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Stothers, R.

Publications and source records attributed to Stothers, R..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

Possible mechanisms for the Hubble-Sandage /S Doradus/ variables

The unexplained long-term variability of the light of the brightest nonexplosive stars known, namely the Hubble-Sandage (or S Doradus) variables, is investigated in terms of the following mechanisms: (1) episodic, nearly catastrophic mass loss, perhaps accompanied by temporary shrinkage of the stellar radius, in a luminous supergiant; (2) flickering of a hydrogen-burning or helium-burning shell; (3) hydrogen flashing in an evolved stellar core; (4) pulsation of a luminous supergiant envelope near the Eddington limit of radiative stability; (5) pulsation of a dense circumstellar gas or dust cloud; (6) overturning of giant convection cells (or some other kind or nonradial oscillation) in a luminous supergiant envelope; or (7) vibrational instability and mass outflow in an extremely massive main-sequence star. It is believed that the second and third suggestions can all but definitely be ruled out and that the sixth and seventh are rather unlikely. Preference is given to the notion that a sudden, massive outflow of matter may sometimes occur from the surface of an evolved supergiant of very high mass.

Stothers, R.↗
NASA NTRS1982

BL Herculis stars - Theoretical models for field variables

Type II Cepheids with periods between 1 and 3 days, commonly designated as Bl Herculis stars, have been modeled here with the aim of interpreting the wide variety of light curves observed among the field variables. Previously modeled globular cluster members are used as standard calibration objects. The major finding is that only a small range of luminosities is capable of generating a large variety of light curve types at a given period. For a mass of approximately 0.60 solar mass, the models are able to reproduce the observed mean luminosities, dispersion of mean luminosities, periods, light amplitudes, light asymmetries, and phases of secondary features in the light curves of known BL Her stars. It is possible that the metal-rich variables (which are found only in the field) have luminosities lower than those of most metal-poor variables. The present revised mass for BL Her, a metal-rich object, is not significantly different from the mean mass of the metal-poor variables.

Carson, R.↗
NASA NTRS1981

A search for apsidal motion in 4U0115+63

The measurement of apsidal motion provides one of the few experimental tests of models of stellar interiors. Binary X-ray pulsars are suited for a potentially important application of the apsidal motion test because of their generally close orbits and the precision with which their orbits can often be measured. The orbit of the X-ray pulsar 4U0115+63 was determined by Rappaport et al. (1978). The orbital parameters were determined with sufficient precision to make possible a measurement of apsidal motion if a second observation of the source could be made. However, 4U0115+63 has not been observed to be active since its 1978 outburst. An analysis has, therefore, been conducted of the archival Uhuru data of the first recorded outburst of this source in early 1971. The results of this analysis are combined with the 1978 observations. It is concluded that apsidal motion would have been detectable if the companion were a rapidly rotating star with a mass not less than 30 solar masses.

Kelley, R. L.↗
NASA NTRS1981

Analytic solutions of the radial pulsation equation for rotating and magnetic star models

Eddington's (1926) form of wave equation for small-amplitude, radial, adiabatic stellar pulsations of spherically symmetric, gaseous stars is generalized to include the effects of axial rotation and tangled magnetic fields. Equilibrium quantities possessing dimensions are affected, and the relative importance of rotation and magnetism in affecting pulsation characteristics of the models depends on the choices of gamma and the type of model. Solutions are obtained in closed form for adiabatic pulsation periods of fine analytic stellar models, and nonadiabatic stability criteria are determined by means of the one-zone stellar model. Results are discussed for a range of physical parameters such as rotational angular momenta, central condensations, and magnetic energies; and applications are made to the case of classical Cepheids and other variable giant stars.

Stothers, R.↗
NASA NTRS1981

RR Lyrae stars - A theoretical study of Bailey types a and b

The full-amplitude behaviour of theoretical models of RR Lyr stars pulsating in the fundamental mode has been explored with the use of Carson's radiative opacities. A detailed comparison of the models with metal-poor RR Lyr stars of Bailey types a and b suggests that these stars have properties that lie somewhere in the following ranges: mass/solar mass = 0.55-0.65, Y = 0.2 0.3, and log (luminosity/solar luminosity) = 1.6-1.7. In obtaining these results, the only data that are used are velocity curves and light curves. Independent information, however, seems to corroborate the values found here. The use of Los Alamos opacities gives somewhat similar results, although the derived masses are smaller by about 20%.

Stothers, R.↗
NASA NTRS1981

Convective overshooting in the evolution of very massive stars

Possible convective overshooting in stars of 30-120 solar masses are considered, including a merger between the convective core and the intermediate zone, and penetration by the outer convection zone into the hydrogen-shell region when the star is a supergiant. Convective mixing between the core and inner envelopes is found to lead to a brief renewal of hydrogen burning in the core, and a moderate widening of the main sequence bond in the H-R diagram. Deep penetration by the outer convection zone is found to force the star out of the red supergiant configuration and into a configuration near the main sequence. This would account for the apparent spread of the uppermost part of the main sequence and the concentration of luminous supergiants towards earlier spectral types. In addition, heavy mass loss need not be assumed to achieve the points of agreement, and are tentatively considered unimportant from an evolutionary point of view.

Stothers, R.↗
NASA NTRS1980

Axial rotation, tangled magnetic fields, and theoretical models of very massive stars

A simple method of computing theoretical models of very massive stars endowed with fast axial rotation and tangled magnetic fields is described and used in the present paper. Both of the two perturbing (nongravitational) forces induce changes in the luminosity and radius that are studied as functions of zero-age chemical composition, opacity, and evolutionary state of the interior. The central condensation of the star is found to have a significant influence on shifts of the upper main-sequence band in the H-R diagram if the perturbing force is concentrated in the stellar envelope (but not if the perturbing force is distributed so as to be approximately proportional to gravity everywhere); the layers of the envelope that contribute most heavily to the central condensation lie approximately at a radius fraction of r/R = 0.5. It is shown that fast uniform rotation and intense envelope magnetic fields lead to probably the largest possible shifts of the main-sequence band in the H-R diagram that rotation and magnetic fields can induce. These displacements are, however, too small to account for the total width of the observed main-sequence band at luminosities brighter than log (L/solar luminosity) = 4.5.

Stothers, R.↗
NASA NTRS1980

Giant solar flares in Antarctic ice

A new hypothesis proposes an explanation for the presence of four prominent spikes in a long time record of the NO3(-) concentration inside the Antarctic ice. This solar flare hypothesis suggests that the ionizing radiation necessary in the spike formation could have come from extremely powerful solar flares. It is proposed that these flares would have occurred during the times of the largest maxima in the solar cycle. The solar flare hypothesis is compared with the supernova hypothesis.

Stothers, R.↗
NASA NTRS1980

Influence of a stellar wind on the evolution of a star of 30 solar masses

A coarse grid of theoretical evolutionary tracks was calculated for a 30 solar mass star to determine the role of mass loss in the evolution of the star during core He burning. The Cox-Stewart opacities were applied, and the rate of mass loss, criterion for convection, and initial chemical composition were taken into consideration. Using the Schwarzschild criterion, the star undergoes little mass loss during core He burning and remains a blue supergiant separated from main sequence stars on the H-R diagram. The stellar remnant consists of the original He core and may appear bluer than equally luminous main sequence stars; a variety of possible evolutionary tracks can be obtained for an initial solar mass of 30 with proper choices of free parameters.

Stothers, R.↗
NASA NTRS1980

Hydromagnetics and period changes in RR Lyrae stars

Balazs-Detre and Detre have drawn an interesting parallel between the observed time scales of variability in the sun and in RR Lyrae stars. Additional information is presented here to support their conjecture that an analog of the solar magnetic cycle is operating in RR Lyrae stars. Rough considerations of the expected changes of photospheric radius and of magnetic energy content during a magnetic cycle suggest that the pulsation periods of these stars should also change in time. Within the large observational and theoretical uncertainties, the predicted period changes are compatible with those observed.

Stothers, R.↗
NASA NTRS1980

Age of the Vela pulsar PSR 0833-45

It is shown that previous astrophysical methods of dating the young pulsar in Vela, PSR 0833-45, have effectively given upper limits to the true age. A value of 5000-8000 yr is found here to be not inconsistent with all available observational data. The historical significance of such a young age is briefly discussed.

Stothers, R.↗
NASA NTRS1980

The apsidal motion test in 4U 0900-40

Recent SAS 3 observations of 4U 0900-40 have confirmed the apparent eccentricity of the orbit. Arguments are presented to show that the measured eccentricity is not a spurious effect resulting from systematic reflection of the X-ray beam within the binary system. The longitude of periastron is not observed to have advanced in the 3.5 years since its original determination. The limit on apsidal motion is omega not greater than 3.8 deg/yr which, in turn, restricts the apsidal motion constant to log k not greater than -2.5. These limits are used to constrain the physical parameters of the companion star HD 77581.

Rappaport, S.↗
NASA NTRS1979

Magnetic Cepheids

The role of a magnetic field in the pulsational behavior of classical Cepheids has been studied by computing linearized models of pulsating stellar envelopes pervaded by a well-tangled magnetic field. It is found that the 'pulsational' masses of Cepheids that are implied by the theoretical values of the quantities Q(0), P(1)/P(0), and (possibly) P(2)/P(0) can be brought into line with the large 'evolutionary' masses, if the pressure due to the postulated magnetic field is assumed to be comparable to the thermodynamic pressure everywhere in the pulsating layers. The field strengths that are required - several hundred gauss at the surface and several ten thousand gauss at the base - are quite reasonable from the point of view of the observed magnetic fields in Cepheids. The influence of the magnetic field on the location of the blue edge of the instability strip in the H-R diagram is estimated to be very slight.

Stothers, R.↗
NASA NTRS1979

Stellar evolution at high mass including the effect of a stellar wind

The effect of a stellar wind on the evolution of stars in the mass range from 15 to 120 solar masses is investigated. All the stellar models are constructed with the use of Cox-Stewart opacities. Four possible cases of mass loss are considered: (1) no mass loss at all; (2) substantial mass loss from stars in all stages of evolution; (3) heavy mass loss from red supergiants only; and (4) sudden and very heavy mass loss from luminous yellow supergiants. The assumption of mass loss during the main-sequence phase of evolution is found to lead to a lowering of the luminosity and, unless the mass loss is extremely heavy, of the effective temperature as well. A comparison of the adopted mass-loss rates with observed rates suggests that stellar winds are probably not an important factor in the evolution of main-sequence stars and supergiants unless the initial masses are greater than about 30 solar masses.

Stothers, R.↗
NASA NTRS1979

Solar activity cycle during classical antiquity

Early accounts of phenomena that may be identified as auroral displays have been abstracted from reports of unusual celestial prodigies in the classical literature. An extensive catalog of ancient aurorae and a new mathematical method of analyzing fragmentary time series of observations have been used to demonstrate, provisionally, that an auroral cycle actually existed in antiquity, at least during the 2nd century BC, and that it had an average length and amplitude comparable with those of the modern auroral cycle. On the reasonable supposition that solar activity has always been the factor responsible for aurorae, it can be concluded that the solar cycle two millennia ago was very similar to what it is today.

Stothers, R.↗
NASA NTRS1978

Stellar winds and the evolution of luminous stars

The effect of a stellar wind on the evolution of stars in the mass range 7-60 solar masses has been investigated for stellar models in which Carson's opacities have been employed. Several cases of mass loss have been considered. It is found that the assumption of heavy mass loss from both blue and red supergiants can account well for the relevant observations of OBN stars, WN stars, and very luminous supergiants of all spectral types. But no amount of mass loss can account adequately for the properties of the B supergiants of lowest luminosity. A critical comparison is made between the present results and some earlier results based on the adoption of Cox-Stewart opacities.

Stothers, R.↗
NASA NTRS1978

Bump Cepheids - The mass anomaly as an opacity effect

Nonlinear calculations of models for bump Cepheids indicate that most of the basic features of these stars can be reproduced well by models having normal to nearly normal evolutionary masses and normal helium abundances, if Carson's opacities are adopted. The trends of the properties of the models with various physical parameters are otherwise very similar to those obtained by other authors with the Los Alamos opacities (which have led to Cepheid masses of half the evolutionary masses). Difficulties with the resonance hypothesis of Simon and Schmidt as an explanation of the bump are discussed.

Sastri, V. K.↗