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Stellar scintillation has been measured for apertures as low as 0.012 m by using photoncounting techniques. The scintillation measurements have been made for various aperture sizes in quick succession in order to determine experimentally the effects of aperture averaging. The results of the experiment are compared with the predictions of a model proposed earlier in the literature to quantify the phenomenon.
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.
The JSC/SRL Balloon-borne Ultraviolet Stellar Spectrometer (BUSS) comprises a 40 cm telescope, an echelle spectrograph and a SEC Vidicon detector. Operating at an altitude of 40 km this instrument provides spectra of stars with 0.1 A resolution in the 2000-3400 A region. Observations made on 19 May 1976, are discussed.
Approximate signal-to-noise ratios are calculated for 12 stellar occultations by the rings of Uranus during 1977-1980. Four of the stars are apparently bright enough to permit observation of the epsilon-ring occultations with a large telescope. For the best of these events, occultations by rings alpha through delta should also be observable with a large telescope, and epsilon-ring occultations should be detectable with smaller telescopes. Formulas for the signal-to-noise ratios are given to aid potential observers in evaluating the quality of the results they can expect to obtain with their own photometric equipment.
Multiaperture infrared photometric observations of the central regions of 51 early-type galaxies and of the integrated light of five globular clusters are presented. These data are compared with selected optical observations and with various model predictions. The main results of the work are: (1) the observed parameters for the brighter galaxies, particularly the CO index and the V-K color, agree with the predictions of stellar synthesis models characterized by giant-dominated populations with mass/visual luminosity ratios less than 10; (2) the galaxian broad-band colors tend to redden with increasing luminosity and decreasing aperture size; (3) for the globular clusters, there is evidence that the integrated colors become redder with increasing metallicity; and (4) in bright galaxies the relative changes of U-V, V-J, and J-K as functions of radius may differ from the relative changes as functions of luminosity at a fixed radius.
Data from high-resolution SEC vidicon spectroscopy with a ground-based telescope (for the Ca II K line) and from spectral scans made with the BUSS ultraviolet balloon spectrograph (for the Mg II h and k lines) are used to derive models of the chromospheres and upper photospheres of three G-K giants. The models are based on partial-redistribution analyses of the Ca II K line wings and cores and on the fluxes in the Mg II lines. The photospheres thus computed are hotter than predicted by radiative-equilibrium models. The minimum-to-effective temperature ratio is found to decrease with decreasing effective temperature, while the mass column density at the top of the chromosphere increases with decreasing stellar surface gravity. The computed pressure at the chromosphere top in the primary member of the Capella spectroscopic binary system is 70 times smaller than the transition-region pressure derived by Haisch and Linsky (1976), which suggests that additional terms must be included in the transition-region energy equations for giant stars. Estimates of the Ca II and hydrogen column densities are made for the circumstellar envelope of Aldebaran.
A prototype telescope system with six movable elements has been used with sharpness detection procedures to observe objects as dim as fifth magnitude. The real-time image sharpening technique does not rely on the use of a reference star. By correcting for atmospherically induced phase perturbations, the sharpening system has permitted restoration of stellar images to the diffraction limit (in one dimension) for a 30-cm telescope. Application of the system to the separation of double stars is reported.
Copernicus UV observations of O VI in early-type stars are used to infer the existence of coronae or hot stellar winds. Attention is given to the optical doublet of O VI at wavelengths 3811 and 3834, which is seen in the spectra of some central stars in planetary nebulae, and taken to be evidence of a coronal wind. It is noted (from observations of NGC 6751) that hot gas in the coronal model produces radiation at energies above 54 eV, which can affect He II Zanstra temperatures. He(+) ionizing radiation in population I early-type stars is also taken as an indication of hot winds.
The nonequilibrium thermodynamic basis for modeling a stellar structure, subatmosphere, and atmosphere is formulated, along with the thermodynamic characterization of a star as a concentration of matter and energy in the interstellar medium. The analytical formulation of nonequilibrium thermodynamic models is discussed in the case where the nonisothermal storage modes are kinetic modes.
Coronal heating by resonant absorption of Alfvenic surface waves (quiescent), and magnetic tearing instabilities (impulsive), is discussed with emphasis on three principles which may have application to late-type evolved stars. (1) If sq B/8 pi greater than sq. rho V is observed 2 in a stellar atmosphere, then the observed magnetic field must originate in an interior dynamo. (2) Low mass loss rates could imply the presence of closed magnetic flux loops within the outer atmosphere which constrain hydrodynamic flows when the magnetic body forces exceed the driving forces. (3) given that such magnetic loops effect an enhancement of the local heating rate, a positive correlation is predicted between the existence of a corona and low mass loss rates. These principles are applied to the M giant star HD 4174, which is purported to have a kilogauss magnetic field. Several of its spectroscopic peculiarities are shown to be consistent with the above principles, and further observational checks are suggested.
Six 128 x 128 CID imagers fabricated on bulk silicon and with thin polysilicon upper-level electrodes were tested in a star tracking mode. Noise and spectral response were measured as a function of temperature over the range of +25 C to -40 C. Noise at 0 C and below was less than 40 rms carriers/pixel for all devices at an effective noise bandwidth of 150 Hz. Quantum yield for all devices averaged 40% from 0.4 to 1.0 microns with no measurable temperature dependence. Extrapolating from these performance parameters to those of a large (400 x 400) array and accounting for design and processing improvements, indicates that the larger array would show a further improvement in noise performance -- on the order of 25 carriers. A preliminary evaluation of the projected performance of the 400 x 400 array and a representative set of star sensor requirements indicates that the CID has excellent potential as a stellar tracking device.
Geometrical considerations are presented for analyzing use of the stellar occultation technique on long duration earth atmosphere monitoring missions. The simulated mission data are presented for three representative orbits. Bright near infrared stars are used as examples of how extensive global longitude-latitude coverage is obtained by performing occultation measurements on several stars during the course of a one year mission. A comparison is made with similar missions using the sun as a light source.
The OSO 8 satellite, launched on June 21, 1975, contains two X-ray polarimeters. These polarimeters use mosaic crystals of graphite to yield polarization-sensitive Bragg reflection of stellar X-rays. The crystals reflect a narrow energy bandwidth centered at 2.6 and 5.2 keV. The polarimeter background signal is minimized by mounting the crystals on parabolic surfaces which focus the diffracted X-rays onto small-area beryllium-window proportional counters. This technique permits the observation of low-intensity X-ray sources and reduces the possibility of systematic background effects which could lead to a false signature of polarization. A description of the instrument is given, and preliminary results for the Crab Nebula and Cyg X-1 are presented.
The stellar-evolution program developed by Sweigart (1973) was used to compute main sequence turnoff, red giant sequences, and horizontal branch sequences for many values of helium abundance and heavy-element abundance. The immediate aim has been to produce a self-consistent set of evolutionary sequences for wide ranges in the composition and mass. The long-term aim involves application to cluster HR diagrams and the integrated properties of galaxies. Turnoff sequences, red giant sequences, and horizontal branch sequences are discussed.
Computer programs have been developed for use in collecting and processing data from a PDS scanning microdensitometer. The goal is to obtain fast and simple algorithms for handling an entire astronomical photograph with one-pass digitization. This capability is realized by a real-time detection scheme that provides a data compression of a factor of 100, and a processing program that produces a catalog of magnitudes, color indices, and positions for up to 90,000 multicolor stellar images.
Ultraviolet stellar fluxes are presented in graphs and tables for 164 bright stars in the spectral region from 1200 to 3600 A. The spectra represent a subset of OAO 2 spectrometer data on file at the National Space Science Data Center. The monochromatic flux is given in units of erg per (sq cm-s-A) with a spectral resolution of about 22 A in the region from 3600 to 1850 A and of approximately 12 A in the region from 1850 to 1160 A.
Substantial and systematic departures from ionization equilibrium can occur in the solar transition region and corona when mass outflows are present. Modeling calculations illustrate the general characteristics of the ionization balance in such regions. The presence of nonequilibrium conditions suggests a natural explanation for the extended region of EUV line emission that is observed above the solar limb. Comparison with observations of a coronal hole on the disk indicates that outflow may not start until temperatures of about 250,000 K are reached. Additional consequences include a diminution of the density discrepancy between ultraviolet and radio observations of coronal holes, and potential effects on the energy balance in solar and stellar atmospheres undergoing mass loss.
The evolution of initially spherical, uniform-density, and uniform-velocity-dispersion stellar systems is investigated by means of a three-dimensional N-body computer model. Some 100,000 simulation stars are used to follow the collapse and relaxation of the system for various amounts of angular momentum in solid-body rotation. For initially low values of the angular momentum satisfying the Ostriker-Pebbles stability criterion the systems quickly relax to an axisymmetric shape and resemble elliptical galaxies in appearance. The maximum flattening for these systems is equivalent only to an E2 system. For larger values of the initial angular momentum bars develop, and the systems undergo a much more drastic evolution. The apparent rotational and random velocities of the barred systems are very sensitive to the viewing direction. An additional complication is the frequent misalignment of the apparent major axis with the direction that reflects the maximum rotation.