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Nelson, L. A.

Publications and source records attributed to Nelson, L. A..

12 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1996

On the Evolution, Numbers, and Characteristics of Close-Binary Supersoft Sources

The ability to perform detailed evolutionary calculations is essential to the development of a well-defined and testable binary model. Unfortunately, traditional evolutionary calculations cannot be used to follow a significant fraction of possible close-binary supersoft sources (CBSS's). It is therefore important to examine the in-put physics carefully, to be sure that all relevant and potentially important physical processes are included. In this paper we continue a line of research begun last year, and explore the role that winds are expected to play in the evolution of CBSS's. We find that at least a subset of the systems that seemed to be candidates for common envelope evolution may survive, if radiation emitted by the white dwarf drives winds from the system. We study the effects of winds on the binary evolution of CBSS's, and compute the number and characteristics of CBSS's expected to be presently active in galaxies such as our own or M31.

DiStefano, R.↗
NASA NTRS1993

On the Li and Be tests for brown dwarfs

We present the results of stellar evolution calculations which show quantitatively how the measured abundances of Li and Be in low-mass stellar objects can be used to discriminate between brown dwarfs and low-mass main-sequence stars. The evolution of B, although less useful, is also studied. We define a transition mass range, below which at least 50 percent of the light element remains at the end of nuclear burning, and above which no more than 10 percent remains. We find that the transition mass range for Li burning is 0.059-0.062 solar mass, while for Be the range is 0.075-0.077 solar mass. Using these results, we then examine the factors (e.g., age and luminosity) that affect our ability to identify low-luminosity objects as brown dwarfs. In particular, we show that the Li test would be well suited for brown dwarf candidates located in nearby open clusters with ages in the range of 2 x 10 exp 8 to 5 x 10 exp 8 yr.

Nelson, L. A.↗
NASA NTRS1993

The detectability of brown dwarfs - Predictions and uncertainties

In order to determine the likelihood for the detection of isolated brown dwarfs in ground-based observations as well as in future spaced-based astronomy missions, and in order to evaluate the significance of any detections that might be made, we must first know the expected surface density of brown dwarfs on the celestial sphere as a function of limiting magnitude, wavelength band, and Galactic latitude. It is the purpose of this paper to provide theoretical estimates of this surface density, as well as the range of uncertainty in these estimates resulting from various theoretical uncertainties. We first present theoretical cooling curves for low-mass stars that we have computed with the latest version of our stellar evolution code. We use our evolutionary results to compute theoretical brown-dwarf luminosity functions for a wide range of assumed initial mass functions and stellar birth rate functions. The luminosity functions, in turn, are utilized to compute theoretical surface density functions for brown dwarfs on the celestial sphere. We find, in particular, that for reasonable theoretical assumptions, the currently available upper bounds on the brown-dwarf surface density are consistent with the possibility that brown dwarfs contribute a substantial fraction of the mass of the Galactic disk.

Nelson, L. A.↗
NASA NTRS1987

The evolutionary status of 4U 1820-30

The evolution of an ultracompact binary wherein mass transfer is driven by the emission of gravitational radiation is calculated, emphasizing the systematic study of how the rate of change in orbital period depends on the various system parameters. Analytic scaling laws are derived describing how the mass of the secondary, the mass transfer rate, and the rate of change of period depend on the binary system parameters, and these simple relations are applied to the 4U 1820-30 system. Detailed numerical evolutionary calculations are performed utilizing Zapolsky-Salpeter (1969) models for cold stars, but allowing for the possibility that the secondary has not yet reached a completely degenerate configuration. The results are used to set significant constraints on the deviation from a completely degenerate configuration of the secondary, the systemic mass-loss parameters, and the mass of the neutron star in the 4U 1820-30 system.

Rappaport, S.↗
NASA NTRS1986

The evolution of very low mass stars

The results of numerical evolutionary calculations for stars with masses in the range of 0.01-0.10 solar mass are presented. The stellar models by which these stars are followed from the early stages of contraction through deuterium burning to the very late stages of degenerate cooling at ages comparable to that of the Galaxy are described, and the methodology used to investigate the major uncertainties in the input physics are discussed. It is found that, for brown dwarfs with masses substantially below the end of the hydrogen-burning main sequence, the evolution of the effective temperature and bolometric luminosity are fairly well determined, despite the residual uncertainties in the input physics. In particular, the evolution is remarkably insensitive to the choice of the atmospheric opacity law at low temperatures and to the amount of mismatch in specific entropy across the stellar envelope. The minimum mass for a star to attain main-sequence status is moderately sensitive to the assumed input physics, particularly the surface boundary conditions.

Nelson, L. A.↗
NASA NTRS1986

The evolution of ultrashort period binary systems

A discussion is presented concerning the results of detailed evolutionary calculations in which a very low mass and hydrogen-depleted semiattached binary star containing a collapsed object can reach an exceptionally short orbital period while sustaining a relatively high mass transfer rate. The observed properties of such systems can be understood under the assumption that they contain moderately to severely hydrogen-defficient secondary stars that are neither fully degenerate nor burning He. It is noted that for extremely hydrogen-depleted stars, the assumption of chemical homogeneity becomes untenable. Attention is given to the binary systems 4U 1626-67, 4U 1916-05, and G61-29.

Nelson, L. A.↗
NASA NTRS1985

Evolution of rotationally and tidally distorted low-mass, close binary systems - Implications for the minimum orbital period of cataclysmic variables

A (1 + 0.4) solar mass close binary system consisting of a compact primary and a red dwarf secondary has been evolved numerically. Such a binary system should effectively model cataclysmic variables for which a minimum orbital period cutoff of about 81 minutes has been observed. The influence of gravitational radiation losses which drive Roche lobe overflow has been studied, and the effects of rotational and tidal distortion have also been incorporated in the calculation. The evolution of the He-3 abundance, which has been suggested as a possible explanation for the upper limit of the apparent orbital period gap exhibited by cataclysmic variables, is also considered. It is found that both the distortional effects and the He-3 chemical profile can play an important role in determining the subsequent evolution of these systems. Specifically, when distortion is included, the theoretical minimum orbital period is increased by about 10 percent, yielding better agreement with observations.

Nelson, L. A.↗
NASA NTRS1985

Clock synchronization by accelerated observers - Metric construction for arbitrary congruences of world lines

Clock synchronization in an arbitrarily accelerated observer congruence is considered. A general solution is obtained that maintains the isotropy and coordinate independence of the one-way speed of light. Attention is also given to various particular cases including, rotating disk congruence or ring congruence. An explicit, congruence-based spacetime metric is constructed according to Einstein's clock synchronization procedure and the equation for the geodesics of the space-time was derived using Hamilton-Jacobi method. The application of interferometric techniques (absolute phase radio interferometry, VLBI) to the detection of the 'global Sagnac effect' is also discussed.

Henriksen, R. N.↗
NASA NTRS1985

On the nature of the companion to Van Biesbroeck 8

Results of the first numerical evolutionary calculations for very low-mass stars with ages up to the age of the Galaxy are presented. The calculations shed new light on the nature of the recently discovered object VB 8B, the companion to Van Biesbroeck's star no. 8, and support the identification of this object as a low-mass star in which hydrogen burning was never able to establish thermal equilibrium. For plausible ages of this binary stellar system, the mass of VB 8B is estimated to be in the range 0.04-0.08 solar mass. The object is supported primarily by degenerate electron pressure; its present values of central density and temperature are found to be 500-2000 g/cu cm and 1.0-1.5 million K. An orbital period of more than about 40 yr is predicted for the binary system. The presented evolutionary tracks are also applicable to the interpretation of observations of other very low-luminosity dwarfs.

Nelson, L. A.↗
NASA NTRS1972

Development of a carbon cloth heat post module for thermal vacuum testing of a spinning spacecraft

Development of a high temperature, high power density, isothermal, rapid transient, infrared thermal module is described. The investigation of various alternative approaches is discussed, as well as the rationale leading to the choice of a carbon cloth concept. Evolution of module design configurations and a summary of the development test results are included, along with a detailed description of the final design, which incorporated a black plate emitter radiantly heated by the carbon cloth. The final module configuration met design criteria during a 200-hour thermal vacuum performance test at 1000 F operating temperature without contaminating a solar cell array.

Levine, M. B.↗