave emission by a distorted rotating fluid star is computed. The distortion is supposed to be symmetric around some axis inclined with respect to the rotation axis. In the general case, the gravitational radiation is emitted at two frequencies: $\Omega$ and $2\Omega$, where $\Omega$ is the rotation frequency. The obtained formul\ae\ are applied to the specific case of a neutron star distorted by its own magnetic field. Assuming that the period derivative $\dot P$ of pulsars is a measure of their magnetic dipole moment, the gravitational wave amplitude can be related to the observable parameters $P$ and $\dot P$ and to a factor $\beta$ which measures the efficiency of a given magnetic dipole moment in distorting the star. $\beta$ depends on the nuclear matter equation of state and on the magnetic field distribution. The amplitude at the frequency $2\Omega$, expressed in terms of $P$, $\dot P$ and $\beta$, is independent of the angle $\alpha$ between the magnetic axis and the rotation axis, whereas at the frequency $\Omega$, the amplitude increases as $\alpha$ decreases. The value of $\beta$ for specific models of magnetic field distributions has been computed by means of a numerical code giving self-consistent models of magnetized neutron stars within general relativity. It is found that the distortion at fixed magnetic dipole moment is very dependent of the magnetic field distribution; a stochastic magnetic field or a superconductor stellar interior greatly increases $\beta$ with respect to the uniformly magnetized perfect conductor case and might lead to gravitational waves detectable by the VIRGO or LIGO interferometers. The amplitude modulation of the signal induced by the daily rotation of the Earth has been computed and specified to the case of the Crab pulsar and VIRGO

Meteor stream activity. 2: Meteor outbursts

Abstract: In the past two centuries, alert amateur and professional meteor astronomers have documented 35 outbursts of 17 individual meteor streams well enough to allow the construction of a homogeneous set of activity curves These curves add to similar profiles of the annual streams in a previous paper (Paper 1) This paper attempts to define the type and range of phenomena that classify as meteor outbursts from which the following is concluded: Outbursts are associated with the return of the comet to perihelion (near-comet type outbursts), but occur also when the parent comet is far from perihelion and far from the Earth (far-comet type) All outbursts of a given type only, depending on encounter geometry The activity curves, expressed in terms of Zenith Hourly Rates (ZHR), have a shape that is generally well described by: ZHR = ZHR(sub max) 10(sup(-B (the absolute value of lambda (sub dot in a circle) - lambda (sup max) (sub dot in a circle))) The steepness of the slopes varies from an exponent of B = 7 to B = 220 per degree of solar longitude, with a typical value of B = 30 In addition, most near-comet type outbursts have a broader component underlying the main peak with B approximately 1 - 7The duration Delta t is approximately 1/B of the main peak is almost independent of location near the comet, while the background component varies considerably in duration and relative intensity from one return to another The two components in the activity curve are due to two distinct structures in the dust distribution near the parent comet, where the main component can be due to a sheet of dust that emanates from the IRAS dust trail This brings the total number of distinct structures in meteor streams to four, including the two structures from the annual stream activity in Paper 1

A new derivation of the tensile strength of cometary nuclei: Application to comet Shoemaker-Levy 9

Abstract: The splitting of comets as exemplified by comet Shoemaker-Levy 9, when it passed near Jupiter, is a common phenomenon. Multiple splitting is also not an uncommon occurrence. It is clear that the comet nucleus is fragile, i.e., its tensile strength is small compared with that of solid materials. We show that aggregates of sub-micron interstellar dust particles presumed to consist of a silicate core, an inner mantle of complex organic refractory molecules, and an outer mantle dominated by H2O ice (Greenberg, 1982) provide the basis for a quantitative derivation of the tensile strength of comet SL9 using molecular interactions at the contact interfaces. In fact, using a mean particle size representing interstellar dust as it would appear in its final presolar state one derives a tensile strength which describes remarkably well the multiple splitting phenomenon. This derivation of the tensile strength of a particle aggregate resulting from molecular interactions is quite general and can be applied to physical situations involving any sorts of aggregates as well as those representing comet nuclei.

Redshift distortions of clustering: a Lagrangian approach.

Abstract: We study the effects of peculiar velocities on statistical measures of galaxy clustering. These effects occur when distances to the galaxies are estimated from their redshifts. It is assumed that the clustering pattern results from the gravitational instability of initially Gaussian, small-amplitude perturbations of a Friedman–Lemaitre cosmological model. Explicit expressions are given for an arbitrary density parameter of the model, both when the cosmological constant, , is zero, and when the model is spatially flat, + =3H2 = 1. Kaiser (1987) had analyzed the redshift distortion of the two–point correlation function. This function determines the variance of the density field distribution function and can be computed using linear perturbation theory. We show here how to compute higher order moments in redshift space, paying special attention to the skewness, or third moment of the density field, and its Fourier space counterpart, the bispectrum. This calls for a (weakly) non–linear analysis. We rely on a perturbative expansion of particle trajectories in Lagrangian coordinates, using the formalism introduced by Moutarde et al. (1991) and further developed by Bouchet et al. (1992, 1994). This formalism extends to higher orders the Zel’dovich first order (i.e. linear) solution (1970). The lowest non-vanishing contribution to the skewness comes from the first and second-order terms in perturbation theory. Therefore, using Zel’dovich approximation would not be self-consistent and would yield inaccurate results. We show that a physically consistent and quantitatively accurate analysis of the growth skewness in redshift space can be obtained from second-order Lagrangian theory. With practical applications to redshift surveys in mind, we also study the effects of spatial smoothingof the evolved density field. The necessary formalism was developed by Juszkiewicz and Bouchet (1991) and Juszkiewicz et al. (1993a). Here we give the first complete account of these calculations; we also extend the formalism by explicitly taking redshift distortions into account. We give analytic expressions for the gravitationSend offprint requests to: E. Hivon ally induced skewness as a function of the power spectrum and of , for a spherical top-hat and a Gaussian smoothing filter. We compare our analytical predictions with measurements performed in numerical simulations, and find good agreement. These results should then prove useful in analyzing large scale redshift surveys. In particular, our results, in conjunction with the recent suggestion of Fry (1994), may solve a well known problem which always arises in conventional dynamical determinations of the mean density of the universe. Such studies produce estimates of which are coupled with the parameters describing the bias in the galaxy distribution. As a result, a biased = 1 model is dynamically indistinguishable from an open, unbiased, one. For the first time, it may become possible to break this degeneracy, and decouple the estimates of linear and non-linear bias from the estimates of and .

Time-variation of Newton's constant and the age of globular clusters.

Abstract: A variation of Newton's constant $G$ over cosmological time scales would modify the main-squence time of globular cluster (GC) stars. We have calculated the evolution of low-mass stars typical for GCs both for standard non-varying $G$ and under the assumption of a linear variation of $G$. The age of the isochrones resulting from the latter models then was chosen such that the isochrones mimicked the standard ones at the turnoff. Assuming that the true age of GCs is between $8$ and $20\,\rm Gyr$, and because their apparent age is between $14$ and $18\,\rm Gyr$, we find that today $-35{\times}10^{-12}\,{\rm yr}^{-1}\la\dot{G}/G\la 7{\times}10^{-12}\,{\rm yr}^{-1}$. The upper limit (gravity weaker in the past) is competitive with direct present-day bounds from celestial mechanics. Within independently determined $\dot{G}/G$ limits a time-varying $G$ as an explanation for the discrepancy between the cosmic expansion age and the apparent GC ages is conceivable.

The stellar kinematics of galactic disks.

Abstract: Stellar velocity dispersion measurements of a sample of 12 galactic disks are summarized. The observed radial functionality is parameterized such that one dispersion value is assigned to each galaxy. Comparison of the galaxy dispersion with absolute magnitude and maximum rotation reveals that the dispersion is larger for the more massive systems; the relation between dispersion and intrinsic brightness of the old disk population appears to be linear. Combination of the data for face-on and inclined systems makes the conclusion plausible that the ratio between vertical and radial dispersion in external systems equals 0.6, as for the solar neighbourhood. From the vertical disk dispersion the maximum rotation of a disk can be calculated once the ratio of scalelength to scaleheight (h/z0) is known. This ratio is derived as a function of disk brightness from the observed dispersion for a simple one colour, one mass-to-light ratio disk model. It appears to be rather constant, possibly increasing towards the fainter systems. Then, for realistic h/z0 values, the stellar velocity dispersions only allow the disk to have a maximum rotation of on average 63% of the observed maximum rotation. The disk is then still dominant in the central parts of the galaxy but generally the maximum disk hypothesis predicting a maximum disk rotation of 85-90% of the observed, does not apply. Exploring the consequences for the Tully-Fisher relation, it is found that this relation for disks only must be positioned at lower rotational velocities than what is observed. A dark halo and bulge must supply the additional rotation. A relation is calculated between Toomre's Q parameter and the mass-to-light ratio for a disk. When this relation is projected onto the observed velocity dispersion - maximum rotational velocity data it is found that the same M/L ratio for galactic disks implies that the Q value is also equal for all disks, and vice versa. A universal Q value can indeed be expected when a process of self regulation is responsible. for the appearance of regular spiral structure. For an (M/L)B of two which is calculated for the one colour disk model from the observed dispersion one finds Q to range between 2 and 2.5. The latter coincides with the general stability criterion for galaxies as derived in numerical experiments. Finally, the effect of a dark halo on the observable velocity dispersion has been investigated. It appears that the hitherto adopted radially decreasing dispersion proportional to the square root of the surface density, as expected for an isolated disk, is a good approximation. This is certainly valid for radii within two scalelengths for which dispersions have been measured.

On the masses of neutron stars.

Abstract: We analyze the currently available observations of X-ray binaries in a consistent way, to re-determine the masses of the neutron stars in these systems. In particular, our attention is focussed on a realistic and consistent assessment of observational uncertainties and sources of systematic error. Confidence limits for these new mass estimates are generally less optimistic than previously assumed. The available observations, including data on six radio pulsars, do not firmly constrain the equation of state of neutron star matter. In particular, a firm upper mass limit cannot yet be established. An improvement of the accuracy of optical data holds the key to further progress.