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Silk, J.

Publications and source records attributed to Silk, J..

At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1978

Fragmentation of molecular clouds

Fragmentation is generally considered to be the initial process that a molecular cloud must undergo before stars can form. Yet its role in determining the final mass spectrum remains obscure. It appears that gravitational fragmentation, considered as a unique process, is unsatisfactory. Both fragmentation and complementary physical processes are, therefore, discussed. One of the principle aims of the discussion is to indicate how stars of solar mass (and more generally, how the initial mass spectrum of stars) can form. Attention is given to the evidence for fragmentation, opacity-limited fragmentation, magnetic flux-limited fragmentation, fragmentation induced by molecule formation and excitation, protostellar heat input, fragment coalescence, accretion, binary formation, and probabilistic theories.

Silk, J.↗
NASA NTRS1977

The correlation function for density perturbations in an expanding universe. II - Nonlinear theory

A formalism is developed to find the two-point and higher-order correlation functions for a given distribution of sizes and shapes of perturbations which are randomly placed in three-dimensional space. The perturbations are described by two parameters such as central density and size, and the two-point correlation function is explicitly related to the luminosity function of groups and clusters of galaxies

Mcclelland, J.↗
NASA NTRS1977

The correlation function for density perturbations in an expanding universe. I - Linear theory

The evolution of the two-point correlation function for adiabatic density perturbations in the early universe is studied. Analytical solutions are obtained for the evolution of linearized spherically symmetric adiabatic density perturbations and the two-point correlation function for these perturbations in the radiation-dominated portion of the early universe. The results are then extended to the regime after decoupling. It is found that: (1) adiabatic spherically symmetric perturbations comparable in scale with the maximum Jeans length would survive the radiation-dominated regime; (2) irregular fluctuations are smoothed out up to the scale of the maximum Jeans length in the radiation era, but regular fluctuations might survive on smaller scales; (3) in general, the only surviving structures for irregularly shaped adiabatic density perturbations of arbitrary but finite scale in the radiation regime are the size of or larger than the maximum Jeans length in that regime; (4) infinite plane waves with a wavelength smaller than the maximum Jeans length but larger than the critical dissipative damping scale could survive the radiation regime; and (5) black holes would also survive the radiation regime and might accrete sufficient mass after decoupling to nucleate the formation of galaxies.

Mcclelland, J.↗
NASA NTRS1977

Graphite grain surface reactions in interstellar and protostellar environments

Surface reactions on warm (at least 60 K) graphite grains between lattice C atoms and impinging H and O atoms within or in the vicinity of H II regions can provide a prolific source of H2CO and other interstellar molecules. It is proposed that similar reactions in the primitive solar nebula led to the formation of the organic molecules found in carbonaceous chondrites. From this and related evidence, it is argued that most of the solid material in the solar system may have originated as interstellar grains.

Barlow, M. J.↗
NASA NTRS1977

Large-scale inhomogeneity of the Universe - Spherically symmetric models

Dust-filled Tolman-Bondi (1934, 1947) spherically symmetric cosmological models are studied with application to superclustering of galaxies. Asymptotic expressions are given for the density and curvature contrast, valid near the singularity and at long times. The density profile of infalling material is expressed in terms of a single arbitrary function (the distribution of total energy per unit mass). Measurement of the distribution of excess galaxy counts and deviations from the Hubble flow in the vicinity of a supercluster could provide a determination of the cosmological deceleration parameter.

Silk, J.↗
NASA NTRS1977

On the fragmentation of cosmic gas clouds. III - The initial stellar mass function

Radiation from the first opaque protostellar fragments to form in a nonhomologously collapsing molecular cloud can provide a significant heat source in regions where the matter remains relatively diffuse over the initial free-fall time scale. This energy input provides a negative feedback that gradually inhibits fragmentation as larger protostellar fragments form that are also more effective radiators. A highly simplified model is described that yields an expression for the initial mass function in stellar clusters. Dynamical dissipation, driven by the more massive protostars, tends to decouple the gas and grain temperatures and steepen the mass function. Fragmentation of less massive stars appears to proceed more efficiently when the heavy-element abundance is decreased, indicating the possibility of an inverse correlation between gradients in Z and M/L. Other possible observational tests of fragmentation are also suggested.

Silk, J.↗
NASA NTRS1977

On the fragmentation of cosmic gas clouds. II - Opacity-limited star formation

Opacity-limited fragmentation of gravitationally collapsing gas clouds is reexamined with inclusion of realistic dust-grain opacities and cooling rates. A minimum fragment size of approximately 0.01 solar mass is found in spherical collapse, but may be the same or larger for spheroidal collapse. A simple analytic expression is given for the characteristic protostellar mass above which further fragmentation can occur during the early opaque phases of dynamical protostellar collapse. The effects of rotation, fragment collisions, and magnetic fields are also discussed.

Silk, J.↗
NASA NTRS1977

X-rays from massive black holes

Tidal disruption of nondegenerate stars is shown to provide an important heat source for gas accreting onto massive black holes (about 10 to 100 million solar masses) in galactic nuclei. Turbulent dissipation by accreting gas streams can also be significant, and the heat input suffices to give rise to temperatures above about 1 billion K, resulting in an appreciable hard X-ray luminosity.

Meszaros, P.↗
NASA NTRS1977

On the fragmentation of cosmic gas clouds. I - The formation of galaxies and the first generation of stars

The formation of galaxies is studied in the context of the fragmentation of massive diffuse ionized gas clouds. A semiquantitative discussion of the role of cooling indicates that there is a characteristic mass of a galaxy (of the order of 500 billion solar masses) which can fragment out of an ionized medium of density not exceeding 10 to the -25th power g/cu cm, whereas more massive galaxies attain a characteristic radius of the order of 60 kpc. The relationship between binding energy and mass is investigated for spheroidal galaxies. The role of H2 and Ly-alpha cooling in the fragmentation of primordial gas clouds is evaluated, and relatively qualitative arguments indicate that the first stars have masses of at least 0.3 solar mass with a characteristic mass of approximately 20 solar masses. When the average heavy-element abundance by mass exceeds about 0.00001, heavy-element cooling prevails over Ly-alpha cooling, and subsequent fragmentation forms second-generation stars of low characteristic mass.

Silk, J.↗
NASA NTRS1977

Sputtering in interstellar shocks - A model for heavy element depletion

High-velocity (at least 100 km/s) shock fronts are found to provide an environment where grains can be destroyed by thermal sputtering. Application is made to supernova remnants. Sputtering of refractory grains in shocks associated with high-velocity clouds leads to cosmic abundances of heavy elements and if followed by deceleration of the high-velocity gas and dilution with ambient depleted matter. A correlation of depletion with systematic velocity is predicted. It is proposed that there is a residual underlying depletion associated with refractory grain formation and a subsequent trapping of gas-phase metallic species by adsorption. Sputtering of adsorbed monolayers in intermediate-velocity shocks (not exceeding 50 km/s) can account for a range of depletions in diffuse interstellar matter.

Barlow, M. J.↗
NASA NTRS1976

Ultraviolet absorption lines associated with the Vela supernova remnant

Two stars behind the Vela supernova remnant and two stars offset from the remnant have been observed with the UV spectrometer aboard the Copernicus satellite. Over 200 interstellar atomic and molecular absorption features between 1000 and 1400 A have been identified and measured for radial velocity and equivalent width. In many cases, additional information was obtained by studying the detailed shapes of the recorded profiles. Most of the stars show several absorption components, with clouds of the highest radial velocity appearing in the spectra of stars behind the remnant. For each component, column densities were derived using velocity dispersion parameters which yielded the most self-consistent results. Qualitatively, the gas toward the remnant exhibits a number of unusual properties, when compared with normal interstellar material. First, abnormally high radial velocities were evident. Second, the degree of ionization of some elements suggested the existence of ionizing processes significantly more potent than those found in general regions of space. Finally, an investigation of electron densities shows that much of the gas, especially that at high velocity, must exist in the form of relatively thin sheets or filaments. If cosmic abundances prevail, the column densities of high-velocity excited material suggest that H-alpha emission measures could be as large as 100 sq cm/cu pc.

Jenkins, E. B.↗
NASA NTRS1976

Accretion by galaxy clusters and the relationship between X-ray luminosity and velocity dispersion

A relationship between X-ray luminosity and cluster velocity dispersion is derived, which improves on the results of Solinger and Tucker (1972) in several important respects, and a simple model for primordial gas accretion by a cluster of galaxies is described. It is argued that the accreted gas may be causally related to the occurrence of a central active radio or giant cD or galaxy in X-ray clusters. Accretion and subsequent dissipation lead to a decrease with time in the amount of hot gas and in the X-ray luminosity as well as to an increase in mass of the dominant central galaxy.

Silk, J.↗
NASA NTRS1976

H2 recombination on interstellar grains

From a consideration of relevant theoretical and experimental data it is concluded that H atoms (but not H2 molecules) will be chemisorbed on interstellar graphite grains, with H2 formation proceeding efficiently for graphite grain temperatures less than 70 K. It is argued that graphite grains will act as the principal sites for H2 formation, with a formation rate of about 4 to the minus 17th cu cm per sec. Heating by H2 molecules formed by surface recombination is analyzed in the context of the available experimental data, and a heating rate is derived and compared with other suggested cloud heating mechanisms. It is concluded that H2 recombination will provide the largest heat source in diffuse clouds if the albedo of interstellar dust in the 912-1200 A region is high (about 0.9), whereas if the albedo in this wavelength region is lower (about 0.5), photoelectron ejection from grains will tend to predominate, and can explain observed cloud temperatures with a carbon depletion factor of approximately 2, a factor attributable to a normal interstellar abundance of graphite grains.

Barlow, M. J.↗
NASA NTRS1976

Ionising flux of cosmic background radiation

The indirect method of estimating the diffuse metagalactic flux of ionizing radiation proposed by Sunyaev (1969) is reconsidered in the light of further studies of the interaction of this radiation with galactic gas. An upper limit is derived for the intensity of the metagalactic background radiation to which the neutral interstellar medium is exposed. This limit on the ionizing radiation flux severely restricts the emission from a galactic corona containing gas in the temperature range 100,000 to 1,000,000 K. An upper limit of 10 to the 29.2 erg/sec/Hz is obtained for the mean luminosity radiated by a quasar in the energy band 40-170 eV. The 21-cm observations examined indicate that further than about 30 kpc from the center of the galaxy self-shielding by H II is possible only when the critical metagalactic ionizing flux is not exceeded.

Silk, J.↗
NASA NTRS1976

Encounters of spherical galaxies. I - Galaxy models with one stellar population. II - Galaxy models with two stellar populations

Close encounters between two spherical galaxies of equal size and consisting of only one stellar population are calculated using models of elliptical galaxies constructed according to King's (1966) method. The mass loss and the change in internal energy are computed under the assumption that the stars do not change their velocity or density distributions during the encounters. The results for a specific case are compared with the calculations of Gallagher and Ostriker (1972), who employed the observed brightness distribution and the derived density distribution of the E1 galaxy NGC 3379. For models with one stellar population, the results suggest that the radius of a galactic halo would have to be at least 200 kpc for appreciable mass loss to occur over the history of a galaxy in a rich cluster. The calculations are then extended to include a halo population characterized by a high central velocity dispersion. In this case, it is found that the halo population of sufficiently large galaxies can be dispersed without appreciably affecting the main population. It is suggested that the missing mass of many clusters of galaxies may be located in an intergalactic sea of faint stars making up an envelope for the centrally located gE galaxies.

Biermann, P.↗