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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS1976

An intense extreme-ultraviolet source in Cetus

The detection of a second EUV (100-1000 A) probable white dwarf star with the EUV telescope aboard the Apollo Soyuz mission is reported. The positional error box includes Feige 24, a binary system consisting of a very blue white dwarf and a dMe star. A grid of LTE pure-hydrogen, high-surface-gravity model atmospheres, is used, in combination with the EUV data and optical spectrophotometry, to derive the stellar parameters, yielding an effective temperature of 60,000 K, a radius of 17,000 km at 100 pc, and a luminosity of 7(d/100 pc) squared solar luminosities. The Feige 24 models appear to rule out white dwarfs with pure hydrogen atmospheres if the interstellar helium abundance and ionization are not anomalous.

Margon, B.↗
NASA NTRS1981

The ultraviolet spectrum of the magnetic degenerate star GD229

The ultraviolet spectrum obtained with the IUE satellite of the magnetic degenerate white dwarf star GD 229 is presented. The ultraviolet energy distribution measured by the short- and long-wavelength cameras covering the range 1170-3200 A, when combined with optical data, indicates a best fit blackbody temperature of 16,000 K, which is significantly lower than previous estimates. A broad absorption trough in the 2000-3000 A region is observed which may be produced by opacity in the fundamental cyclotron frequency for surface magnetic fields from 3.2 to 5 x 10 to the 8th gauss. The absence of a strong Lyman-alpha sigma + component suggests that the primary atmospheric constituent is not hydrogen, with various absorption features probably attributable to Zeeman components of neutral helium, or possibly Mg II, C II and other metal lines. The superposition of many strong Zeeman features is also an alternative interpretation of the absorption in the 2000-3000 A interval.

Green, R. F.↗
NASA NTRS1979

Middle- and far-ultraviolet observations of hot white dwarfs

Observations of HD 149499 B, the previously unrecognized white dwarf star found by the Skylab ultraviolet objective-prism survey, are discussed. Spectra of the binary HD 149499 AB were obtained by the short wavelength and long wavelength cameras of the IUE, and converted into fluxes. The absorption line strengths of the white dwarf secondary indicate a He/H composition ratio greater than or equal to 1. Comparison with model atmospheres of the IUE data suggests a temperature of around 93,000 K for HD 149499 B, while comparison with visual magnitude data indicates a temperature of 82,000 K. Either temperature value would make the star the hottest white dwarf for which reasonably reliable temperature estimates are available. Future work intended includes the repetition of the Skylab ultraviolet experiment on Spacelab for the detection of a complete sample of hot white dwarfs and subdwarfs.

Parsons, S. B.↗
NASA NTRS1986

Narrow-band, slowly varying decimetric radiation from the dwarf M flare star YZ Canis Minoris

Observations of slowly varying radiation from the dwarf M star YZ Canis Minoris with a maximum flux density of 20 mJy and narrow-band frequency structure at frequencies near 1465 MHz are presented. Possible explanations for this radiation are examined. Thermal gyroresonant radiation would require impossibly large coronal loops and magnetic field strengths. The narrow-band structure cannot be explained by continuum emission processes such as thermal bremsstrahlung, thermal gyroresonant radiation, or nonthermal gyrosynchrotron radiation. Coherent burst mechanisms seem to be required.

Lang, K. R.↗
NASA NTRS2003

Target Characterization and Follow-Up Observations in Support of the Kepler Mission

A variety of experiments were carried out to investigate the number and characteristics of the stars to be included in the Kepler Input Catalog. One result of this work was the proposal that the 2MASS Catalog of astrometry and photometry in the infrared be used as the primary source for the initial selection of candidate target stars, because this would naturally decrease the number of unsuitable hot blue stars and would also increase the number of desirable solar-type dwarf stars. Another advantage of the 2MASS catalogue is that the stellar positions have more than adequate astrometric accuracy for the Kepler target selection. The original plan reported in the Concept Study Report was to use the parallaxes and multi-band photometry from the FAME mission to provide the information needed for reliable separation of giants and dwarfs. As a result of NASA's withdrawal of support for FAME an alternate approach was needed. In November 2002 we proposed to the Kepler Science Team that a ground-based multi-band photometric survey could help alleviate the loss of the FAME data. The Science Team supported this proposal strongly, and we undertook a survey of possible facilities for such a survey. We concluded that the SAO's 4Shooter CCD camera on the 1.2-m telescope at the Whipple Observatory on Mount Hopkins, Arizona, showed promise for this work.

Latham, David W.↗
NASA NTRS2023

High Tide or Riptide on the Cosmic Shoreline? A Water-Rich Atmosphere or Stellar Contamination for the Warm Super-Earth GJ 486b from JWST Observations

Planets orbiting M-dwarf stars are prime targets in the search for rocky exoplanet atmospheres. The small size of M dwarfs renders their planets exceptional targets for transmission spectroscopy, facilitating atmospheric characterization. However, it remains unknown whether their host stars’ highly variable extreme-UV radiation environments allow atmospheres to persist. With JWST, we have begun to determine whether or not the most favorable rocky worlds orbiting M dwarfs have detectable atmospheres. Here, we present a 2.8—5.2 µm JWST NIRSpec/G395H transmission spectrum of the warm (700 K, 40.3× Earth’s insolation) super-Earth GJ 486b (1.3 R⊕ and 3.0 M⊕). The measured spectrum from our two transits of GJ 486b deviates from a flat line at 2.2 − 3.3σ, based on three independent reductions. Through a combination of forward and retrieval models, we determine that GJ 486b either has a water-rich atmosphere (with the most stringent constraint on the retrieved water abundance of H2O > 10% to 2σ) or the transmission spectrum is contaminated by water present in cool unocculted starspots. We also find that the measured stellar spectrum is best fit by a stellar model with cool starspots and hot faculae. While both retrieval scenarios provide equal quality fits (χ2ν = 1.0) to our NIRSpec/G395H observations, shorter wavelength observations can break this degeneracy and reveal if GJ 486b sustains a water-rich atmosphere.

JWST↗
NASA NTRS1971

The role of neutron stars in the acceleration of cosmic rays.

Two cases of cosmic ray acceleration are considered, including the acceleration of the material from the surface of a neutron star and the acceleration of the material in an EM wave interaction with the supernova ejecta. It is assumed that the neutron star mass would probably be somewhat less than the limiting mass of white dwarf stars, 1.4 solar masses. It is found that neutron stars may be effective in accelerating helium and iron to the highest cosmic ray energies, and that at lower energies they may accelerate supernova debris into the cosmic ray fluxes throughout the galaxy.

Cameron, A. G. W.↗
NASA NTRS1990

Brown dwarfs as dark galactic halos

The possibility that the dark matter in galactic halos can consist of brown dwarf stars is considered. The radiative signature for such halos consisting solely of brown dwarfs is calculated, and the allowed range of brown dwarf masses, the initial mass function (IMF), the stellar properties, and the density distribution of the galactic halo are discussed. The prediction emission from the halo is compared with existing observations. It is found that, for any IMF of brown dwarfs below the deuterium burning limit, brown dwarf halos are consistent with observations. Brown dwarf halos cannot, however, explain the recently observed near-IR background. It is shown that future satellite missions will either detect brown dwarf halos or place tight constraints on the allowed range of the IMF.

Adams, Fred C.↗
NASA NTRS1990

White dwarf evolution - Cradle-to-grave constraints via pulsation

White dwarf evolution, particularly in the early phases, is not very strongly constrained by observation. Fortunately, white dwarfs undergo nonradial pulsation in three distinct regions of the H-R diagram. These pulsations provide accurate masses, surface compositional structure and rotation velocities, and help constrain other important physical properties. We demonstrate the application of the tools of stellar seismology to white dwarf evolution using the hot white dwarf star PG 1159-035 and the cool DAV (or ZZ Ceti) stars as examples. From pulsation studies, significant challenges to the theory of white dwarf evolution emerge.

Kawaler, Steven D.↗
NASA NTRS1994

Radio evidence for nonthermal particle acceleration on stars of late spectral type

Radio evidence for nonthermal particle acceleration on stars of late spectral type (G, K, and M) is reviewed, including the pre-main-sequence T Tauri stars, the dwarf M flare stars, and the RS CVn binary stars. Both the long-lasting radio emission and the transient radio flares can be attributed to nonthermal electrons accelerated in magnetically confined coronae, radiating by either incoherent gyrosynchrotron radiation or by coherent processes. Related observations of the Sun provide the framework for interpreting the radio emission of late-type stars. We review recent Very Long Base Interferometry (VLBI) observations of single and binary stars, observations of rapid acceleration and dynamic spectra of dwarf M flare stars, and models for long-lasting radio emission from dwarf M flare stars and RS CVn stars. The VLBI data provide evidence for nonthermal particle acceleration in large-scale magnetospheres several times larger than the stellar radius for T Tauri and RS CVn stars. In contrast, rapid radio flares on dwarf M stars require sources much smaller than the visible star in size, and this is consistent with the VLBI data. Dynamic spectra suggest particle acceleration by relativistic electron beams on dwarf M stars. The long-lived radio radiation of dwarf M stars and RS CVn stars requires more-or-less continuous particle acceleration over periods of hours, perhaps by persistent low-amplitude flaring.

Lang, Kenneth R.↗
NASA NTRS1994

Analysis of helium-rich subdwarf O stars. 1: NLTE models, methods, and fits for 21 Palomar Green survey sdOs

Atmospheric parameters for 21 helium-rich hot subdwarf O stars from the Palomar Green survey are found from fits of non-local thermodynamic equilibrium (NLTE) models to optical spectra. About 250 new NLTE models in the parameter range T(sub eff) from 35,000 to 65,000 K, log (g) from 4.0 to 6.5, and epsilon(He) from 50% He to 99% He have been calculated. A fit for each object is presented. Estimated distances and luminosities are calculated, assuming a mass of 0.5 solar masses. Large distances above the Galactic plane are found, and this implies that the majority of the sdO stars belong to a different stellar population than the sdB, planetary nebulae and white dwarf stars. Possibilities may include the halo and a 'thick disk.' Kinematical data from the literature are also discussed in view of the found distances. Five of the stars have been observed, modeled, and analyzed by Dreizler et al., and significant differences exist between their results and ours for these stars. The reason for these differences is not yet known, but the problem does not alter the conclusions about population membership that we present.

Thejll, P.↗
NASA NTRS1973

The abundance and age distributions of 500 F stars in the solar neighbourhood

The uvby catalog of Perry has been used, in conjunction with theoretical uvby colors and stellar evolutionary tracks, to obtain the metal abundance distribution and age distribution of F and G type dwarf stars within 100 pc of the sun. Eighty-three per cent of stars with spectral type F5-G2 have metal content within a factor of 2.5 of the solar abundance, and only 2% have less than one-quarter the solar abundance. The metal abundance distribution, which contains more metal-rich stars than found in previous investigations of F and G dwarfs, has also been compared with the observed metal abundance distribution of G and K giants and the model galaxy calculations of Truran and Cameron (1971). A comparison of the age distribution with evolutionary calculations implies that the rate of star formation in the galaxy may not have decreased by a large factor in the last 10 billion years.

Clegg, R. E. S.↗
NASA NTRS2018

A Revised Exoplanet Yield from the Transiting Exoplanet Survey Satellite (TESS)

The Transiting Exoplanet Survey Satellite (TESS) has a goal of detecting small planets orbiting stars bright enough for mass determination via ground-based radial velocity observations. Here, we present estimates of how many exoplanets the TESS mission will detect, the physical properties of the detected planets, and the properties of the stars that those planets orbit. This work uses stars drawn from the TESS Input Catalog Candidate Target List and revises yields from prior studies that were based on Galactic models. We modeled the TESS observing strategy to select approximately 200,000 stars at 2-minute cadence, while the remaining stars are observed at 30-minute cadence in full-frame image data. We placed zero or more planets in orbit around each star, with physical properties following measured exoplanet occurrence rates, and used the TESS noise model to predict the derived properties of the detected exoplanets. In the TESS 2-minute cadence mode we estimate that TESS will find 1250 ± 70 exoplanets (90% confidence), including 250 smaller than 2 R(sub ⊕). Furthermore, we predict that an additional 3100 planets will be found in full-frame image data orbiting bright dwarf stars and more than 10,000 around fainter stars. We predict that TESS will find 500 planets orbiting M dwarfs, but the majority of planets will orbit stars larger than the Sun. Our simulated sample of planets contains hundreds of small planets amenable to radial velocity follow-up, potentially more than tripling the number of planets smaller than 4 R(sub ⊕) with mass measurements. This sample of simulated planets is available for use in planning follow-up observations and analyses.

Barclay, Thomas↗
NASA NTRS2010

Kepler Planet Detection Mission: Introduction and First Results

The Kepler Mission is designed to determine the frequency of Earth-size and rocky planets in and near the habitable zone (HZ) of solar-like stars. The HZ is defined to be the region of space where a rocky planet could maintain liquid water on its surface. Kepler is the 10th competitively-selected Discovery Mission and was launched on March 6, 2009. Since completing its commissioning, Kepler has observed over 156,000 stars simultaneously and near continuously to search for planets that periodically pass in front of their host star (transit). The photometric precision is approximately 23 ppm for 50% of the 12th magnitude dwarf stars for an integration period of 6.5 hours. During the first 3 months of operation the photometer detected transit-like signatures from more than 200 stars. Careful examination shows that many of these events are false-positives such as small stars orbiting large stars or blends of target stars with eclipsing binary stars. Ground-based follow-up observations confirm the discovery of five new exoplanets with sizes between 0.37 andl.6 Jupiter radii (R(sub J)) and orbital periods ranging from 3.2 to 4.9 days. Ground-based observations with the Keck 1, Hobby-Ebberly, Hale, WIYN, MMT, Tillinghast, Shane, and Nordic Optical Telescopes are used to vet the planetary candidates and measure the masses of the putative planets. Observations of occultations and phase variations of hot, short-period planets such as HT-P-7b provide a probe of atmospheric properties. Asteroseismic analysis already shows the presence of p-mode oscillations in several stars. Such observations will be used to measure the mean stellar density and infer the stellar size and age. For stars too dim to permit asteroseismology, observations of the centroid motion of target stars will be used to measure the parallax and be combined with photometric measurements to estimate stellar sizes. Four open clusters are being observed to determine stellar rotation rates as a function of age and spectral type. Many different types of stellar variability are observed with unprecedented precision and over a wide range of time scales. Solar-like photometric variability of thousands of field stars is being studied to determine how photometric variability and the stellar rotation rates change with stellar age and metallicity. A wide variety of other astrophysical phenomena have also been observed. The data are being analyzed at the Ames Research Center and archived at the MAST at STScI. The Kepler Mission also supports a vigorous Guest Observer Program.

Borucki, William↗
NASA NTRS1992

The collapse of white dwarfs to neutron stars

The observable consequences of an accreting white dwarf collapsing directly to a neutron star are considered. The outcome depends critically upon the nature of the wind that is driven by neutrino absorption in the surface layers as the dwarf collapses. Unlike previous calculations which either ignored mass loss or employed inadequate zoning to resolve it, a characteristic mass-loss rate of about 0.005 solar mass/s and an energy input of 5 x 10 exp 50 ergs/s is found. Such a large mass-loss rate almost completely obscures any prompt electromagnetic display and certainly rules out the production by this model of gamma-ray bursts situated at cosmological distances. The occurrence of such collapses with the Milky Way Galaxy might, however, be detected and limited by their nucleosynthesis and gamma-ray line emission. To avoid the overproduction of rare neutron-rich isotopes heavier than iron, such events must be very infrequent, probably happening no more than once every thousand years.

Woosley, S. E.↗
NASA NTRS1995

Determining the atmospheric structure and dynamics of the FK Comae Star HD32918

The results of UV observations taken with the International Ultraviolet Explorer (IUE) satellite and microwave observations obtained with the Australia Telescope during an observing campaign of the rapidly rotating K0 dwarf star HD 197890, nicknamed 'Speedy Mic' are presented. This star was recently recognized as a powerful, transient EUV source by the ROSAT WFC, and subsequent investigation showed it to be a ZAMS or possibly a PMS dwarf which may be a member of the Local Association. Our observations show it to have strong, variable UV emission lines near the 'saturation' levels. The radio observations show a level of 'quiescent' emission consistent with other rapidly rotating stars, but there is no evidence for the large flux variations that normally characterize the time history of such objects.

Robinson, R. D.↗