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At least 127 records · Page 7
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NASA NTRS1991

The formation of primordial binaries in globular clusters by star-disk interactions

The formation of primordial binaries in globular clusters is examined using simple numerical models. Clusters of protostars collapse until their velocity dispersion rises sufficiently to reverse the infall and the cluster reaches equilibrium. During the collapse, interactions between stars and protostellar disks lead to stellar capture. It is found that binary fraction of a few percent typically result. Binary formation is terminated when the velocity dispersion rises to a point at which most encounters result in disk destruction rather than capture. As a result, much gas is returned to the cluster ISM, limiting the star formation efficiency to a value significantly below 100 percent.

Murray, Stephen D.↗
NASA NTRS1992

Constraints on the power spectrum of the primordial density field from large-scale data - Microwave background and predictions of inflation

It is shown here that, by using galaxy catalog correlation data as input, measurements of microwave background radiation (MBR) anisotropies should soon be able to test two of the inflationary scenario's most basic predictions: (1) that the primordial density fluctuations produced were scale-invariant and (2) that the universe is flat. They should also be able to detect anisotropies of large-scale structure formed by gravitational evolution of density fluctuations present at the last scattering epoch. Computations of MBR anisotropies corresponding to the minimum of the large-scale variance of the MBR anisotropy are presented which favor an open universe with P(k) significantly different from the Harrison-Zeldovich spectrum predicted by most inflationary models.

Kashlinsky, A.↗
NASA NTRS1992

The evolution of a primordial binary population in a globular cluster

A simple model is presented for the evolution of a primordial binary population in a globular cluster. Monte Carlo simulations are given for an initial population of 50,000 binaries against a fixed background population of 500,000 single stars in a tidally truncated cluster model. Individual histories of all binaries are followed through mass segregation, scattering recoil, escape from the cluster, or coalescence. It is found that most binaries are destroyed by binary-binary interactions, with the rest escaping in the point-mass approximation. In a more realistic model, the majority of the rest merge. At any instant, most of the remaining binaries are drifting in toward the center before their first strong encounter. A typical binary spends most of its active life in or near the cluster core. The few binaries which receive a recoil sufficient to place them in the halo past the half-mass radius remain there long enough to make a significant contribution to the radial binary distribution.

Hut, Piet↗
NASA NTRS1992

Generation of dynamo magnetic fields in the primordial solar nebula

The present treatment of dynamo-generated magnetic fields in the primordial solar nebula proceeds in view of the ability of the combined action of Keplerian rotation and helical convention to generate, via alpha-omega dynamo, large-scale magnetic fields in those parts of the nebula with sufficiently high, gas-and magnetic field coupling electrical conductivity. Nebular gas electrical conductivity and the radial distribution of the local dynamo number are calculated for both a viscous-accretion disk model and the quiescent-minimum mass nebula. It is found that magnetic fields can be easily generated and maintained by alpha-omega dynamos occupying the inner and outer parts of the nebula.

Stepinski, Tomasz F.↗
NASA NTRS1992

On determining the primordial helium abundance from the spectra of H II galaxies

A data set of 53 H II galaxies and giant H II regions whose observed abundances have reliable formal uncertainties reveals that the O/H-He/H relation used to determine the primordial helium abundance (PBA) has a highly significant physical dispersion. Consideration of possible sources of scatter indicates that it is almost certainly one-sided, i.e., upward from a minimum value of He/H at each O/H. The presence of an asymmetric dispersion casts doubt on values of PBA obtained by extrapolating ridge-line fits to the O/H-He/H relation. The loss of oxygen in SN explosions will lead naturally to a positive dispersion in He/H. The observed dispersions in He/H can be explained by chemical evolution models in which 0-95 percent of the newly synthesized oxygen is lost. It is concluded that in H II galaxies and giant H II regions PBA can be reliably determined only by fitting to the lower envelope of the O/H-He/H relation.

Campbell, Alison↗
NASA NTRS1993

Primordial black holes in globular clusters

It has recently been recognized that significant numbers of medium-mass back holes (of order 10 solar masses) should form in globular clusters during the early stages of their evolution. Here we explore the dynamical and observational consequences of the presence of such a primordial black-hole population in a globular cluster. The holes initially segregate to the cluster cores, where they form binary and multiple black-hole systems. The subsequent dynamical evolution of the black-hole population ejects most of the holes on a relatively short timescale: a typical cluster will retain between zero and four black holes in its core, and possibly a few black holes in its halo. The presence of binary, triple, and quadruple black-hole systems in cluster cores will disrupt main-sequence and giant stellar binaries; this may account for the observed anomalies in the distribution of binaries in globular clusters. Furthermore, tidal interactions between a multiple black-hole system and a red giant star can remove much of the red giant's stellar envelope, which may explain the puzzling absence of larger red giants in the cores of some very dense clusters.

Sigurdsson, Steinn↗
NASA NTRS1993

Improving the reconstruction of the velocity potential and primordial density fluctuations by choice of smoothing windows

Recently considerable attention has been focused on improving algorithms for restoring primordial density fluctuations in the universe or investigating large-scale velocity fields by going to higher order approximations relating nonlinear states to their initial conditions. An alternative approach is to investigate the use of alternative smoothing windows within the context of first-order approximations. I present evidence that the universally used Gaussian smoothing window is far from optimal. A sharp truncation P(k) = 0 for k greater than k(c) leads to a much more direct connection to initial conditions for either 'Gaussianization' methods or use of the Zel'dovich approximation, as discussed in the text.

Melott, Adrian L.↗
NASA NTRS1994

On determining the spectrum of primordial inhomogeneity from the COBE DMR sky maps: Method

The natural approach to a spectral analysis of data distributed on the sky employs spherical harmonic decomposition. A common problem encountered in practical astronomy is the lack of full sky coverage in the available data. For example, the removal of the Galactic plane data from the Cosmic Background Explorer (COBE) Differential Microwave Radiometer (DMR) sky maps compromises Fourier analysis of the cosmic microwave background (CMB) temperature distribution due to the loss of orthogonality of the spherical harmonics. An explicit method for constructing orthonormal functions on an incomplete (e.g., Galaxy-cut) sphere is presented. These functions should be used in the proper Fourier analysis of the COBE DMR sky maps to provide the correct input for the determination of the spectrum of primordial inhomogeneity. The results of such an analysis are presented in an accompanying Letter. A similar algebraic construction of appropriate functions can be devised for other astronomical applications.

Gorski, Krzysztof M.↗
NASA NTRS1994

On determining the spectrum of primordial inhomogeneity from the COBE DMR sky maps: Results of two-year data analysis

A new technique of Fourier analysis on a cut sky has been applied to the two-year Cosmic Background Explorer (COBE) Differential Microwave Radiometer (DMR) 53 and 90 GHz sky maps. The Bayesian power spectrum estimation results are consistent with the Harrison-Zel'dovich n = 1 model. The maximum likelihood estimates of the usual parameters defining the power spectrum of primordial perturbations are n = 1.22 (1.02) and Q(sub rms-PS) = 17 (20) microK including (excluding) the quadrupole. A spectral-index-independent normalization is naturally expressed for the two-year maps in terms of the multipole amplitude a(sub 9) = 8.2 (8.3) microK (to approximately 12 sigma significance). The marginal likelihood function on n obtained by intergration with respect to a(sub 9) renders n = 1.17 +/- 0.31 (0.96 +/- 0.36).

Gorski, K. M.↗
NASA NTRS1994

The range of core size of postcollapse clusters supported by primordial binaries

We investigate how the core size of globular clusters in a postcollapse phase driven by primordial binaries depends upon the parameters of the distribution of the binary binding energy and the abundance of the binaries. We show that the variation of these quantites increases the plausible range for the core size.

Vesperini, E.↗
NASA NTRS1995

Giant branch mixing and the ultimate fate of primordial deuterium in the Galaxy

The observed cosmic abundances of light elements are most consistent with each other, and with the predictions of big bang nucleosynthesis, if, contrary to the usual assumption, galactic chemical evolution reduces (D = He-3)/H with time. Chemical evolution models which accomplish this require that low-mass stars destroy He-3 in the envelope gas that they return to the interstellar medium. A simple argument based on the rates of limiting nuclear reactions shows that the same giant branch mixing process which appears to be needed to explain the observed C-12/C-13 and C/N ratios in 1-2 solar mass stars would indeed also probably destroy He-3 by a large factor in the bulk of the envelope material. The conclusion is that Galactic He-3/H estimates should not be trusted for setting an upper limit on primordial (D = He-3)/H. This removes the strongest lower bound on the cosmic baryon density from big bang nucleosynthesis and the only argument for abundant baryonic dark matter.

Hogan, Craig J.↗
NASA NTRS1997

Positrons from quantum evaporation of primordial black-holes

The unconfirmed prediction of quantum evaporation of primordial black holes (PBHs) is considered together with the related unanswered questions of whether PBHs ever existed and whether any could still exist. The behavior of the positrons from PHBs is modeled in relation to three facts. Firstly, the integrated emitted number spectrum of positrons is six to eight times larger than that of photons. Secondly, positrons emitted from PBHs lose energy and annihilate, producing a prominent line at 511 keV which is redshifted by the expansion of the universe. Thirdly, these photons may be detectable in the X-ray and low gamma ray energy ranges. The model predicts a flux which is significantly inferior to the instrument sensitivities of the foreseeable future.

Durouchoux, P.↗
NASA NTRS1997

Laboratory Simulations: The Primordial Comet Mantle

Laboratory data are needed to understand the formation of organics in cometary and precometary materials and for deciding on the fate of the volatiles. Appropriate experiments were described in the talk at Milipitas. Because of its importance for the comet sample return mission, I discuss here the relevance of this data for predicting the thickness, nature, and ability to survive of the cosmic-ray produced primordial comet mantle ('crust'). That part of the mantle which becomes predominantly refractory is approx. 30 gm/sq cm thick. The tensile strength of this outer mantle is such that it might survive the comet's entrance into the inner solar system. In addition, important modifications to the ices occur to depths approx. 300 gm/cu cm. Based on this it is expected that a deep probe is needed to obtain minimally altered material.

Johnson, R. E.↗
NASA NTRS1999

The Evolution of the Martian Hydrosphere and Its Implications for the Fate of a Primordial Ocean

The existence of a primordial ocean in the northern plains of Mars appears to have been an inevitable consequence of the hydraulic and thermal conditions that existed during the Early Noachian. In this abstract we demonstrate that the progressive crustal assimilation of this early surface reservoir of H2O (punctuated by possible episodes of less extensive flooding) was a natural consequence of the planet's subsequent climatic and geothermal evolution. Additional information is contained in the original extended abstract.

Clifford, S. M.↗
NASA NTRS2001

The Evolution of the Martian Hydrosphere: Implications for the Fate of a Primordial Ocean and the Current State of the Northern Plains

In this paper we consider the hydraulic and thermal conditions that gave rise to the elevated source regions of the Late Hesperian outflow channels and explore their implications for the evolution of the Martian hydrosphere. We find that if the outflow channel floodwaters were derived from a subpermafrost aquifer, then it implies that, throughout the planet's first billion years of evolution, as much as one third of its surface was covered by standing bodies of water and ice. Following the development of the global dichotomy, the bulk of this water would have existed as an ice-covered ocean in the northern plains. We demonstrate that the progressive crustal assimilation of this early surface reservoir of H2O (punctuated by possible episodes of less extensive flooding) was a natural consequence of the planet's subsequent climatic and geothermal evolution-potentially cycling the equivalent of a km-deep global ocean of water through the atmosphere and subsurface every approx. 10(exp 9) years. In response to the long-term decline in planetary heat flow, the progressive coldtrapping of H2O into the growing cryosphere is expected to have significantly depleted the original inventory of groundwater-a development that could well explain the apparent decline in outflow channel activity observed during the Amazonian. Although primarily a theoretical analysis, our findings appear remarkably consistent with the geomorphic and topographic evidence that Mars once possessed a primordial ocean and that a substantial relic of that body continues to survive as massive ice deposits within the northern plains. Confirmation of the presence of such deposits, combined with the potential detection of a global-scale groundwater system, would provide persuasive support for the validity of this analysis.

Clifford, Stephen M.↗
NASA NTRS1984

Delayed fertilization of anuran amphibian (Xenopus) eggs leads to reduced numbers of primordial germ cells

Several media were tested for the extent to which they promoted high fertilization efficiencies in ovulated, stripped Xenopus eggs. One medium was selected for maintaining eggs in a 'delayed fertilization' (DelF) condition. DelF eggs displayed several unusual characteristics, including shift of the center of gravity, prominent sperm entrance site, and occasional polyspermy. The frequency of normal pattern formation varied according to the length of time eggs were maintained in the DelF condition. Various developmental abnormalities were observed during gastrulation, neurulation, and organogenesis. Most abnormalities appeared, however, to be related to morphogenesis of the endoderm. Primordial germ cell (PGC) development was examined in DelF eggs which displayed normal external morphological features at the swimming tadpole stage. PGC counts were usually normal in short-duration (eg, 5 hr) DelF eggs, but frequently substantially reduced or completely diminished in longer-duration (eg, 25h) tadpoles. Six spawnings were compared and shown to exhibit considerable variability in fertility, morphogenesis, and PGC development. Yolk platelet shifts and developmental parameters were examined in two additional spawnings. The subcortical cytoplasm in which the germ plasm is normally localized appeared to be disrupted in longer duration DelF eggs. That observation may account for low PGC counts in DelF tadpoles.

NASA Discipline Number 40-20↗
NASA NTRS1984

Topology of the germ plasm and development of primordial germ cells in inverted amphibian eggs

Inverted Xenopus eggs have reduced numbers of primordial germ cells (PGCs). The extent of the reduction varies from spawning to spawning. Histologic examination revealed that PGC counts were lowest in inverted eggs which displayed the greatest amount of shift in the vegetal mass of large yolk platelets, although the germ plasm itself always remained localized in the egg's original vegetal hemisphere. Even at blastulation the germ plasm continued to be localized in the egg's original vegetal hemisphere. In many cases, however, it was confined to the periphery of the embryo, which probably accounts for the reduced PGC number in some tadpoles. In other cases it may have been dispersed and therefore not detectable in histologic analyses. Although the altered site of involution in inverted embryos did not influence PGC development, subsequent cell movement patterns apparently did. Those embryos which displayed the largest degree of pattern reversal at the tail-bud stage also exhibited the most extreme reduction in PGC numbers. A brief cold shock (4 degrees C, 10 min) prior to first cleavage leads to a further reduction in PGC numbers in inverted embryos, probably as a result of the displacement of the germ plasm away from its original vegetal pole location.

Cell Polarity/physiology↗