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At least 145 records · Page 8
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NASA NTRS1985

Intermediate and narrow band photometry of Epsilon Aurigae

Intermediate band blue (4530A), far red (7790A) and H-alpha intermediate and narrow band photoelectric observations of the peculiar, 27 year eclipsing binary, Epsilon Aurigae were made from December 1981 through the present (December 1984). BD +42 1170 served as the primary comparison star because of its angular proximity to the variable star. The analysis of this data along with other available photometry was undertaken to study the characteristics of the low amplitude, semi-regular light variations that appear inside and outside of eclipse. It appears that these short term light variations arise from nonradial pulsations of the luminous f supergiant in the system. Furthermore, the semi-regular light variations found for Epsilon Aurigae are similar to those found for other luminous A-F supergiants. Also, the preliminary results from the analyses of the light variations produced by the eclipse of the F-supergiant by the mysterious cooler component is discussed.

Donovan, R. A.↗
NASA NTRS2002

Asteroseismology from Space: The Delta Scuti Star Theta(sup 2) Tauri Monitored by the WIRE Satellite

The bright variable star theta(sup 2) Tau was monitored with the star camera on the Wide-Field Infrared Explorer satellite. Twelve independent frequencies were detected down to the 0.5 mmag amplitude level. Their reality was investigated by searching for them using two different algorithms and by some internal checks: both procedures strengthened our confidence in the results. All the frequencies are in the range 10.8-14.6 cd(exp -1). The histogram of the frequency spacings shows that 81% are below 1.8 d; rotation may thus play a role in the mode excitation. The fundamental radial mode is not observed, although it is expected to occur in a region where the noise level is very low (55 mu mag). The rms residual is about two times lower than that usually obtained from successful groundbased multisite campaigns. The comparison of the results of previous campaigns with the new ones establishes the amplitude variability of some modes.

Poretti, E.↗
NASA NTRS2002

Asteroseismology from Space: The Delta Scuti Star Theta(sup 2) Tauri Monitored by the WIRE Satellite

The bright variable star (theta)(sup 2) Tau was monitored with the star camera on the Wide-Field Infrared Explorer satellite. Twelve independent frequencies were detected down to the 0.5 mmag amplitude level. Their reality was investigated by searching for them using two different algorithms and by some internal checks: both procedures strengthened our confidence in the results. All the frequencies are in the range 10.8 - 14.6 cd(exp -1). The histogram of the frequency spacings shows that 81% are below 1.8 d; rotation may thus play a role in the mode excitation. The fundamental radial mode is not observed, although it is expected to occur in a region where the noise level is very low (55 (micro)mag). The rms (root mean square) residual is about two times lower than that usually obtained from successful groundbased multisite campaigns. The comparison of the results of previous campaigns with the new ones establishes the amplitude variability of some modes.

Poretti, E.↗
NASA NTRS2002

The Sun: Our Nearest Star

We have in our celestial backyard, a prime example of a variable star. The Sun, long thought to be "perfect" and unvarying, began to reveal its cycles in the early 1600s as Galileo Galilei and Christoph Scheiner used a telescope to study sunspots. For the past four hundred years, scientists have accumulated data, showing a magnetic cycle that repeats, on average, every eleven (or twenty-two) years. In addition, modern satellites have shown that the energy output at radio and x-ray wavelengths also varies with this cycle. This talk will showcase the Sun as a star and discuss how solar studies may be used to understand other stars.

Adams, M. L.↗
NASA NTRS2006

The Asymmetrical Wind of the Candidate Luminous Blue Variable MWC 314

We present the results of long-term spectropolarimetric and spectroscopic monitoring of MWC 314, a candidate Luminous Blue Variable star. We detect the first evidence of H alpha variability in MWC 314, and find no apparent periodicity in this emission. The total R-band polarization is observed to vary between 2.21% and 3.00% at a position angle consistently around approximately 0 degrees, indicating the presence of a time-variable intrinsic polarization component, hence an asymmetrical circumstellar envelope. We find suggestive evidence that MWC 314's intrinsic polarization exhibits a wavelength-independent magnitude varying between 0.09% and 0.58% at a wavelength-independent position angle covering all four quadrants of the Stokes Q-U plane. Electron scattering off of density clumps in MWC 314's wind is considered as the probable mechanism responsible for these variations.

Wisniewski, John P.↗
NASA NTRS1981

The detection of X-ray variability in O stars

Seven O stars known to have strong, and sometimes variable, stellar winds have been observed repeatedly with the Imaging Proportional Counter on the Einstein Observatory, in a program designed to determine whether the X-ray fluxes from these stars are variable. In three cases, definite changes were seen, either on a time scale of a year (Iota Ori and Delta Ori) or five days (15 Mon). In two of these cases, the X-ray spectrum was harder when the overall flux was higher, indicating that some of the fluctuations may take place in a hot (approximately 10 to the 7th K) emitting region at the bottom of the winds.

Snow, T. P., Jr.↗
NASA NTRS1987

A search for RR Lyrae variables in NGC 147

An analysis of repeated deep CCD observations of a field in the Local Group dwarf elliptical galaxy NBC 147 has resulted in the detection of 34 variable stars. The periods and shapes of light curves obtained for 13 of these variables show nine of them to be RR Lyrae stars. An intensity-averaged phase-weighted-mean g magnitude of 25.15 (+ 0.17 or - 0.15) is found which indicates a distance modulus to NGC 147 of 23.85 (+ 0.22 or - 0.20), or 589 kpc.

Saha, A.↗
NASA NTRS1988

Chromospherically active stars. IV - HD 178450 = V478 Lyr: An early-type BY Draconis type binary

It is shown that the variable star HD 178450 = V478 Lyr is a chromospherically active G8 V single-lined spectroscopic binary with a period of 2.130514 days. This star is characterized by strong UV emission features and a filled-in H-alpha absorption line which is variable in strength. Classified as an early-type BY Draconis system, it is similar to the BY Dra star HD 175742 = V775 Her. The unseen secondary of HD 178450 has a mass of about 0.3 solar masses and is believed to be an M2-M3 dwarf.

Fekel, Francis C.↗
NASA NTRS1989

Variability of X-ray emission from OB stars

The variability in soft X-ray emission of 12 OB stars is studied. Two different methods of analysis, one more suitable for detecting short-term variations, the other aimed at detecting long time-scale variations, are applied to all stars in the sample. The long-term variability analysis shows that Cyg-OB2 8A Zeta Pup and Delta Ori exhibit significant count rate variations between different data sections. Similar variations are marginally detected in 15 Mon; the count rate variations for the other eight stars are consistent with statistical fluctuations. The light curve of Cyg-OB2 8A suggests the existence of two different emission levels. The short-term variability analysis detects marginal variability in Tau Sco with an effective amplitude of about 30 percent and a time scale of about 50 s. The upper limits to the effective short-term variability amplitude for all other sample stars are in the 10-30 percent range.

Collura, A.↗
NASA NTRS1979

The list of the stars from the Two-micron sky survey: A preliminary catalog by G. N. Neugebauer et al (IRC), the identification of which is not correct

Two tables are published. The first presents the numbers of stars according to the Catalog IRC for which all or part of the identification with the Catalogs BS-HR, GC or DM is incorrect. The second table presents the numbers of stars in the IRC catalog, for which the identifications with variable stars printed with them are erroneous, whereas other identifications are usually correct.

Kukarkin, B. V.↗
NASA NTRS1994

A spectrum of the veiled T Tauri star CY Tau

We present a flux calibrated spectrum of the star listed as CY Tau in the `General Catalog of Variable Stars 4th ed.' in the spectral range 3700 A less than or equal to lambda less than or equal to 6400 A with a resolution of approximately equals 15 A showing the Balmer-Series from H(sub beta) to H(sub 10) as well as the CaII H (in blend with H(sub epsilon) and K lines in emission. Apart from the emission lines the spectrum is composed of a continuum equivalent to that of an ordinary pre-main sequence star (i.e. a `naked' T Tau) of spectral type M2 V with emission lines plus a `blue' veiling continuum that can be described as black body radiation of temperature T(sub BL) approximately equals 7000K due to accretion onto a boundary layer at a rate of M-dot(sub acc) greater than or approximately = 2.18 10(exp -8) solar mass/a.

Stuewe, J. A.↗
NASA NTRS1977

Satellite observations of new infrared sources

Preliminary results are reported for satellite observations of about 1000 IR sources at a wavelength of 2.7 microns. The observations were made in the declination zone between +10 and -10 deg to a limiting flux of 4 by 10 to the -16th power W/sq cm per micron. Of 83 previously unobserved sources, twenty are identified with late-type stars having visual magnitudes of 6 to 9. The nine sources with available luminosity classifications are found to be giant stars, and flux densities at 2.2 and 3.6 microns are predicted for four unreddened stars among these. Three other detected sources are positively identified with red variable stars; twenty more are tentatively identified as such. It is suggested that some of the 40 blank-field 2.7-micron sources detected at high galactic latitude may be Mira variables with thick dust shells located at large distances from the galactic plane.

Heinsheimer, T. F.↗
NASA NTRS1989

Chromospherically active stars. VI - HD 136901 = UV CrB: A massive ellipsoidal K giant single-lined spectroscopic binary

The variable star HD 136901 = UV CrB is a chromospherically active K2 III single-lined spectroscopic binary with an orbital period of 18.665 days. It has modest-strength Ca H and K emission and UV features, while H-alpha is a strong absorption feature containing little or no emission. The inclination of the system is 53 + or - 12 deg. The v sin i of the primary is 42 + or - 2 km/s, resulting in a minimum radius of 15.5 + or - 0.8 solar. When compared with the Roche lobe radius, this results in a mass ratio of 2.90 or larger. Additional constraints indicate that the secondary has a mass between 0.85 and 1.25 solar. Thus, the mass of the primary is at least 2.5 solar and probably is in the range 2.5-4 solar.

Fekel, Francis C.↗
NASA NTRS1997

Asteroseismology of White Dwarf Stars

The primary purpose of this investigation has been to study various aspects of multimode pulsations in variable white dwarfs. In particular, nonlinear interactions among pulsation modes in white dwarfs (and, to some extent, in other variable stars), analysis of recent observations where such interactions are important, and preliminary work on the effects of crystallization in cool white dwarfs are reported.

Hansen, Carl J.↗
NASA NTRS2006

Correlated Temporal and Spectral Variability in Neutron Star and Black Hole X-Ray Binaries

The variability of neutron star and black hole X-ray sources has several dimensions, because of the roles played by different important time-scales. The variations on time scales of hours, weeks, and months, ranging from 50% to orders of magnitude, arise out of changes in the flow in the disk. The most important driving forces for those changes are probably various possible instabilities in the disk, though there may be effects with other dominant causes. The changes in the rate of flow appear to be associated with changes in the flow's configuration, as the accreting material approaches the compact object, for there are generally correlated changes in both the X-ray spectra and the character of the faster temporal variability. There has been a lot of progress in tracking these correlations, both for Z and Atoll neutron star low-mass X-ray binaries, and for black hole binaries. I will discuss these correlations and what they tell us about the physical states of the systems.

Swank, J.↗
NASA NTRS1984

Far-infrared observations of a star-forming region in the Corona Australis dark cloud

A high-resolution far-IR (40-250-micron) survey of a 0.9-sq-deg section of the core region of the Corona Australis dark cloud (containing very young stellar objects such as T Tauri stars, Herbig Ae and Be stars, Herbig-Haro objects, and compact H II regions) is presented. Two extended far-IR sources were found, one associated with the Herbig emission-line star R CrA and the other with the irregular emission-line variable star TY CrA. The two sources have substantially more far-IR radiation than could be expected from a blackbody extrapolation of their near-IR fluxes. The total luminosities of these sources are 145 and 58 solar luminosity, respectively, implying that the embedded objects are of intermediate or low mass. The infrared observations of the sources associated with R CrA and TY CrA are consistent with models of the evolution of protostellar envelopes of intermediate mass. However, the TY CrA source appears to have passed the evolutionary stage of expelling most of the hot dust near the central source, yielding an age of about 1 Myr.

Cruz-Gonzalez, I.↗
NASA NTRS1972

Stellar evolution.

Aspects of normal stellar evolution are discussed together with evolution near the main sequence, stellar evolution from main sequence to white dwarf or carbon ignition, the structure of massive main-sequence stars, and problems of stellar stability and stellar pulsation. Other subjects considered include variable stars, white dwarfs, close binaries, novae, early supernova luminosity, neutron stars, the photometry of field horizontal-branch stars, and stellar opacity. Transport mechanisms in stars are examined together with thermonuclear reactions and nucleosynthesis, the instability problem in nuclear burning shells, stellar coalescence, and intense magnetic fields in astrophysics. Individual items are announced in this issue.

Chiu, H.-Y.↗