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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 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

New measurements of photospheric magnetic fields in late-type stars and emerging trends

The magnetic fields of late-type stars are measured using the method of Saar et al. (1986). The method includes radiative transfer effects and compensation for line blending; the photospheric magnetic field parameters are derived by comparing observed and theoretical line profiles using an LTE code that includes line saturation and full Zeeman pattern. The preliminary mean active region magnetic field strengths (B) and surface area coverages for 20 stars are discussed. It is observed that there is a trend of increasing B towards the cooler dwarfs stars, and the linear correlation between B and the equipartition value of the magnetic field strength suggests that the photospheric gas pressure determines the photospheric magnetic field strengths. A tendency toward larger filling factors at larger stellar angular velocities is also detected.

Saar, S. H.↗
NASA NTRS1984

Stellar chromospheres and coronae in the Ursa Major Cluster stars

IUE spectra of 18 proposed members of the Ursa Major Cluster and Einstein X-ray images of 11 of these stars are discussed. Thirteen stars, six in the nucleus and seven in the extended comoving stream, are probably true members of the UMa Cluster in that their bright ultraviolet and X-ray surface fluxes indicate youth. Four stars, one in the nucleus and three in the stream, exhibit weak ultraviolet and/or X-ray emission suggesting that they are old field stars that have the same space motion as the UMa Cluster. The X-ray surface fluxes of the UMa Cluster stars appear to be brighter than those of the Hyades Cluster stars, consistent with their relative ages. It is argued that chromospheres and transition regions could be present in dwarf stars hotter than B-V of about 0.30, but are unobservable in IUE spectra due to the rapid increase in photospheric flux in hotter stars.

Simon, T.↗
NASA NTRS1991

Asteroseismology of the DOV star PG 1159 - 035 with the Whole Earth Telescope

Results are reported from 264.1 hr of nearly continuous time-series photometry on the pulsating prewhite dwarf star (DPV) PG 1159 - 035. The power spectrum of the data set is completely resolved into 125 individual frequencies; 101 of them are identified with specific quantized pulsation modes, and the rest are completely consistent with such modal assignment. It is argued that the luminosity variations are certainly the result of g-mode pulsations. Although the amplitudes of some of the peaks exhibit significant variations on the time scales of a year or so, the underlying frequency structure of the pulsations is stable over much longer intervals. The existing linear theory is invoked to determine, or strongly constrain, many of the fundamental physical parameters describing this star. Its mass is found to be 0.586 solar mass, is rotation period 1.38 days, its magnetic field less than 6000 G, its pulsation and rotation axes to be aligned, and its outer layers to be compositionally stratified.

Winget, D. E.↗
NASA NTRS1983

An infrared study of the eclipsing dwarf nova U Geminorum

One series of data comprising the present J, H, K broadband IR photometry of the cataclysmic binary system U Geminorum over the course of two orbital cycles was obtained several days before an outburst, while a second was gathered when the system had almost completely returned to quiescence from that outburst. In quiescence, the M4-M5.5 red dwarf star supplies most of the IR luminosity of the system, while the light curves at this time are tidally induced ellipsoidal variations of the red dwarf which exhibit an amplitude such that the red dwarf must fill, or nearly fill, its Roche lobe. This shows directly that Roche lobe overflow is important in cataclysmic variable stars. The range of the disk IR colors after outbursts suggest that cool and optically thick gas, together with optically thin gas, supply most of the IR light of the disk.

Berriman, G.↗
NASA NTRS2004

Spitzer Space Telescope Observations of the aftermath of Microlensing Event MACHO-LMC-5

We have carried out photometry of the microlensing event MACHO-LMC-5 with Spitzer Infrared Array Camera (IRAC) 10 years after the magnification of the LMC source star was recorded. This event is unique in the annals of gravitational microlensing: the lensing star itself has been observed using the Hubble Space Telescope (once with WFPC2 and twice with ACS/HRC). Since the separation between the source and lens at the epoch of the Spitzer observations was ~0".24, the two stars cannot be resolved in the Spitzer images. However, the IRAC photometry clearly establishes that the lens is an M5 dwarf star from its infrared excess, which in turn yields a mass of ~0.2 Mө. This demonstrates the potential of Spitzer to detect the lenses in other gravitational microlensing events.

Stern, Daniel↗
NASA NTRS1998

NGST: Exploring the Fossil Record of Galaxy Formation

During this grant period the investigator has accomplished the following: developed and researched the science case for high resolution optical imaging with NGST (Next Generation Space Telescope); presented to the ASWG (Ad-Hoc Science Working Group) on the use of NGST to image and measure the proper motions of white dwarf stars that could account for the MACHO events toward the LMC (Large Magellanic Cloud); wrote proposals for the Design Reference Mission. Three proposals have been written on the following topics: Measuring the faint end of the white dwarf luminosity function to get an independent measure of the age of the oldest stars, measurement of the stellar luminosity function over the full range of age, abundance, and population type, and deep imaging of Local Group halos to measure the relative ages of the oldest stars in the Milky Way and other galaxies; he has introduced simulations of crowded field stellar photometry on a hypothetical population in the M31 halo. Using a physically correct luminosity function and a surface brightness of 24.5 mag/sq. arcsec, NGST imaging in 10 hours easily measures the turnoff and reaches 3 mags down the main sequence in the halo of M31; and has delivered talks at the NGST workshop in Liege, and at the meeting in Paris entitled "Connecting the Distant Universe with the Local Fossil Record".

Rich, R. Michael↗
NASA NTRS2020

The Multiplanet System TOI-421: A Warm Neptune and a Super Puffy Mini-Neptune Transiting a G9 V Star in a Visual Binary

We report the discovery of a warm Neptune and a hot sub-Neptune transiting TOI-421 (BD-14 1137, TIC 94986319), a bright (V = 9.9) G9 dwarf star in a visual binary system observed by the Transiting Exoplanet Survey Satellite (TESS) space mission in Sectors 5 and 6. We performed ground-based follow-up observations—comprised of Las Cumbres Observatory Global Telescope transit photometry, NIRC2 adaptive optics imaging, and FIbre-fed Echellé Spectrograph, CORALIE, High Accuracy Radial velocity Planet Searcher, High Resolution Échelle Spectrometer, and Planet Finder Spectrograph high-precision Doppler measurements—and confirmed the planetary nature of the 16 day transiting candidate announced by the TESS team. We discovered an additional radial velocity signal with a period of five days induced by the presence of a second planet in the system, which we also found to transit its host star. We found that the inner mini-Neptune, TOI-421 b, has an orbital period of P(b) = 5.19672 ± 0.00049 days, a mass of M(b) = 7.17 ± 0.66 Mꚛ, and a radius of R(b) = 2.68 (+0.19,-0.18) Rꚛ, whereas the outer warm Neptune, TOI-421 c, has a period of P(c) = 16.06819 ± 0.00035 days, a mass of M(c) = 16.42 (+1.06,-1.04) Mꚛ, a radius of R(c) = 5.09{+0.16,-0.15) Rꚛ, and a density of ρ(c) = 0.685 (+0.080,-0.072) g/cu. cm. With its characteristics, the outer planet (ρ(c) = .685 {+0.080,-0.072) g/cu cm) is placed in the intriguing class of the super-puffy mini-Neptunes. TOI-421 b and TOI-421 c are found to be well-suited for atmospheric characterization. Our atmospheric simulations predict significant Lyα transit absorption, due to strong hydrogen escape in both planets, as well as the presence of detectable CH4 in the atmosphere of TOI-421 c if equilibrium chemistry is assumed.

Ilaria Carleo↗
NASA NTRS1979

Population statistics of faint stellar and non-stellar objects

A disc and halo population model is constructed to fit star counts and color data down to V approximately 23 at absolute value of b = 90 deg. This model is used to predict star counts and colors down to V approximately 30. Deviations from these extrapolated relationships provide constraints on the number of faint quasars and black dwarf stars. It is shown that extra-galactic globular clusters start contributing significantly to star counts at V approximately 25 and are more numerous than stars for V 31. Morphological studies of galaxies with approximately 0.5, were made with the space telescope. Significant constraints on theoretical models that describe the evolution of clusters of galaxies are provided.

Vandenbergh, S.↗
NASA NTRS1983

Magnetic fields and activity of the sun and stars - An overview

Recent work on the observation and theory of solar and stellar magnetic field activity and its relation to stellar activity is reviewed, emphasizing those aspects relevant to the problem of activity of red dwarf stars. New observational facts relevant to understanding the root cause of stellar surface activity are summarized and theoretical questions concerning the underlying physical basis for the observed correlations between stellar activity, rotation, and magnetic fields are addressed. These include dyanamo theory and the rotation-activity connection as well as flux tube dynamics and plasma heating.

Rosner, R.↗
NASA NTRS1986

Brown dwarfs and Jovian planets: A comparison

The recent detection of a subluminous companion to the M dwarf star VB8 has renewed interest in the characteristics of objects spanning the mass range from Jupiter to hydrogen burning stars. Atmospheric and interior models were constructed for objects in this mass regime, up to 30 Jupiter masses, with emphasis on understanding the relationship of brown dwarfs such as the VB8 companion to the better-studied Jovian planets. The atmospheric model solves the equation of radiative transfer assuming frequency dependent molecular opacity sources H2, He, H2O, CO, and CH4 which are important by virtue of the high cosmic abundance of their constituent atoms. Condensation of cosmochemically important materials, iron and silicates, in the atmosphere is possible, and the effect of such grains as opacity sources is assessed. The luminosity of the object is presumed due to degenerate cooling following a collapse phase and possibly deuterium burning and an interior model is constructed using as an outer boundary condition the temperature and pressure level at which the atmosphere becomes convective. The interior model is analogous to Jupiter, with a large liquid metallic-hydrogen core and a thinner molecular-hydrogen envelope. The oxidation state of carbon in the outer envelope of a brown dwarf of similar age to Jupiter is a function of the object's mass. This makes the wavelength dependence of the atmospheric opacity sensitive to the carbon to oxygen ratio, since the abundance of the primary source of molecular opacity, H2O, decreases as more oxygen is tied up as CO.

Lunine, J. I.↗
NASA NTRS2001

Kepler Mission: A Search for Terrestrial Planets

The Kepler Mission is a search for terrestrial planets by monitoring a large ensemble of stars for the periodic transits of planets. The mission consists of a 95-cm aperture photometer with 105 square deg field of view that monitors 100,000 dwarf stars for four years. The mission is unique in its ability to detect Earth-size planets in the habitable zone of other stars in the extended solar neighborhood. An Earth-size transit of a solar-like star causes a change in brightness of about 100 ppm. Laboratory testing has demonstrated that a total system noise level of 20 ppm is readily achievable on the timescale of transits. Earth-like transits have been created and reliably measured in an end-to-end system test that has all known sources of noise including, spacecraft jitter. To detect Earth-size planets, the photometer must be spaceborne; this also eliminates the day-night and seasonal cycle interruptions of ground-based observing. The photometer will stare at a single field of stars for four years, with an option to continue for two more years. This allows for detection of four transits of planets in Mars-like orbits and detection of planets even smaller than Earth especially for short period orbits, since the signal to noise improves as the square root of the number of transits observed. In addition to detection of planets, Kepler data are also useful for understanding the activity cycles and rotation rates of the stars observed. For the 3,000 stars brighter than mv= 11.4 p-mode oscillations are measured. The mission has been selected as one of three candidates for NASA's next Discovery mission.

Koch, D.↗
NASA NTRS1994

High-Speed Hubble Space Telescope Ultraviolet photometry of two DB white dwarfs: Nonradial and radial pulsations

We observed two DB white dwarf stars with the High Speed Photometer aboard the Hubble Space Telescope. The two targets, the nonradial pulsator GD 358, and PG 0112+104, a non-pulsating white dwarf with similar temperature, were each observed for 1800 s with a time resolution of 10 ms. We used the F140LP configuration, which gives a broadband response in the UV between 1400 and 3000 A. The data clearly show the long period (about 700 s) pulsations in GD 358. Comparison with optical observations obtained two weeks earlier shows that the amplitude of the pulsations in the UV is approximately 1.4 times higher, consistent with nonradial pulsations due solely to temperature changes at constant radius. The high time resolution of these observations allows us to search for high-frequency pulsations (such as p modes or high overtone radial modes). No firm evidencefor high-frequency pulsations was seen in either object between 1 and 12 Hz. Correlation analysis of GD 358 shows no clear signal of multifrequency high overtone radial pulsations at the 0.00075 mag level, with no individual modes above the 0.0016 mag level. Upper limits for PG 0112+104 are approximately 2 times higher than for GD 358. Implications of this study for the theory of white dwarf pulsations are discussed.

Kawaler, Steven D.↗
NASA NTRS1974

Electron capture in carbon dwarf supernovae

The rates of electron capture on heavier elements under the extreme conditions predicted for dwarf star supernovae have been computed, incorporating modifications that seem to be indicated by present experimental results. An estimate of the maximum possible value of such rates is also given. The distribution of nuclei in nuclear statistical equilibrium has been calculated for the range of expected supernovae conditions, including the effects of the temperature dependence of nuclear partition functions. These nuclide abundance distributions are then used to compute nuclear equilibrium thermodynamic properties. The effects of the electron capture on such equilibrium matter are discussed. In the context of the 'carbon detonation' supernova model, the dwarf central density required to ensure core collapse to a neutron star configuration is found to be slightly higher than that obtained by Bruenn (1972) with the electron capture rates of Hansen (1966).-

Mazurek, T. J.↗
NASA NTRS2021

Origins Space Telescope: From First Light to Life

The Origins Space Telescope ( Origins ) is one of four science and technology definition studies selected by National Aeronautics and Space Administration (NASA) in preparation of the 2020 Astronomy and Astrophysics Decadal survey in the US. Origins will trace the history of our origins from the time dust and heavy elements permanently altered the cosmic landscape to present-day life. It is designed to answer three major science questions: How do galaxies form stars, make metals, and grow their central supermassive black holes from reionization? How do the conditions for habitability develop during the process of planet formation? Do planets orbiting M-dwarf stars support life? Origins operates at mid- to far-infrared wavelengths from ~ 2.8 to 588 μm, is more than 1000 times more sensitive than prior far-IR missions due to its cold (~ 4.5 K) aperture and state-of-the-art instruments.

Spaceborne astrophysics↗
NASA NTRS1986

Definition and empirical structure of the range of stellar chromospheres-coronae across the H-R diagram: Cool stars

Major advances in our understanding of non-radiative heating and other activity in stars cooler than T sub eff = 10,000K has occured in the last few years. This observational evidence is reviewed and the trends that are now becoming apparent are discussed. The evidence for non-radiatively heated outer atmospheric layers (chromospheres, transition regions, and coronae) in dwarf stars cooler than spectral type A7, in F and G giants, pre-main sequence stars, and close bindary systems is unambiguous, as is the evidence for chromospheres in the K and M giants and supergiants. The existence of non-radiative heating in the outer layers of the A stars remains undetermined despite repeated searches at all wavelengths. Two important trends in the data are the decrease in plasma emission measure with age on the main sequence and decreasing rotational velocity. Variability and atmospheric inhomogeneity are commonly seen, and there is considerable evidence that magnetic fields define the geometry and control the energy balance in the outer atmospheric layers. In addition, the microwave observations imply that non-thermal electrons are confined in coronal magnetic flux tubes in at least the cool dwarfs and RS CVn systems. The chromospheres in the K and M giants and supergiants are geometrically extended, as are the coronae in the RS CVn systems and probably also in other stars.

Linsky, J. L.↗
NASA NTRS1982

Electron-cyclotron masers as the source of certain solar and stellar radio bursts

The theory of electron-cyclotron masers as they might apply in the astrophysical context is developed, and it is suggested that such masers provide an explanation for the very bright emission known to be associated with certain kinds of radio bursts observed on the Sun and other stars. Some of the observed properties of solar and stellar radio bursts that seem to require amplification of the radiation are summarized, including millisecond solar spikes, RS CVn binaries, and flares on M dwarf stars. Recent developments in the theory of electron-cyclotron masers are summarized and the current theory is applied to electrons with a loss cone anisotropy, estimating the growth rate and saturation levels. In the interpretation of solar microwave spikes and RS CVn binaries, the mechanisms of gyromagnetic absorption, maser at the second harmonic, polarization, and angular distribution are examined in the light of the theory.

Melrose, D. B.↗
NASA NTRS2020

An Evolutionary System of Mineralogy. Part I: Stellar Mineralogy (>13 to 4.6 Ga)

Minerals preserve records of the physical, chemical, and biological histories of their origins and subsequent alteration, and thus provide a vivid narrative of the evolution of Earth and other worlds through billions of years of cosmic history. Mineral properties, including trace and minor elements, ratios of isotopes, solid and fluid inclusions, external morphologies, and other idiosyncratic attributes, represent information that points to specific modes of formation and subsequent environmental histories—information essential to understanding the co-evolving geosphere and biosphere. This perspective suggests an opportunity to amplify the existing system of mineral classification, by which minerals are defined solely on idealized end-member chemical compositions and crystal structures. Here we present the first in a series of contributions to explore a complementary evolutionary system of mineralogy—a classification scheme that links mineral species to their paragenetic modes. The earliest stage of mineral evolution commenced with the appearance of the first crystals in the universe at >13 Ga and continues today in the expanding, cooling atmospheres of countless evolved stars, which host the high-temperature (T > 1000 K), low-pressure (P < 10-2 atm) condensation of refractory minerals and amorphous phases. Most stardust is thought to originate in three distinct processes in carbon- and/or oxygen-rich mineral-forming stars: (1) condensation in the cooling, expanding atmospheres of asymptotic giant branch stars; (2) during the catastrophic explosions of supernovae, most commonly core collapse (Type II) supernovae; and (3) classical novae explosions, the consequence of runaway fusion reactions at the surface of a binary white dwarf star. Each stellar environment imparts distinctive isotopic and trace element signatures to the micro- and nanoscale stardust grains that are recovered from meteorites and micrometeorites collected on Earth’s surface, by atmospheric sampling, and from asteroids and comets. Although our understanding of the diverse mineral-forming environments of stars is as yet incomplete, we present a preliminary catalog of 41 distinct natural kinds of stellar minerals, representing 22 official International Mineralogical Association (IMA) mineral species, as well as 2 as yet unapproved crystalline phases and 3 kinds of non-crystalline condensed phases not codified by the IMA.

Robert M. Hazen↗
NASA NTRS1983

Chromospheric and coronal emissions from the giants in the Hyades

The visible Ca II K and International Ultraviolet Explorer spectra from the four K0 III stars (theta-1, gamma, delta, and epsilon Tau) in the Hyades, along with a field giant of similar spectral type, beta Gem, have been measured. Among the Hyades giants, the range of the high-temperature emissions can be a factor of 6 or more for C IV and an order of magnitude in the X-ray luminosity measured by Stern et al. (1981). For these presumably coeval cluster giants with similar macroscopic parameters, such as age, mass, effective temperature, gravity, and projected rotational velocity, the observed range in chromospheric and coronal emissions is not easily explained by the dominant factors thought to control these emissions in cool stars. It is possible that the emissions are time variable, on a time scale longer than six months. This would be the first evidence that giants undergo magnetic activity cycles similar to those of dwarf stars.

Baliunas, S. L.↗