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Endal, A. S.

Publications and source records attributed to Endal, A. S..

32 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1980

Probable detection of climatically significant change of the solar constant

It is suggested that the decrease in the solar radius inferred from solar eclipse observations made from 1715 to 1979 reflects a variation of the solar constant that may be of considerable climatic significance. A general, time-averaged relationship between changes in the solar constant and changes in the solar radius is derived based on a model of the contraction and expansion of the convective zone. A preliminary numerical calculation of radius changes due to changes in the mixing length of the solar envelope is presented which indicates that a decrease in solar radius of 0.5 arcsec, as observed in the last 264 years, would correspond to a decrease of 0.7% in the solar constant, a value of large climatic significance. Limitations of the observational method and the numerical approach are pointed out, and required additional theoretical and observational efforts are indicated.

Sofia, S.↗
NASA NTRS1980

Rotational history of the sun: Spin-down of the interior by circulation currents and fluid instabilities

A number of astronomical observations show that solar-type stars begin the main-sequence stage with surface rotation rates which are much greater than that of the sun. The subsequent decrease in the surface rotation rate is due to the braking torque exerted by magnetically-coupled mass loss (the solar wind). The direct braking action of the solar wind should be confined to the convective envelope so the rotation of the radiative interior remains an open question. After reviewing the relevant astronomical data, we describe how angular momentum could be transported out of the radiative interior by fluid instabilities and estimate the time scales for such transport. This picture is used to construct an evolutionary model of the sun, which predicts the present rotation of the radiative interior. The results of such a model are interpreted in terms of the measured oblateness of the solar surface.

Endal, A. S.↗
NASA NTRS1979

The evolution of rotating stars. III - Predicted surface rotation velocities for stars which conserve total angular momentum

Predicted surface rotation velocities for Population I stars at 10, 7, 5, 3, and 1.5 solar masses are presented. The surface velocities were computed for angular momentum with no radial redistribution, complete redistribution, and partial redistribution as predicted by consideration of circulation currents in rotating stars. Near the main sequence, rotational effects can reduce the moment of inertia of a star, so nonrotating models underestimate the expected velocities for evolving stars. On the red giant branch, angular momentum redistribution reduces the surface velocity by a factor of 2 or more, relative to the velocity expected for no radial redistribution. This removes the discrepancy between predicted and observed rotation rates for the K giants and makes it unlikely that these stars lose significant amounts of angular momentum by stellar winds. Calculations indicate that improved observations of the red giants in the Hyades cluster can be used to determine how angular momentum is redistributed by convection

Endal, A. S.↗
NASA NTRS1979

Death throes of massive stars

The data regarding the optical outburst in the case of the formation of a type II supernova are incomplete in the sense that supernovae are invariably discovered at or shortly after peak optical intensity so only the decline from maximum brightness has been recorded in any detail. The possibility of an observation of the preceding part of the optical intensity curve, would be greatly facilitated if the occurrence of a supernova could be predicted on the basis of the presupernova behavior of the star. It is shown that neutrino emission in the final stages of nuclear burning could, in principle, provide such a warning, though the current limitations are severe. The reported results are based on a study of the final stages of evolution of a 15-solar mass star.

Sofia, S.↗
NASA NTRS1977

Detectable gravitational radiation from stellar collapse

The problem of astrophysical sources of detectable gravitational radiation is considered from the stellar-evolution viewpoint. Calculations are presented which indicate that the final stages of evolution may well be dominated by rapidly rotating, collapsing cores which develop nonaxisymmetric configurations. Such events emit large amounts of gravitational radiation which should be detectable in the near future.

Endal, A. S.↗
NASA NTRS1976

The evolution of rotating stars. I - Method and exploratory calculations for a 7-solar-mass star

A method is developed which allows the evolution of rotating stars to be studied well beyond the main-sequence stage. Four different cases of redistribution of angular momentum in an evolving star are considered. Evolutionary sequences for a 7-solar-mass star, rotating according to these different cases, were computed from the zero-age main-sequence to the double-shell-source stage. Each sequence was begun with a (typical) equatorial rotational velocity of 210 km/s. On the main sequence, the effects of rotation are of minor importance. However, as the core contracts during later stages, important effects arise in all physically plausible cases. The outer regions of the cores approach critical velocities and develop unstable angular-velocity distributions. The effects of these instabilities should significantly alter the subsequent evolution.

Endal, A. S.↗
NASA NTRS1976

A model for the X-ray nova A0620-00

The model involves a white dwarf accreting mass from a late-type subgiant companion. The transient behavior of the X-ray source is explained by the instability to mass loss of the companion (as in Algol-type binaries). The brightening, spectrum, and decay timescale of the optical counterpart are explained in terms of re-emission of X-radiation intercepted by the subgiant. It was concluded that A0620-00 can provide an excellent test case for numerical models of stellar atmospheres irradiated by an external X-ray flux.

Endal, A. S.↗
NASA NTRS1976

The evolution of rotating stars. 1: Method and exploratory calculations for a 7 solar mass star

A method was developed which allows us to study the evolution of rotating stars beyond the main sequence stage. Four different cases of redistribution of angular momentum in an evolving star are considered. Evolutionary sequences for a 7 solar mass star, rotating according to these different cases, were computed from the ZAMS to the double shell source stage. Each sequence was begun with a (typical) equatorial velocity of 210 km/sec. On the main sequence, the effects of rotation are of minor importance. As the core contracts during later stages, important effects arise in all physically plausible cases. The outer regions of the cores approach critical velocities and develop unstable angular velocity distributions. The effects of these instabilities should significantly alter the subsequent evolution.

Endal, A. S.↗
NASA NTRS1976

The nature of the X-ray nova A0620-00

A model is presented for the X-ray nova A0620-00. Identification with a nova can be ruled out on two counts. A binary consisting of a late-type subgiant near the Roche lobe, irradiated by an accreting compact companion is shown, however, to be in agreement with all known observations. Photometry of the optical object should be pursued since variability on an approximately eight hour period is expected.

Endal, A. S.↗
NASA NTRS1976

A model for the X-ray nova A0620-00

A model is proposed for the transient X-ray source A0620-00 involving a white dwarf accreting mass from a late-type subgiant companion. The transient behavior of the X-ray source is explained by the instability to mass loss of the companion (as in Algol-type binaries). The brightening, spectrum, and decay timescale of the optical counterpart are explained in terms of re-emission of X-radiation intercepted by the subgiant. A0620-00 should provide an excellent test case for numerical models of stellar atmospheres irradiated by an external X-ray flux.

Endal, A. S.↗
NASA NTRS1975

On the lower mass limit for the carbon detonation scenario

The lower mass limit for carbon detonation (about 4 solar masses) is due to an argument by Paczynski (1970) that stars in the range between 1.4 and 3.5 solar masses will lose their envelopes prior to carbon detonation. This argument is based on two criteria for mass loss: (1) the envelope is unstable to adiabatic oscillations, and (2) the total energy of the envelope is positive. Recent hydrodynamic calculations of red-giant envelopes have shown that the envelopes are highly nonadiabatic and that the second criterion is not a sufficient condition for mass loss. Thus, a calculation which takes into account the nonadiabatic nature of the envelope is required in order to determine the lower mass limit for carbon detonation. This lower mass limit is directly related to the formation mechanisms and statistics of supernovae and pulsars.

Endal, A. S.↗
NASA NTRS1975

Theoretical studies of massive stars. I - Evolution of a 15-solar-mass star from the zero-age main sequence to neon ignition

The evolution of a star with mass 15 times that of the sun from the zero-age main sequence to neon ignition has been computed by the Henyey method. The hydrogen-rich envelope and all shell sources were explicitly included in the models. An algorithm has been developed for approximating the results of carbon burning, including the branching ratio for the C-12 + C-12 reaction and taking some secondary reactions into account. Penetration of the convective envelope into the core is found to be unimportant during the stages covered by the models. Energy transfer from the carbon-burning shell to the core by degenerate electron conduction becomes important after the core carbon-burning stage. Neon ignition will occur in a semidegenerate core and will lead to a mild 'flash.' Detailed numerical results are given in an appendix. Continuation of the calculations into later stages and variations with the total mass of the star will be discussed in later papers.

Endal, A. S.↗
NASA NTRS1975

Carbon-burning nucleosynthesis with convection

The effect of convection on carbon-burning nucleosynthesis is explored with a limited network of reactions. Convection is simulated by a series of networks at fixed mass points in the core of an evolving 15 solar mass star. Complete mixing is always assumed. Comparison to single network calculations show that the 'half-energy' approximation of Arnett yields reasonable results, although the abundances of nuclei which are created by beta-decays of unstable nuclei tend to be underestimated, by this approximation.

Endal, A. S.↗