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At least 541 records · Page 30
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NASA NTRS1973

Interstellar extinction in the ultraviolet from emission stars

Since many of the analyses of interstellar extinction in the ultraviolet depend on observations of stars with emission lines, it is important to know whether the slope of the reddening line is on the average the same for normal stars and for emission stars. An investigation of normal and emission stars ranging in spectral class from O5 to O9 (as observed by the Celescope experiment on board OAO 2) indicates that normal and emission O stars do not have the same average color-excess ratio and that this characteristic is particularly prominent at 2300 A. This implies that interstellar extinction in the ultraviolet cannot be mapped unambiguously by using O stars unless all those in the sample are known not to be emission-line objects.

Haramundanis, K. L.↗
NASA NTRS1973

Broad-band 20-micron photometry of 76 stars

Observations in the 16 to 28 micron spectral band made with an infrared photometer mounted at the Cassegrain f/10 focus of a 2.24-m telescope are discussed. The stars observed belonged to classes B through M and included several carbon and S stars. Most of them are Mira, semiregular, or irregular variables. The remaining 12 stars are nonvariable objects of which the brightest (both in the visible and infrared) are alpha-Boo and alpha-Tau. Since 16 of the Mira stars varied during the period of observation, it is concluded that they are probably intrinsic variables at 20 microns. There is no conclusive evidence for variation of any other star. Because the variation at this wavelength originates largely in circumstellar shells for most of these stars, the nonvariability suggests that these shells are stable phenomena, at least on a time scale of a year.

Morrison, D.↗
NASA NTRS1973

A reflection model for eclipsing binary stars

A highly accurate reflection model has been developed which emphasizes efficiency of computer calculation. It is assumed that the heating of the irradiated star must depend upon the following properties of the irradiating star: (1) effective temperature; (2) apparent area as seen from a point on the surface of the irradiated star; (3) limb darkening; and (4) zenith distance of the apparent centre as seen from a point on the surface of the irradiated star. The algorithm eliminates the need to integrate over the irradiating star while providing a highly accurate representation of the integrated bolometric flux, even for gravitationally distorted stars.

Wood, D. B.↗
NASA NTRS1975

CCD star trackers

The application of CCDs to star trackers and star mappers is considered. Advantages and disadvantages of silicon CCD star trackers are compared with those of image dissector star trackers. It is concluded that the CCD has adequate sensitivity for most single star tracking tasks and is distinctly superior in multiple star tracking or mapping applications. The signal and noise figures of several current CCD configurations are discussed. The basic structure of the required signal processing is described, and it is shown that resolution in excess of the number of CCD elements may be had by interpolation.

Goss, W. C.↗
NASA NTRS1974

An infrared survey of RW Aurigae stars

An infrared photometric survey of 89 RW Aur type variables in both hemispheres has been made. JHKL magnitudes and colors are listed. The RW Aur variables include a small number of highly reddened late-type stars. All T Tauri and hot Orion population stars show infrared excesses and the infrared properties mark certain field stars as being young. The greatest infrared excesses are found for A and F stars while young variable B stars usually show no excesses. The location of the RW Aur stars in the two-color H-K, K-L diagram favor dust re-radiation over free-free emission as the mechanism responsible for the infrared excess. A weak correlation of H-K with emission class links the occurrence of circumstellar dust and gas shells.

Glass, I. S.↗
NASA NTRS1975

Metal abundances of RR Lyrae stars in galactic globular clusters

Results are reported for image-tube scanner measurements of Preston's (1959) metal-abundance indicator and metallic-line absorption in the G-band region (4285 to 4317 A) of 47 RR Lyrae stars in 12 galactic globular clusters and 67 RR Lyrae field stars. The relationship (Fe/H) is defined and determined from curve-of-growth analyses of 13 RR Lyrae field stars, while a comparison of metal-abundance vs metallic-line absorption relationships derived for the field and cluster stars indicates that both groups can be calibrated in the same way. Based on this calibration, (Fe/H) values are deduced for the globular-cluster stars and the clusters themselves. It is found that the alpha-rich phenomenon is present in these stars and is confined to calcium and titanium.

Butler, D.↗
NASA NTRS1976

A precision star tracker utilizing advanced techniques and materials

An image dissector star tracker has been developed which operates in the photon counting mode making it possible to utilize all digital electronics. A unique pulse processing circuit allows bright stars to be tracked as well as dim stars. Thermal mechanical stability has been greatly enhanced by fabricating a housing with graphite/epoxy composite material with a linear coefficient of thermal expansion near zero. Test results indicate the +10 Mv stars can be acquired and tracked, while position variation with star intensity is less than 2 arc seconds from 2.5 Mv to +10 Mv. The noise equivalent angle for a +8 Mv star is 3 arc seconds. Polynominal correction for remaining cross-coupling and nonlinearity reduced error over total 1 x 1 deg field to 1.5 arc seconds.

Gates, R. F.↗
NASA NTRS1976

Evidence for mass loss at moderate to high velocity in Be stars

Ultraviolet spectra of intermediate resolution have been obtained with Copernicus for 12 objects classified as Be or shell stars and for 19 additional early B dwarfs. Some of these spectra show marked asymmetries in certain resonance lines, especially the Si IV doublet at 1400 A, indicating the presence in some cases of outflowing material with maximum velocities of nearly 1000 km/s. Direct evidence for mass loss at these velocities is seen for the first time in dwarf stars as late as B1.5; the only objects later than B0.5 which show this effect are Be or shell stars. Among the stars considered, there is a correlation between the presence of mass-loss effects and projected rotational velocity, suggesting that the ultraviolet flux from B1-B2 dwarfs is sufficient to drive high-velocity stellar winds only if rotational effects reduce the effective gravity near the equator. The mass-loss rate for one of the most active Be stars, 59 Cyg, is crudely estimated to be one billionth or one ten-billionth of a solar mass per year. The data suggest that the extended atmospheres associated with Be-star phenomena may be formed by mass ejection.

Snow, T. P., Jr.↗
NASA NTRS1976

On the distance to the candidate star coincident with A0620-00

High-dispersion echelle spectra of the candidate star for the transient X-ray source A0620-00 are discussed. No absorption or emission lines attributed to the star were found under visual examination of the spectra, but interstellar sodium D lines were clearly visible. Results are presented for measurements of the profiles and central velocities of the sodium absorption lines, and other observations of interstellar features in the general direction of the star are summarized. While the exact distance to the star cannot be determined due to the patchy distribution of the interstellar gas, it is suggested that the star is located between 1.5 and 2.5 kpc; 2 kpc is adopted as the most reasonable value. It is noted that the possibility cannot be ruled out that the star lies beyond 3 kpc and is seen through a hole in the distant neutral hydrogen gas.

Gull, T. R.↗
NASA NTRS1977

A comparison of lyman alpha and HeI lambda 10830 line structure and variations in early-type star atmospheres

Fabry-Perot interferometric profiles for fifty of the early-type stars including supergiants, eclipsing binaries, Bp and Ap stars, Be and shell stars, and variable stars have been obtained. Results for beta Persei (Algol) just before primary and secondary eclipses show strong emission profiles lasting about 0.1 phase. An absorption line was seen during secondary eclipse. Bright supergiant stars (O9-A2) show time-variable, complicated absorption/emission profiles similar to those obtained for the Be/shell stars.

Meisel, D. D.↗
NASA NTRS1976

A new interpretation of luminous blue stars

A major revision of current theoretical ideas about the brightest blue stars must be made if Carson's (1976) radiative opacities are adopted in stellar models. Unlike earlier opacities, these exhibit a large 'bump' due to CNO ionization, which leads to very strong central condensation, convective instability, and pulsational instability in hot diffuse stellar envelopes. Despite a number of theoretical uncertainties, the new picture of the structure of very luminous stars is reasonably successful in accounting for a variety of previously unexplained observations. The stellar models for the phase of core hydrogen burning predict large radii and rather cool effective temperatures for O stars and a spreading out of the main-sequence band in the H-R diagram toward luminous cool supergiants for masses higher than about 20 solar masses. In massive X-ray binary systems, circular orbits and supergiant-like visual companions are expected to be quite common. Long-period variability is predicted to exist for massive blue supergiants of luminosity class Ia. The models for helium stars predict large radii and rather cool effective temperatures for Wolf-Rayet stars, as well as multimodal pulsational instability and, possibly, surface turbulence for these stars.

Stothers, R.↗
NASA NTRS1976

Copernicus ultraviolet observations of mass-loss effects in O and B stars

Far-UV spectra of 47 O, B, and A stars obtained with the Copernicus satellite are examined for P Cygni profiles. For all 40 stars with displaced absorption lines, values are given for the velocities of the short-wavelength edge, the line center, and the emission peak (if present). Parts of the spectra of 42 stars are reproduced, evidence for mass motions in ground-based spectra is discussed, and the best available data are summarized on the wavelengths and oscillator strengths of most lines likely to show mass-loss effects in either visual or UV spectra. The main conclusions are that: (1) the far-UV transitions, especially resonance lines, show that mass flow is present over a much wider group of stars than revealed by visible data on subordinate lines; (2) most of the line shifts imply mass motion away from the stars; (3) mass flow occurs in all but one star brighter than a bolometric magnitude of -6.0; and (4) the observed terminal velocities generally exhibit no significant correlation with temperature, luminosity, gravity, rotational velocity, or line strength.

Snow, T. P., Jr.↗
NASA NTRS1976

Nonlinear coupling between pulsation and convection in late type stars

A simple idealized nonlinear model applicable to long-period variable stars has been formulated which assumes that the convective envelope of M giants is composed of giant convection cells comparable in size to the stellar radius. The simplicity of this model essentially constitutes a physical analog to the strong dynamic coupling that occurs if the convective envelope of the star undergoes both modes of motion. As shown implicitly in the time scales associated with these motions, the coupling produces asymmetrical fluctuations of the entire star, the mean velocity of which is comparable to the escape velocity of the star at particular values of the ratio of the pulsation and convection time scales. It is suggested that this can account for the mass loss from late-type stars and the circumstellar dust shells that are associated extensively with long-period variables. For critical values of the pulsation and convection time scales, the solutions correspond to the rapid expansion of the entire convective envelope and are the basis of a mechanism that simulates the manner in which pulsating stars ballistically accelerate their convective shells to form planetary nebulae.

Anand, S. P. S.↗
NASA NTRS1977

Scanner K-line photometry of Orion stars

Results are presented for two-channel scanner measurements of calcium K-line strengths in 39 Orion sword and belt stars. Values of the calcium k index and its associated standard error are given for each observed star, and the K-line strengths are compared with those of K-line standard stars and Hyades stars. Plots of k index against reddening-corrected color and of k-index deviation against metal-strength index deviation are provided which show that the Orion sword and belt stars do not differ significantly in their calcium and metal abundances from general field stars.

Hesser, J. E.↗
NASA NTRS1976

Characteristics of IR variable stars as observed from orbit

A selection of infrared variable stars was studied at wavelength 2.7 microns during 1971-1975 with data from U.S. Air Force satellites. Stars observed in this program are classified as long-period variable stars, semiregular variables, and irregular variables and are among the strongest stellar sources at this wavelength. In addition, a few new, as yet unclassified variable stars were identified during the course of the investigation. Time scales of reproducible variations range from a few weeks to a few years, and amplitudes of variation are as large as a factor of three for stars with periods of order one year. The minimum infrared flux density of a long-period star repeats accurately from one cycle to the next, whereas the maximum flux density was found to be unstable. The correlation of 2.7 micron and radio emission line data from one, well-studied long-period variable is consistent with the hypothesis that the H2O and OH circumstellar masers are saturated, if pumped by the stellar infrared flux near 2.7 microns.

Maran, S. P.↗
NASA NTRS1978

Gamma-ray lines from neutron stars as probes of fundamental physics

The detection of gamma-ray lines produced at the surface of neutron stars will serve to test both the strong and gravitational interactions under conditions unavailable in terrestrial laboratories. Observation of a single redshifted gamma-ray line, combined with an estimate of the mass of the star will serve as a strong constraint on allowable equations of state of matter at supernuclear densities. Detection of two redshifted lines arising from different physical processes at the neutron star surface can provide a test of the strong principle of equivalence. Expected fluxes of nuclear gamma-ray lines from accreting neutron stars were calculated, including threshold, radiative transfer and redshift effects. The most promising probes of neutron star structure are the deuterium formation line and the positron annihilation line. Detection of sharp redshifted gamma-ray lines from X-ray sources such as Cyg X-1 would argue strongly in favor of a neutron star rather than black hole identification for the object.

Brecher, K.↗
NASA NTRS1978

On the angular momentum loss of late-type stars

The observed surface angular velocity of main-sequence stars shows a sharp decrease at about spectral type F6. It is suggested that stars more massive than F6 cannot experience an appreciable angular-momentum loss because their convection zones cannot sustain a magnetic dynamo: without a magnetic field the angular-momentum loss is very small. The influence of rotation on the convective motions is essential for the existence of a solar-type dynamo. Rotation can influence these convective motions only if the typical convective time is larger than the rotation time. For main-sequence stars of different masses and chemical compositions the dimensionless parameter (convective velocity/sum's angular velocity times mixing length in the lower part of the convection zone) is evaluated. It is shown that this parameter increases very sharply for stars whose mass exceeds that defined by the relation log(star mass/solar mass) is of the order of 0.1. Thus even for large angular velocities, magnetic dynamos are not feasible if log(star mass/solar mass) appreciably exceeds 0.1.

Durney, B. R.↗
NASA NTRS1978

Star formation in molecular clouds

A general overview is presented of the findings of molecular line and infrared astronomy regarding star formation. Beginning with the statement that stars, and therefore, planets, form from molecular clouds, it is argued that the growing body of data on conditions in molecular clouds should serve as input parameters for theoretical calculations of star and planet formation. The fact that the gravitational potential energy of molecular clouds far exceeds their thermal energy implies that the required rate of star formation is easily explained. Indeed the most important problem in this field is that the rate of star formation predicted from molecular cloud studies is too high. Until this problem is understood, additional modes of inducing star formation, while they may be occurring, only add to the existing difficulties.

Evans, N. J., II↗