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At least 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

On the radio variability of V410 Tauri

The radio flux density of the star V410 Tauri has been monitored at 6 and 2 cm wavelengths at monthly intervals over a one-year period with the VLA. In contrast to earlier observations, the radio emission was typically at the level of roughly 1 mJy with flat or negative spectral indices. No large radio 'flare' events were detected. However, a periodicity analysis of the quiescent data shows statistically significant evidence for a modulation of the radio emission, with a period of 0.933 days, or one-half the optical period reported by Vrba, et al. (1988). If the modulation is real, it would imply that (1) the radio emission is produced either in localized zones on two opposite hemispheres of the star, or in a single, presumably high-latitude, structure that remains visible for an entire rotation period; (2) this zone or zones must be comparable to the stellar radius in size; (3) the energy spectrum of the radiating electrons is stable over long times (greater than 1 yr) but the absolute intensity level in the quiescent state may vary by factors of a few; and (4) the emission solid angle varies with viewing angle of the emission zone. It is suggested that gyrosynchrotron radiation from closed-loop magnetic structures may be responsible for the quasisteady low-level radio emission reported here. A geometric model for the radio emission structure may explain qualitatively the modulation of the radio emission. By analogy with the premain-sequence star DoAr 21, this same physical mechanism may produce both the 'quiescent' and 'flaring' radio emission. The monitoring results suggest that strong radio outbursts, such as were previously observed for V410 Tau, are relatively rare.

Bieging, John H.↗
NASA NTRS1991

Elemental abundance analyses with DAO spectrograms. VII - The late normal B stars Pi Ceti, 134 Tauri, 21 Aquilae, and Nu Capricorni and the use of Reticon spectra

This paper presents elemental abundance analyses of sharp-lined normal late B stars. These stars exhibit mostly near-solar abundances, but each star also shows a few abundances which are a factor of 2 less than solar. The coadded photographic spectrograms are supplemented with Reticon data. A comparison of 261 equivalent widths on 2.4 A/mm spectra of sharp-lined B and A stars shows that the Reticon equivalent widths are about 95 percent of the coadded equivalent mean. The H-gamma profiles of the coadded and Reticon spectra for eight sharp-lined stars show generally good agreement. The generally high quality of the coadded data produced from 10 or more spectrograms is confirmed using the REDUCE graphics-oriented computed reduction code. For five stars, metal lines which fall in the gap between the U and V plates are analyzed using Reticon data.

Adelman, Saul J.↗
NASA NTRS2000

A third star in the T Tauri system

New speckle holographic images of the T Tauri infrared companion (T Tauri IRC; T Tauri S) reveal it to be a double system with a sky-projected separation of 0. 05, corresponding to a linear distance of 7 AU.

visual circumstellar matter stars↗
NASA NTRS1994

10 micron imaging of UZ Tauri: Evidence for circumstellar disk clearing due to a close companion star

We present 10 micrometer images of the multiple T Tauri star system UZ Tau taken with the Berkeley mid-infrared array camera at the United Kingdom Infrared Telescope (UKIRT) and evidence that UZ Tau E and W are a common proper pair. The mid-infrared emission is resolved for the first time into the two components UZ Tau E and UZ Tau W. The mid-infrared excess deduced for UZ Tau W appears to be much lower than that observed for UZ Tau E. This excess emission is consistent with an optically thin circumstellar disk in the case of UZ Tau W, whereas UZ Tau E's excess is consistent with an optically thick disk. We suggest that the close binary star pair in UZ Tau W is responsible for the observed difference between UZ Tau E and W's mid-infrared excess. In the proposed model, the binary star interacts with the local circumstellar disk environment and clears out much of the material inside its orbital radius (approximately 50 AU). As a result, the hot dust, observed at mid-infrared wavelengths, in UZ Tau W is suppressed compared to its 'wider' companion UZ Tau E. This scenario can also plausibly account for differences observed in UZ Tau E's and UZ Tau W's optical line strengths and profiles.

Chez, A. M.↗
NASA NTRS1975

The ultraviolet spectrum of zeta Tauri

This report describes a study of the ultraviolet spectrum of the B-shell star, zeta Tauri. The observational material consists of high-dispersion spectrograms obtained from a rocket experiment launched in November 1972. The spectra cover the wavelength interval from 1100 to 2050 A with a resolution of about 0.1 A. The UV stellar lines indicate that the star has an effective temperature as high as 27,000 K, the stellar atmosphere appears to have serious abundance deficiencies in carbon and silicon, and the velocity field in the atmosphere is complicated but shows evidence for outward acceleration and differential rotation.

Heap, S. R.↗
NASA NTRS1992

Whole Earth Telescope observations of V471 Tauri - The nature of the white dwarf variations

Time-series photometric observations of the binary star V471 Tauri were conducted using the Whole Earth Telescope observing network. The purpose was to determine the mechanism responsible for causing the 555 and 277 s periodic luminosity variations exhibited by the white dwarf in this binary. Previous observers have proposed that either g-mode pulsations or rotation of an accreting magnetic white dwarf could cause the variations, but were unable to decide which was the correct model. The present observations have answered this question. Learning the cause of the white dwarf variations has been possible because of the discovery of a periodic signal at 562 s in the Johnson U-band flux of the binary. By identifying this signal as reprocessed radiation and using its phase to infer the phase of the shorter wavelength radiation which produces it, made it possible to compare the phase of the 555 s U-band variations to the phase of the X-ray variations. It was found that U-band maximum coincides with X-ray minimum. From this result it was concluded that the magnetic rotator model accurately describes the variations observed, but that models involving g-mode pulsations do not.

Clemens, J. C.↗
NASA NTRS1994

The enigmatic T Tauri radio source

We have analyzed eight high angular resolution images of the prototype low mass pre-main-sequence star T Tauri obtained with the Very Large Array between 1987 and 1990. Our objectives were to confirm a recent report that the radio emission is both variable and circularly polarized, to determine whether this behavior originates in the optical star or in the infrared source lying 0.6 sec to its south, and to identify possible emission mechanisms. No variability or circular polarization was detected in the weak (approximately equals 1 mJy) radio emission associated with the visible star, down to levels of approximately equals 10%. The observed flux can be accounted for by free-free emission from an ionized wind with a mass-loss rate M = 3.7 x 10(exp -8) solar mass yr(exp -1), but a more accurate determination of the spectral index is needed to test the validity of sperical wind models. In sharp contrast, the 3.6 cm emission of the infrared source is variable (approximately equals 4-7 mJy) on time scales less than or equal to 3 days and circularly polarized at low levels of approximately equals 3% - 5%. The polarization was left circular when detected during periods of low radio activity but changed to right circular during a radio outburst, similar to the reversals that have been seen in some RS CVn binary systems. The spectral index is negative during 'quiescence,'and we argue that the emission is nonthermal gyrosynchrotron radiation. The detection of magnetic radio activity in this optically invisible infrared source sparks new interest in clarifying its evolutionary status. Sensitive 3.6 cm images reveal weak emission extending approximately equals 1 sec west of the T Tau system that is probably associated with high-velocity shock-excited gas flowing toward HH-1555. We find no radio evidence for a putative third component north of the visible star.

Skinner, Stephen L.↗
NASA NTRS1988

TH28 (Krautter's star) and its string of Herbig-Haro objects

A high-quality spectrogram of the unusual T Tauri-like star Th28 and its string of Herbig-Haro (HH) objects has been obtained. New velocities and line intensities for the star and the gaseous knots are reported, and data are given for a third HH object located 87 arcsec to the SE along the same collimation axis as defined by the other features. Th28 has a heliocentric velocity of +5 km/s which is close to the velocity of the CO in the area. The star's spectral type is probably in the G8-K2 range.

Graham, J. A.↗
NASA NTRS1987

Chemical condensation in the outflowing matter from the proto-sun and its application to meteorites

A novel model for solar nebula is proposed which is based on the observations of nebular disks around young solar-type stars in their T Tauri phase and the evidence that these disks formed from the material condensed from the outflowing gases from these stars. The model makes it possible to calculate the mass and the angular momentum. From these, parameters and the known photospheric temperature of T Tauri stars, the characteristic features of the nebula (such as radial extent, thickness, and temperature and pressure profiles) can be calculated. The model accounts very well for meteoritic mineralogy. It is suggested that chemical condensation took place in the outflowing matter from the protosun and that the solar nebula formed from this condensed material. The model also accounts for the formation of ices in the outer solar system and for planetary composition.

Trivedi, B. M. P.↗
NASA NTRS1992

Elemental abundances of the B and A stars Gamma Geminorum, 7 Sextantis, HR 4817, and HR 5780

Fine analyses of the B and A stars, Gamma Geminorum, 7 Sextantis, HR 4817, and HR 5780 are performed. Although the data cover rather limited spectral regions, still useful results were obtained. The data were mostly obtained at the KPNO coude feed telescope with CCD TI No. 3, camera 5, and grating A. The He/H ratio of HR 4817 confirms the similarity of many abundance values with those of the peculiar Mn star 53 Tauri. For the most part Gamma Gem, 7 Sex, and HR 5780 have derived abundances similar to those of other normal sharp-lined stars of similar effective temperature.

Adelman, Saul J.↗
NASA NTRS1989

The nature of RV Tauri variables. II- Properties of the peculiar F stars

Basic atmospheric parameters for six RV Tauri variables are considered. The Fe/H abundance ratios for the stars are found to be between -1 and - 1.7, inclusively. Stars of the Preston (1962) type B and C subclasses do not appear segregated by metallicity. In comparison to the cooler Preston type A stars, types B and C are shown to be more metal poor by a factor of 10, on average. The results are consistent with the theory that RV Tau type B and C variables are a direct evolutionary analog to the cluster type II Cepheids.

Cardelli, Jason A.↗
NASA NTRS1972

The nature of the Herbig Ae- and Be-type stars associated with nebulosity.

A quantitative spectrophotometric study of the Herbig Ae and Be stars associated with nebulosity demonstrates that the majority have surface gravities appropriate to pre-main-sequence stars while the remainder have surface gravities characteristic of the zero-age main seee. This evidence, along with their gross spectroscopic and pc similarity to T Tauri-like stars and the fact that all these stars appear to be surrounded by circumstellar dust, places them among the youngest known stellar objects. The geometry of the circumstellar shells is discussed and the evidence seems to favor a nonspherical, possibly disklike shape. Finally, photographs taken with a 1-micron image-tube system reveal a large number of red stellar objects surrounding many of these stars. This suggests that the Herbig Ae and Be objects may be the brightest representatives of very recently formed stellar groups.-

Strom, S. E.↗
NASA NTRS1993

Photometric light curves for ten rapidly rotating stars in Alpha Persei, the Pleiades, and the field

We present the results from a photometric monitoring program of ten rapidly rotating stars observed during 1991 using the FLWO 48-in. telescope. Brightness variations for an additional six cluster stars observed with the Lick 40-in. telescope are also given. The periods and light curves for seven Alpha Persei members, two Pleiades members, and one naked T Tauri field star are reported.

Prosser, Charles F.↗
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 NTRS1981

Spectropolarimetry of Herbig-Haro objects and the exciting star of HH 30

Spectrophotometric and spectropolarimetric observations are presented of three HH objects: HH 11, 30, and 43. Although null polarization results are obtained for HH 11 and 43, the unusually strong continuum of HH 30 is linearly polarized by 3%. The emission lines in this nebula are unpolarized. The electric vector position angle appropriate to the continuum polarization, and the presence of Fe II emission lines apparently from the photosphere of a T Tauri-like star, indicate HL Tau as the exciting/illuminating star for HH 30. Evidence is adduced for a highly anisotropic distribution of circumstellar obscuration around this star.

Cohen, M.↗
NASA NTRS2000

The Visual Orbit of theta(sup 2) Tauri and the Distance to the Hyades

Comparisons of measurements from interferometry, spectroscopy, precision photometry, lunar occultation, and Hipparcos not only checks the accuracy of the distance to the Hyades, but also provides an accurate empirical mass-luminosity relation, and a good estimate of the age of the Hyades.

binaries visual-instrumentation interferometers st↗