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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,135 records · Page 63
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Magnetic activity in pre-main-sequence stars

The extensive new evidence for extremely high levels of magnetic activity on the surface of premain-sequence stars is reviewed and analyzed in the context of solar evolution. Most of these observations concern 'weak' T Tauri stars, in which the surface properties of the young stars are not dominated by circumstellar matter as in 'classical' T Tauri stars. Optical photometry shows that weak T Tauria stars are moderately rapid rotaters with cool starspots covering 5 to 40 percent of the surface. Optical and ultraviolet spectroscopy indicates that their chromospheres are enhanced about 50 times over contemporary solar levels. X-ray and radio intensities are of order 1000 to 100,000 times greater than peak solar levels, and they vary on time scales of days and (in a few cases) hours or minutes. The variability can only be explained in terms of some extremely powerful magnetic-flare model. The large numbers of weak T Tauri stars, probably larger than the population of classical T Tauri stars, strongly suggests that the sun was a weak T Tauri star for at least part of its premain-sequence evolutionary phase. These findings are summarized in a suggested portrait of the sun as a 1 Myr old weak T Tauri star.

Feigelson, Eric D.↗
NASA NTRS2012

Periodic Emission from the Gamma-ray Binary 1FGL J1018.6-5856

Gamma-ray binaries are stellar systems containing a neutron star or black hole with gamma-ray emission produced by an interaction between the components. These systems are rare, even though binary evolution models predict dozens in our Galaxy. A search for gamma-ray binaries with the Fermi Large Area Telescope (LAT) shows that IFGL JI018.6-5856 exhibits intensity and spectral modulation with a 16.6 day period. We identified a variable X-ray counterpart, which shows a sharp maximum coinciding with maximum gamma-ray emission, as well as an 06V f) star optical counterpart and a radio counterpart that is also apparently modulated on the orbital period. IFGL J1018.6-5856 is thus a gamma-ray binary, and its detection suggests the presence of other fainter binaries in the Galaxy.

Celic, O.↗
NASA NTRS1986

IUE observations of magnetically controlled stellar winds in the helium peculiar stars

Dramatic periodic variations in the C IV resonance lines of magnetic helium-weak sn stars HD 5737 = alpha Scl, HD 21699 = HR 1063, and HD 79158 = 36 Lyn are discussed. In all three cases, the 1548,50 doublet is the only non-negligibly variable UV spectral feature. The line profiles are consistent with outflow in a jet-like structure. In HD 21699 this outflow arises from one of the magnetic polar regions. Observations of two additional He-wk sn stars do not reveal strong C IV absorption, implying that the UV characteristics of these stars are less uniform than the optical phenomenology.

Shore, Steven N.↗
NASA NTRS2012

Periodic Emission from the Gamma-Ray Binary 1FGL J1018.6-5856

Gamma-ray binaries are stellar systems containing a neutron star or black hole, with gamma-ray emission produced by an interaction between the components. These systems are rare, even though binary evolution models predict dozens in our Galaxy, A search for gamma-ray binaries with the Fermi Large Area Telescope (LAT) shows that 1FGL ]1018.6-5856 exhibits intensity and spectral modulation with a 16.6 day period. We identified a variable x-ray counterpart, which shows a sharp maximum coinciding with maximum gamma-ray emission, as well as an O6V((f)) star optical counterpart and a radio counterpart that is also apparently modulated on the orbital period. 1FGL ]1018.6-5856 is thus a gamma-ray binary, and its detection suggests the presence of other fainter binaries in the Galaxy.

Source record↗
NASA NTRS1992

Evidence for an old Galactic bulge from RR Lyrae stars in Baade's window - Implications for the formation of the Galaxy and the age of the universe

Recent abundance measurements of RR Lyrae variables in the Baade's window field of the Galactic nuclear bulge are examined, and the observed metallicity distributions of the RR Lyraes in different Galactic radial zones are compared with those predicted for the HB population models. It is shown that the observed systematic variation with Galactocentric distance is only explained if the age of the stellar population increases, in the mean, with decreasing R(G). Thus, the oldest stellar population, the RR Lyraes in the Galactic nuclear bulge, is indeed older than that in the halo.

Lee, Young-Wook↗
NASA NTRS1993

Spatially resolved spectroscopy of AG Carinae, and direct evidence for stellar evolution: The central star of NGC 2392

We have performed spatially-resolved low dispersion long-slit spectroscopy for the circumstellar shell of the luminous blue variable AG Carinae with IUE between lambda lambda 1200 - 3200 A. At all positions a strong dust-scattered stellar continuum is detected. Only a few emission lines, FE II 2600 and O I (?) 1304, have been detected; the emission is stronger on the southern side of the shell. The UV surface brightness and derived dust properties are completely consistent with the groundbased and KAO FIR observations: the grains are large (0.1 - 1 micron), warm (45 - 100 K depending on composition), highly reflecting (C(sub sca)/C(sub abs) approximately equal to 1), and tie up about 10(exp -4) to 10(exp -3) solar mass of material. We also observe similarities of the results for HR Car and LBV's to those reported here.

Altner, Bruce↗
NASA NTRS2021

The Asymmetric Inner Disk of the Herbig Ae Star HD 163296 in the Eyes of VLTI/MATISSE: Evidence for a Vortex?

Context.A complex environment exists in the inner few astronomical units of planet-forming disks. High-angular-resolution observa-tions play a key role in our understanding of the disk structure and the dynamical processes at work.Aims.In this study we aim to characterize the mid-infrared brightness distribution of the inner disk of the young intermediate-massstar HD 163296 from early VLTI/MATISSE observations taken in theL- andN-bands. We put special emphasis on the detection ofpotential disk asymmetries.Methods.We use simple geometric models to fit the interferometric visibilities and closure phases. Our models include a smoothedring, a flat disk with an inner cavity, and a 2D Gaussian. The models can account for disk inclination and for azimuthal asymmetriesas well. We also perform numerical hydrodynamical simulations of the inner edge of the disk.Results.Our modeling reveals a significant brightness asymmetry in theL-band disk emission. The brightness maximum of the asym-metry is located at the NW part of the disk image, nearly at the position angle of the semimajor axis. The surface brightness ratio inthe azimuthal variation is3.5±0.2. Comparing our result on the location of the asymmetry with other interferometric measurements,we confirm that the morphology of ther<0.3au disk region is time-variable. We propose that this asymmetric structure, located in ornear the inner rim of the dusty disk, orbits the star. To find the physical origin of the asymmetry, we tested a hypothesis where a vortexis created by Rossby wave instability, and we find that a unique large-scale vortex may be compatible with our data. The half-lightradius of theL-band-emitting region is0.33±0.01au, the inclination is52◦+5◦−7◦, and the position angle is143◦±3◦. Our models predictthat a non-negligible fraction of theL-band disk emission originates inside the dust sublimation radius forμm-sized grains. Refractorygrains or large (&10μm-sized) grains could be the origin of this emission.N-band observations may also support a lack of smallsilicate grains in the innermost disk (r.0.6au), in agreement with our findings fromL-band data.

J Varga↗
NASA NTRS1990

Spectrophotometry of emission-line stars in the magellanic clouds

The strong emission lines in the most luminous stars in the Magellanic Clouds indicate that these stars have such strong stellar winds that their photospheres are so masked that optical absorption lines do not provide an accurate measure of photospheric conditions. In the research funded by this grant, temperatures and gravities of emission-line stars both in the Large (LMC) and Small Magellanic Clouds (SMC) have been measured by fitting of continuum ultraviolet-optical fluxes observed with IUE with theoretical model atmospheres. Preliminary results from this work formed a major part of an invited review 'The Distribution of Types of Luminous Blue Variables'. Interpretation of the IUE observations obtained in this grant and archive data were also included in a talk at the First Boulder-Munich Hot Stars Workshop. Final results of these studies are now being completed for publication in refereed journals.

Bohannan, Bruce↗
NASA NTRS1985

The magnetically controlled stellar wind of HD 21699

The discovery of a magnetically controlled stellar mass outflow in the helium-weak sn star HD 21699 = HR 1063 is reported. IUE observations show that the C IV resonance doublet is variable on the rotational time scale of about 2.5 days, and that there are no other observable spectrum variations in the UV. The magnetic field reverses sign on the rotational time scale. An interpretation of the observations in terms of magnetically structured jets is presented.

Brown, D. N.↗
NASA NTRS1987

Quasi-thermal models

Non-local thermodynamics equilibrium (LTE) effects in the photosphere; recent research on the chromosphere of the M and C stars; and elementary shock-waves and pulsation theories and their applications to Mira long-period variables are discussed.

Delareza, Ramiro↗
NASA NTRS1988

The location of the Ca II ions in the beta Pictoris disk

Echelle spectra of beta Pictoris have been obtained in the region of the Ca II 8542A line which arises from the metastable 3d 2D5/2 level. A narrow, weak, circumstellar absorption line is seen at the stellar radial velocity. A simple theory is developed of the radiative pumping of the metastable levels of the Ca II ions by the radiation from the star at the H and K lines. The relatively large fractional population, N3(Ca II)/N1(Ca II) of roughly 0.05, observed for the metastable level then requires that the calcium absorbers be largely concentrated within about 1 AU of the star. This result confirms two previous, independent estimates of the location of this gas. A time-variable circumstellar component of the 8542A line also is found at an infall velocity of about 15 km/s.

Hobbs, L. M.↗
NASA NTRS1991

From wind to superwind - The evolution of mass-loss rates for Mira models

Dynamical atmosphere models were calculated for a large grid of variables with Mira-like properties satisfying the Iben radius-luminosity-mass relationship for evolving AGB stars. Their masses ranged from 0.7 to 2.4 solar masses, and their periods from 150 to 800 days. All were fundamental-mode pulsators, had solar metallicity, and included effects of dust. The mass-loss rate increases as an approximately exponential function of time, reaching 0.00001-0.0001 solar masses/yr. Further evolution is dominated by the powerful wind, which strips the star's envelope from the core. This 'superwind', a remarkably robust effect, occurs for all initial stellar masses and all modeling parameters that have been tested. Models with very low metallicity also show the effect, but at higher luminosities, which has intriguing implications for the number of supernovae in early low-metallicity populations and for the chemical evolution of galaxies.

Bowen, G. H.↗
NASA NTRS1997

Oxygen-rich Mira variables: Near-infrared luminosity calibrations. Populations and period-luminosity relations

Hipparcos astrometric and kinematical data of oxygen-rich Mira variables are used to calibrate absolute near-infrared magnitudes and kinematic parameters. Three distinct classes of stars with different kinematics and scale heights were identified. The two most significant groups present characteristics close to those usually assigned to extended/thick disk-halo populations and old disk populations, respectively, and thus they may differ by their metallicity abundance. Two parallel period-luminosity relations are found, one for each population. The shift between these relations is interpreted as the consequence of the effects of metallicity abundance on the luminosity.

Alvarez, R.↗
NASA NTRS1996

The Flare and Quiescent States of M-dwarf Coronae

As part of an ongoing program searching for and analyzing X-ray variable sources in the ROSAT PSPC archive, the lightcurves and spectral analysis for two M- dwarf flare stars are presented; AD Leo and VB 8.

Stringfellow, Guy S.↗
NASA NTRS2004

X-ray Monitoring of eta Carinae: Variations on a Theme

We present monitoring observations by the Rossi X-ray Timing Explorer of the 2-10 keV X-ray emission from the supermassive star eta Carinae from 1996 through late 2003. These data cover more than one of the stellar variability cycles in temporal detail and include especially detailed monitoring through two X-ray minima. We compare the current X-ray minimum which began on June 29, 2003 to the previous X-ray minimum which began on December 15, 1997, and refine the X-ray period to 2024 days. We examine the variations in the X-ray spectrum with phase and with time, and also refine our understanding of the X-ray peaks which have a quasi-period of 84 days, with significant variation. Cycle-to-cycle differences are seen in the level of X-ray intensity and in the detailed variations of the X-ray flux on the rise to maximum just prior to the X-ray minimum. Despite these differences the similarities between the decline to minimum, the duration of the minimum, and correlated variations of the X-ray flux and other measures throughout the electromagnetic spectrum leave little doubt that that the X-ray variation is strictly periodic and produced by orbital motion as the wind from eta Carinae collides with the wind of an otherwise unseen companion.

Corcoran, M. F.↗
NASA NTRS2016

Measuring Solar Doppler Velocities in the He II 30.38 nm Emission Using the EUV Variability Experiment (EVE)

The EUV Variability Experiment (EVE) onboard the Solar Dynamics Observatory has provided unprecedented measurements of the solar EUV irradiance at high temporal cadence with good spectral resolution and range since May 2010. The main purpose of EVE was to connect the Sun to the Earth by providing measurements of the EUV irradianceas a driver for space weather and Living With a Star studies, but after launch the instrument has demonstrated the significance of its measurements in contributing to studies looking at the sources of solar variability for pure solar physics purposes. This paper expands upon previous findings that EVE can in fact measure wavelength shifts during solar eruptive events and therefore provide Doppler velocities for plasma at all temperatures throughout the solar atmosphere from the chromosphere to hot flaring temperatures. This process is not straightforward as EVE was not designed or optimized for these types of measurements. In this paper we describe the many detailed instrumental characterizations needed to eliminate the optical effects in order to provide an absolute baseline for the Doppler shift studies. An example is given of a solar eruption on 7 September 2011 (SOL2011-09-07), associated with an X1.2 flare, where EVE Doppler analysis shows plasma ejected from the Sun in the He II 30.38 nm emission at a velocity of almost 120 km s(exp -1) along the line-of-sight.

Chamberlin, Phillip Clyde↗
NASA NTRS1994

Stellar occultation of polarized light from circumstellar electrons. 4: Detached binary systems

The finite sizes of stars in detached binary systems are incorporated in the single scattering approximation of Brown, McLean, & Emslie (1978). In the absence of scatterer occultation, the predicted polarimetric variability is found to be represented by a terminating two harmonic time series, as previously found by Brown et al. (1978). When the occultation of scatterers by the light sources is included, additional Fourier harmonics are introduced. Model results are illustrated for a spherically symmetric scattering envelope centered on the primary star, in which it is found that for occultation effects to be identified in polarimetric data requires the separation of the system to be less than 10 times the radius of the primary, and for the primary to be the dominant light source.

Fox, G. K.↗