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At least 1,297 records · Page 72
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NASA NTRS1980

The detection of a companion star to the Cepheid variable Eta Aquilae

Ultraviolet spectra have been obtained with IUE of the classical Cepheid Eta Aquilae at several phases in the 7.18 day period. Significant ultraviolet emission is detected at wavelengths less than 1600 A, where little flux is expected from classical Cepheids. Furthermore, the emission at wavelengths less than about 1600 A does not vary with phase. Comparison with model atmosphere flux distributions shows that the nonvariable emission is consistent with the flux expected from a main-sequence companion star with an effective temperature of about 9500 K (A0 V). The observed ultraviolet flux and spectral type are used to compute a distance of 300 pc to the system, in agreement with the distance predicted using the period luminosity relation.

Mariska, J. T.↗
NASA NTRS1980

Mercury in a thin layer in HgMn stars - A test of a diffusion model

Lines of the first three states of ionization of mercury have been observed in Mu Leporis and Chi Lupi using the Copernicus satellite. Lines of Hg II and Hg III have been observed in Alpha Andromedae. There appears to be an absorption feature at every wavelength where there is expected to be a mercury line. The presence of all three states of ionization is likely in Mu Lep and Chi Lup. The relative equivalent widths of the lines of the various states of ionization do not depend on the effective temperature of the stars, in contradiction to what is expected if mercury were uniformly distributed in the atmosphere. It is, however, expected if mercury has been concentrated, by diffusion, in a thin layer, where the radiative forces just equal the gravitational forces on mercury. That mercury should be so concentrated is also required by the explanation of the mercury isotope anomaly proposed by Michaud, Reeves, and Charland.

Megessier, C.↗
NASA NTRS1980

Gigahertz analog modulation and differential delay of GaAlAs lasers - Temperature and current behavior

Gigahertz analog modulation characteristics of broad-area commercially available GaAlAs lasers have been investigated as a function of temperature and current in the vicinity of the upper frequency limit, where the resonance phenomena occur. The optimum temperature for small-signal amplitude modulation was found to be around -15 C for our particular broad-stripe geometry double-heterostructure laser. The Q was found to increase by a factor of 2 and the bandwidth by about 2%; the external quantum efficiency was maximized in this range. The optimum dc current bias was about 2% above the threshold current. Differential delays have also been measured down to a few picosecond accuracy by a unique phase-angle measurement method using a vector voltmeter. Some of the temperature effects observed may be related to mode changes and multimode and superradiance behavior.

Eng, S. T.↗
NASA NTRS1981

Test results of flight guidance for fuel conservative descents in a time-based metered air traffic environment

The NASA developed, implemented, and flight tested a flight management algorithm designed to improve the accuracy of delivering an airplane in a fuel-conservative manner to a metering fix at a time designated by air traffic control. This algorithm provides a 3D path with time control (4D) for the TCV B-737 airplane to make an idle-thrust, clean configured (landing gear up, flaps zero, and speed brakes retracted) descent to arrive at the metering fix at a predetermined time, altitude, and airspeed. The descent path is calculated for a constant Mach/airspeed schedule from linear approximations of airplane performance with considerations given for gross weight, wind, and nonstandard pressure and temperature effects. The flight management descent algorithms are described and flight test results are presented.

Knox, C. E.↗
NASA NTRS1981

Absolute energy curves from late B-type supergiants

Energy curves were determined for six late B and early A type supergiants using IUE data and other ultraviolet and ground based photometry. Effective temperatures and angular diameters are presented as well as estimates of the outflow velocity of the wind. All six stars show a strong Balmer continuum in emission; the Ia supergiants also show an infrared excess which reaches into the visible range. Evidence is found for the presence of a warm mantle as well as for wind from the Ia stars.

Underhill, A. B.↗
NASA NTRS1980

Theoretical profiles for the 1-0 S/1/ H2 line in carbon stars

The spectra of carbon stars have been synthesized from the models of Querci, Querci, and Tsuji and Querci and Querci in the region of the 1-0 S(1) vibration-rotation quadrupole line of H2. The line is shown to be sufficiently strong to be seen against the numerous lines of the CN red system for models with effective temperatures less than about 2700 K. The usefulness of the line as a diagnostic of the atmosphere is discussed, and a comparison with the measured spectra of UU Aur and S Cep is made. It is concluded that the outer atmospheric layers of carbon stars are significantly warmer than the models predict. An additional opacity source in the outer layers is required.

Goorvitch, D.↗
NASA NTRS1980

Observations of the motion and distribution of the ionized gas in the central parsec of the Galaxy. II

Observations of infrared fine-structure line emission from compact clouds of ionized gas within Sgr A West are presented. These clouds have diameters of 0.1-0.5 pc, internal velocity dispersions of 100 km/s (FWHM), and line center velocities up to + or - 260 km/s. Their masses are not accurately determined but are probably between 0.1 and 10 solar masses. They are ionized by radiation like that of stars of effective temperature not greater than 35,000 K. The clouds are shown to have lifetimes of 10,000 yr and so must be generated and dissipated at a rate of a few per 1000 yr. From analysis of the distribution of the velocities of the clouds, a most probable mass distribution is derived which includes a central pointlike mass of several x 10 to the 6th solar masses in addition to several x 10 to the 6th solar masses of stars within 1 pc of the center.

Lacy, J. H.↗
NASA NTRS1980

Far-infrared forbidden O III line emission from the galactic center

The forbidden O III 51.8 micron fine-structure line has been detected in Sgr A West. It appears that the emission arises from the same compact clouds within the central parsec of the Galaxy which are observed in forbidden Ne II. The line intensity is used to derive an effective temperature of 32,000-40,000 K for the radiation field that ionizes the clouds. An upper limit is also reported for the forbidden O III 88.4 micron fine-structure line in Sgr A West. From this upper limit, one can conclude that the average electron density outside the known ionized clouds and within 30 arcsec of the galactic center is less than 40 per cu cm.

Watson, D. M.↗
NASA NTRS1980

IUE ultraviolet spectra and chromospheric models of HR 1099 and UX Arietis

IUE spectra in the region 1150-3200 A of the RS CVn-type variables HR 1099 and UX Arietis are presented and analyzed in terms of chromospheric models. Measurements of Mg h and k lines and Ca II H-K and H alpha spectra are indicated which are found not to be correlated with orbital phase or radio flares and which suggest that the strong emission arises in the K star rather than the G star in these systems. Under the assumption that the UV emission lines are associated with the K star, surface gravities of log g = 3.6 and 3.4 and effective temperatures of 4850 and 5000 K are adopted for HR 1099 and UX Ari, respectively, along with solar metal abundances for each. Model calculations of the chromospheric structure necessary to account for observed C(+), Mg(+), Si(+) and Si(+2) line fluxes are presented which indicate that the transition region pressure lies in the range 0.18-1.0 dynes/sq cm, implying transition regions that are more extended than that of the sun and are not conductively heated. It is noted that pressure scaling laws and the use of Mg II and C II lines as pressure diagnostics may be invalid, possibly due to atmospheric inhomogeneities or gas flows.

Simon, T.↗
NASA NTRS1980

An absorption line in the ultraviolet spectrum of 40 Eridani B

Two excellent low-resolution spectra show an absorption line of equivalent width 3 A, near 1391 A, in the typical DA (hydrogen atmosphere) white dwarf 40 Eri B. The line is confirmed by a high-resolution spectrum and is the first seen in any DA star. Ultraviolet fluxes and the profile of Lyman-alpha confirm an effective temperature near 17,000 K. If the line is Si IV, it requires a temperature near 40,000 K. Unattractive possibilities are a hot circumstellar absorbing envelope dependent on accretion from companions, or formation at large optical depth in a transparent atmosphere with high Si/H. A suggestion that H2 should be considered leads to the possible interpretation as the (0, 5) transition of the Lyman band, formed at small optical depth. The band should be stronger in cooler DAs.

Greenstein, J. L.↗
NASA NTRS1979

The energy distribution of Sirius B

The origin of the X-ray emission observed from the DA white dwarf Sirius B is investigated on the basis of low-resolution short wavelength IUE spectrum. Following a correction for the influence of scattered light from Sirius A, the intensity distribution of Sirius B is obtained as a function of wavelength. Comparison of the continuous energy distribution with model atmosphere energy distributions indicates an effective temperature of 25,500 K, which is not high enough to explain the observed X rays as normal photospheric radiation. In addition, with the exception of an unusual feature in the neighborhood of the C I 1657 line, no clear evidence is found of emission lines indicative of a corona, chromosphere or disk, and the origin of the X rays remains unknown.

Boehm-Vitense, E.↗
NASA NTRS1980

Vector space methods of photometric analysis. II - Refinement of the MK grid for B stars. III - The two components of ultraviolet reddening

This paper discusses systematic errors which arise from exclusive use of the MK system to determine reddening. It is found that implementation of uvby, beta photometry to refine the qualitative MK grid substantially reduces stellar mismatch error. A working definition of 'identical' ubvy, beta types is investigated and the relationship of uvby to B-V color excesses is determined. A comparison is also made of the hydrogen based uvby, beta types with the MK system based on He and metal lines. A small core correlated effective temperature luminosity error in the MK system for the early B stars is observed along with a breakdown of the MK luminosity criteria for the late B stars. The second part investigates the wavelength dependence of interstellar extinction in the ultraviolet wavelength range observed with the TD-1 satellite. In this study the sets of identical stars employed to find reddening are determined more precisely than in previous studies and consist only of normal, nonsupergiant stars. A multivariate analysis of variance techniques in an unbiased coordinate system is used for determining the wavelength dependence of reddening.

Massa, D.↗
NASA NTRS1980

On the detection of other planetary systems - Detection of intrinsic thermal radiation

There is currently no unambiguous observational evidence for the existence of other planetary systems. One possible way to detect and study such systems is infrared observations of continuum blackbody radiation from planets revolving around other stars. It is shown that the effective temperature of large planets revolving around mid- to late-spectral-type main-sequences stars is set by energy sources internal to the planet rather than by equilibrium with the radiation field of the central star, making them easier to detect than had been previously thought. Consideration is given to the two major observational constraints on detecting planetary companions to nearby stars, namely, angular resolution and sensitivity. A comparison is made between the performance of an ambient (T 200 K), single-aperture telescope and a cooled interferometer. In each case the required aperture (baseline) is large (in the 10-m class), but consistent with Shuttle launch capability.

Black, D. C.↗
NASA NTRS1981

Far-ultraviolet energy distributions of the metal-poor A stars HD 109995 and HD 161817

Low-resolution IUE spectra at wavelengths between 1300 and 3400 A of the metal-poor stars HD 109995 (A1p) and HD 161817 (A4p) have been compared with model-atmosphere energy distributions computed by Kurucz (1979). Good overall agreement is found. Effective temperatures, metal abundances, and angular diameters could be determined. Assuming an absolute visual magnitude of 0.7, the previously determined gravity log = 3 yields masses of 0.5 solar masses for both stars. It is found that the theoretical UBV colors calculated earlier agree reaonably well with the ones observed for these stars.

Boehm-Vitense, E.↗
NASA NTRS1981

The far-ultraviolet energy distribution of two globular cluster blue horizontal-branch stars in M13

The far-UV energy distribution of two blue horizontal-branch (BHB) stars detected in the nucleus of M13 is presented. Their continua can be characterized by a model atmosphere of an effective temperature of 15,000 to 20,000 K and log g = 4.5, and it follows that V is in the range 15.9-16.7 for these stars. Comparison with integrated photometry by OAO 2 and by ANS indicates that, at wavelengths smaller than 1800 A, the integrated light is from BHB stars only, of which M13 contains about 200. For these far-UV wavelengths, the color of the BHB stars is also very similar to the integrated color of the elliptical galaxy M87.

De Boer, K. S.↗
NASA NTRS1981

Precursor active galaxies and the cosmic X-ray background

Consideration is given to Carr's theory (1980) that most cosmic X-ray background is produced by thermal X-ray emission associated with black hole accretion disks for compact objects. It is posited that the onset of significant X-rays occurs during galaxy formation when accretion disks are formed by the interaction of young galaxies and massive black holes of pregalactic origin. The study suggests that most cosmic X-ray background could be produced from distant sources with a low ratio of optical to X-ray luminosity. Precursor active galaxy objects involving a massive black hole surrounded by a hot optically thin accretion disk radiating near the Eddington luminosity limit and with an effective temperature of about 2 x 10 to the 9th K would fulfill the condition and produce the correct background spectrum and intensity. This model also associates cosmic X-ray background and nonthermal gamma-ray background to active galaxies, thus implying that massive black holes are associated with their internal dynamics.

Boldt, E.↗
NASA NTRS1981

Observations of strong ultraviolet absorptions in the spectrum of the DC white dwarf LDS 678 B

Evidence from spectroscopic observations that LDS 678 B has strong ultraviolet lines of neutral carbon in its spectrum is presented. The spectra are found to contain strong lines of C I in the ultraviolet and weak He I lines in the visible. The appearance of the star's spectrum and published photometry are consistent with its having an effective temperature near 10,600 K with a helium-dominated atmosphere and a carbon abundance lower than C:He = 0.001. The star's spectra are compared with those of the white dwarfs L879-14 and L145-141, which are cooler than LDS 678B, have C2 bands in the visible and also show strong evidence of strong ultraviolet C I lines. It is concluded that LDS 678 B has a temperature intermediate between the DB and C2 band white dwarfs and that the ultraviolet C I lines can be a useful means for finding carbon in the atmosphere of the cooler white dwarfs.

Wegner, G.↗