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At least 73 records · Page 4
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NASA NTRS1990

On the apparent positions of T Tauri stars in the H-R diagram

The spread in apparent luminosities of T Tauri stars caused by occultation and emission from protostellar disks is investigated. A random distribution of disk inclination angles, coupled with a plausible range of accretion rates, introduces a significant scatter in apparent luminosities for intrinsically identical stars. The observed dispersion of luminosities for K7-M1 Hayashi track stars thought to have disks in Taurus-Auriga is similar to predictions of the simple accretion disk model, which suggets that age determinations form many pre-main-sequence stars are uncertain. The results also suggest that Stahler's 'birthline' for convective track pre-main-sequence stars may be located at slightly lower luminosities than previously thought.

Kenyon, Scott J.↗
NASA NTRS1994

Multiwavelength observations of RV Tauri stars. 4: SX Centauri

We present ultraviolet, optical and infrared observations of the RV Tauri star SX Cen. From a fit of model atmospheres to the data we conclude that Z/Z solar = 0.033, that SX Cen has an extended atmosphere, and that the spectral types implied by the ultraviolet data close to deep minimum are consistent with those implied from optical spectra. Unlike AC Her, there seems to be no silicate component in the circumstellar dust shell, and the star pulsates in purely radial modes. The implication is that, despite their many similarities, SX Cen seems to be at a significantly earlier phase of post-AGB evolution than AC Her. Additional data, particularly in the infrared, are necessary to confirm this conclusion.

Shenton, M.↗
NASA NTRS1983

A large-amplitude photometric periodicity on a T Tauri star

B, V, and R light curvea are presented for the years 1970-1972, during whith the T Tauri star SY Chamaeleontis was observed to undergo precisely periodic brightness oscillations with a 1.6 mag B amplitude and a 6.129-day period. While SY Cha has shown small amplitude irregular variability since 1901, and in 1971 exhibited a sinusoidal light curve in the 13.4-14.2 mag range, this range was found to increase during 1971-1972 to 12.76-14.34 mag. Between 1973 and 1978, the odd behavior pattern was resumed. It is suggested that a localized bright spot formed on, or near, the surface of SY Cha, and that stellar rotation has caused the spot to appear and disapear from the vantage point of the earth.

Schaefer, B. E.↗
NASA NTRS1987

Forbidden line and H-alpha profiles in T Tauri star spectra - A probe of anisotropic mass outflows and circumstellar disks

The results of a high-resolution spectroscopic study of 10 T Tauri stars (TTS) and two Herbig emission stars are presented based on red echelle spectra including the lines of forbidden O I 6300 A, forbidden N II 6584 A, forbidden S II 6716, 6731 A, as well as H-alpha. The forbidden lines display a continuous progression of profile types. The velocity structure in the forbidden lines is critically examined and compared to computed line profiles for a number of different wind models. Constant velocity spherical or conical winds fail to reproduce the observed line profiles, which are better explained by a wind with a latitude-dependent velocity field. A prediction of the wind model correlating the velocity of the reversal at H-alpha and the forbidden line velocity structure as a function of the view angle to the star is explored. Estimates of the average densities and sizes for the TTS forbidden emission regions are presented, and mass-loss rates are computed. IRAS far-IR fluxes are used to estimate the disk sizes.

Edwards, Suzan↗
NASA NTRS1995

A search for T Tauri stars in high-latitude molecular clouds. 2: The IRAS Faint Source Survey catalog

We present a catalog of infrared point sources from the IRAS Faint Source Survey at Galactic latitudes the absolute magnitude of b is greater than or equal to 30 deg. The aim of this paper is to provide a list of possible star-forming sites at high Galactic latitudes in order to address the question of whether or not the translucent molecular clouds (which are most easily identified at high latitudes) are capable of star formation. The primary list of sources has 12, 25, 60, and 100 micron fluxes within the range typical of pre-main-sequence or T Tauri stars. A secondary list has the same range of 12, 25, and 60 micron fluxes, but only upper limits at 100 microns. A total of 127 candidates from the first category and 65 candidates from the second category are identified and their positions and infrared spectral characteristics tabulated. Although the colors and fluxes of these sources are typical of T Tauri or pre-main-sequence stars and YSOs, extragalactic sources and planetary nebulae sometimes have similar colors. These lists provide a starting point for optical spectroscopy or other techniques to positively identify these objects. We can determine an upper limit to the star forming efficiency of high-latitude molecular clouds assuming all the candidates in our sample are pre-main sequence stars of one solar mass. The upper limit of a few tenths of 1% is less than the star-forming efficiency of local dark cloud complexes such as the Taurus-Auriga or rho Ophiuchus clouds.

Magnani, Loris↗
NASA NTRS1987

Spectral energy distributions of T Tauri stars - Disk flaring and limits on accretion

The Adams et al. (1987) conclusion that much of the IR excess emission in the spectral energy distribution of T Tauri stars arises from reprocessing of stellar radiation by a dusty circumstellar disk is presently supported by analyses conducted in light of various models of these stars' spectra. A low mass reprocessing disk can, however, produce these spectra as well as a massive accretion disk. The detection of possible boundary layer radiation in the optical and near-UV regions poses the strongest limits on accretion rates. Disk accretion in the T Tauri phase does not significantly modify stellar evolution.

Kenyon, S. J.↗
NASA NTRS1978

Molecular hydrogen emission from T Tauri stars

Results are presented for a search for molecular hydrogen emission in the v = 1-0 S(1) line at 2.2 microns in five T Tauri stars, including T Tau itself, RY Tau, SU Aur, DK Tau, and HL Tau. Molecular hydrogen emission was detected only in T Tau; the results suggest that the emitting region is probably larger than 5 arcsec (2.5-sigma confidence level) and that the emission is probably centered on the star. Upper limits on the line strengths are given for the other four stars. Various excitation mechanisms for the H2 in T Tau are considered, and the mass-loss rate and neutral-particle density in the shocked region are estimated for the case of shock excitation.

Beckwith, S.↗
NASA NTRS1992

Winds from T Tauri stars. II - Balmer line profiles for inner disk winds

Results are presented of calculations of Balmer emission line profiles using escape probability methods for T Tauri wind models with nonspherically symmetric geometry. The wind is assumed to originate in the inner regions of an accretion disk surrounding the T Tauri star, and flows outward in a 'cone' geometry. Two types of wind models are considered, both with monotonically increasing expansion velocities as a function of radial distance. For flows with large turbulent velocities, such as the HF Alfven wave-driven wind models, the effect of cone geometry is to increase the blue wing emission, and to move the absorption reversal close to line center. Line profiles for a wind model rotating with the same angular velocity as the inner disk are also calculated. The Balmer lines of this model are significantly broader than observed in most objects, suggesting that the observed emission lines do not arise in a region rotating at Keplerian velocity.

Calvet, Nuria↗
NASA NTRS1980

Clumpy molecular clouds - A dynamic model self-consistently regulated by T Tauri star formation

A model is proposed which can account for the longevity, energetics, and dynamical structure of dark molecular clouds. Recent observations of a high space density of T Tauri stars in some dark clouds provide the basis for the assertion that high-velocity winds from these low mass pre-main-sequence stars provide a continuous dynamic input into molecular clouds. It is postulated that as clumps are driven above the Jeans mass, both by coalescence and the enhancement of ram pressure through continuing acceleration by protostellar winds, collapse is followed by the formation of low-mass stars that generate additional protostellar winds. Finally, it is found that star formation occurs on a relatively slow time scale, comparable to the cloud lifetime

Norman, C.↗
NASA NTRS1982

Line profiles of T Tauri stars - Clues to the nature of the mass flow

A set of high resolution observations of the H-alpha Na D, He I 5876 A, and H-gamma lines in several T Tauri stars is presented. Repeated observations of some of these objects yield some indication of the importance of time-dependent effects. The line profiles should be useful for detailed radiative transfer modeling. A cursory survey of the profiles suggests that 'turbulent' velocities in some cases may not be negligible in comparison with the bulk flow speeds and that many emission lines, such as Na D, may be formed in a narrow layer near the stellar photosphere. It is suggested that the effects of geometrical occulation of narrow line-emitting regions in general outflow can account for some of the peculiarities of the line profiles. However, some aspects of the line profiles, such as the large-scale, highly variable redshifted Na D absorption in RW Aur, clearly require more complicated motions.

Hartmann, L.↗
NASA NTRS1980

The formation of a T Tauri star: Observations of the infrared source in L 1551

The 5 to 25 micron observations show that an object previously discovered at 2.2 microns (IRS 5) within the densest part of the L1551 dark cloud is a strong source of radiation from grains as cool as 230 K. The energy distribution resembles that of infrared objects embedded within other molecular cloud cores, but implies a total solar luminosity of only 30. The luminosity of the source and its proximity to other T Tauri stars suggests that it is a 1 to 2 solar mass pre-main sequence star still swaddled within the L1551 cloud. The radio and optical evidence for mass motions around IRS 5 may mean that IRS 5 has been flaring in FU Ori-type eruptions within the last 1000 years.

Harris, S.↗
NASA NTRS1981

The formation of a T Tauri star - Observations of the infrared source in L1551

Observations at 5 to 25 microns show that the object discovered by Strom et al. (1976) at 2.2 microns within the densest part of the L1551 dark cloud is a strong source of radiation from grains as cool as 130 K. The energy distribution resembles that of infrared objects embedded within other molecular cloud cores, but implies a total luminosity of only 30 solar luminosities. The luminosity of the source and its proximity to the other T Tauri stars suggests that it is a 1-2 solar-mass premain-sequences star still swaddled within the L1551 cloud. The radio and optical evidence for mass motions around IRS 5 may mean that IRS 5 has been flaring in FU Ori-type eruptions within the last 1000 years.

Beichman, C.↗
NASA NTRS1980

Emission measures derived from far ultraviolet spectra of T Tauri stars

Spectroscopic diagnostics based on UV emission line observations have been developed to study the solar chromosphere, transition region, and corona. The atmospheric properties that can be inferred from observations of total line intensities include the temperature, by identifying the ionic species present; the temperature distribution of the emission measure, from the absolute intensities; and the electron density of the source, from line intensity ratios sensitive to the electron density. In the present paper, the temperature distribution of the emission measure is estimated from observations of far UV emission line fluxes of the T Tauri stars, RW Aurigae and RU Lupi, made on the IUE. A crude estimate of the electron density of one star is obtained, using density-sensitive line ratios.

Cram, L. E.↗
NASA NTRS1978

Photometric and spectral properties of some T Tauri stars

Photometric and spectroscopic data have been obtained for selected T Tauri members of the Taurus-Aurigae cloud and the Orion complex. A correlation between the intensity ratio of calcium and hydrogen emission lines and the infrared excess at 3.5 microns is found for these stars, which indicates a causal relationship between 'chromospheric activity' and emission processes in the circumstellar shells. It is argued that a comparison with properties of well-studied novae could lead to a better understanding of the physical structure of T Tauri stars.

Warner, J. W.↗
NASA NTRS1994

Submillimeter constraints on dust near Lindroos' post T Tauri stars

Young stars from the dynamical sample identified by Lindroos (A&A, 156,223 (1986)) have been observed at 800 micrometers wavelength using the James Clerk Maxwell Telescope on Mauna Kea. The new data are used to constrain the mass of circumstellar dust around these stars. The Lindroos sample is dominated by low-mass stars of age (3-150) x 10(exp 6) yr, intermediate in age between the T Tauri and main-sequence stars. When combined with previous measurements, the present observations are compatible with the depletion of circumstellar dust on a time scale tau approximately 10(exp 7.0) yr. If this dust is present in circumstellar disks, then tau represents the lifetime of these disks.

Jewitt, David C.↗