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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 649 records · Page 36
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NASA NTRS1991

Direct imaging of nonsolar planets with infrared telescopes using apodized coronagraphs

This research examines the use of modified Lyot coronagraphs with monolithic and segmented infrared telescopic systems for imaging nonsolar planets. These systems are investigated with the aim of reducing the effects of stellar diffracted energy on the planet image in the final image plane. A square telescope objective is best for this purpose. The associated coronagraph is composed of a cross-shaped apodizer in the first focal plane and either a square Lyot stop or circular corner Lyot stops in the corners of the pupil plane. The consequences of segmenting the aperture and the effects of various segment spacings and random piston and tilt errors of the individual segments are examined. A system to correct for the misalignments is proposed.

Mills, James P.↗
NASA NTRS1994

Origins of interstellar and solar system: Carbonaceous materials

Carbon is a crucial atom in cosmochemistry. It is well-established that carbon is synthesized in stellar interiors after the main sequence, is ejected by red giants as small carbonaceous grains during their 'carbon star' phase, resides in the interstellar medium, and was later incorporated into the solar system. The mechanisms of carbon grain formation and later chemical processing are complex because, with only small thermodynamic differences, carbon can take on a bewildering variety of forms: diamond; oxides; carbides; graphite; aliphatic hydrocarbons; polycyclic aromatic hydrocarbons (PAH's); fullerenes; amorphous carbon; and other compounds. These are evidence for many of the forms of carbon found in astronomical observations. We seek to understand the possible astrophysical sites and conditions of the origins of different forms of carbon by combining state-of-the-art capabilities of carbon chemistry with astrophysical modeling. The work is a collaboration between Prof. Frenklach, a leading carbon materials scientist with both laboratory and computer modeling expertise and Prof. Feigelson, an astrophysicist with interests in star formation. The largest effort under this grant was devoted to developing this concept into a comprehensive quantitative model. In addition to explaining the astronomical properties of red giants producing carbonaceous grains, our model also can incorporate recent meteoritic findings. Finally, our induced nucleation grain formation model provides a natural explanation for the widespread presence of PAH emission bands in the Galactic interstellar medium. A brief synopsis of other activities sponsored under this grant and a list of publications from this grant is included.

Feigelson, Eric D.↗
NASA NTRS1995

Photospheric, circumstellar, and interstellar features of HE, C, N. O, and Si in the HST spectra of four hot white dwarf stars

This paper reports on the observations of four hot white dwarf stars with the spectrographs on the Hubble Space Telescope (HST). The higher resolving power and higher signal/noise, in comparison with IUE, reveals a very rich phenomomenology, including photospheric features from heavy elements, circumstellar features, and the first direct detection of accretion onto the white dwarf component of a binary system. Specific results include the following: Our observations of the ultrahot degenerate H1504+65 confirm that it has a photosphere which is depleted in both H and He, and reveals features of C IV and O VI. The spectrum fits previously published models extremely well. The intermediate-temperature DO star PG 1034+001 has an ultraviolet spectrum showing complex profiles of the well-known resonance doublets of C IV, N v, and Si IV. The O V 1371 line shows a clear separation into a photospheric and a circumstellar component, and it is likely that the same two components can explain the other lines as well. The cooler DA star GD 394 has an extensive system of heavy-element features, but their radial velocity is such that it is highly unlikely that they are formed in the stellar photosphere. Time-resolved spectra of the accreting white dwarf in the V 471 Tau binary system are briefly presented here; they do show the presence of C IV, Si IV, and He II. However, the C IV and He II lines are in emission, rather than in aborption as had been expected.

Shipman, Harry L.↗
NASA NTRS1985

The modelling of latitudinal gradients in the solar wind in the outer solar system

A steady, axisymmetric, quasi-radial, global model is developed for thermally driven stellar winds with embedded magnetic fields. The asymptotic, linear results are presented for 0(1) latitudinal variations in the radial magnetic field, mass-loss rate, and radial velocity of the wind. The MHD equations are solved for the latitudinal dependence of the rotational velocity and magnetic field. They are driven by the meridional flows that develop naturally from internal magnetic stresses. Most flows open flux tubes in the stars equatorial plane, redistributing mass and magnetic flux as a function of stellar latitude. The plasma spins up to conserve angular momentum in fields and plasma.

Nerney, S.↗
NASA NTRS1990

Determining gravitational radiation from Newtonian self-gravitating systems

Quadrupole gravitational radiation formulas are tested and compared in a calculation of rotating stellar core collapse. While the standard quadrupole formula (SQF) allows an economical calculation of the waveform in postlinear gravity, it has several shortcomings that become apparent in a finite difference (FD) calculation. These shortcomings are related to the large-moment arm of the quadrupole moment and the two (numerical) time derivatives that separate it from the waveform. These shortcomings lead to high-frequency noise in the SQF waveform. Several alternatives to the SQF are developed. Each is mathematically equivalent to the SQF and applicable to self-gravitating sources. For FD calculations, the new quadrupole formulas are all superior to the SQF and capable of producing waveforms free of the high-frequency noise characteristics of its use.

Finn, Lee Samuel↗
NASA NTRS1992

Coordinated Ginga, IUE, and VLA observations of stellar activity in Sigma(2) Coronae Borealis

Observations of the active cool star binary system Sigma(2) Corona Borealis with the Large Area Counters on the Ginga X-ray satellite over 2.5 d, or about two orbital periods, are reported. Throughout the observations, Sigma(2) CrB exhibited X-ray variability on time scales ranging from tens of minutes to hours. The X-ray light curve shows evidence of flaring during the course of the observations. The summed X-ray pulse height spectrum from flaring events compared with similar spectra from quiescent periods indicates an increase in coronal temperature. Periodogram analysis of the entire X-ray light curve shows a possible periodicity at about 0.40-0.44 d, with no strong signal at the orbital period or 1/2 of the orbital period. Coordinated multiwavelength observations of IUE and the VLA indicate both UV and microwave flaring during one X-ray outburst. The quiescent microwave spectrum is not consistent with a thermal gyrosynchrotron process; nonthermal gyrosynchrotron radiation is argued to be the most likely emission process.

Stern, R. A.↗
NASA NTRS1979

Isotopic heterogeneities in the solar system - Special report

The paper discusses the nature of isotopic anomalies in solar system material with emphasis on correlated anomalies in refractory elements. Evidence for the existence of short-lived radioactive nuclides in the early solar system is used to estimate the time scale for the last injection of freshly synthesized nuclear material. It is shown that the early solar nebula was incompletely mixed and contained debris which was injected from a stellar source at most a few million years prior to the formation of the solar system. The average solar system material consists of ambient interstellar material deficient in certain nuclear species to which a small fraction of freshly synthesized material was added. The isotopic variations reflect slightly different proportions of nuclei from different stellar sources which were locally well mixed and homogenized prior to or during the formation of the early solar nebula condensates.

Wasserburg, G. J.↗
NASA NTRS1994

FRESIP: A Discovery Mission Concept To Find Earth-Sized Planets Around Solar Like Stars

The current nebular theory postulates that planets are. a consequence of the formation of stars from viscous accretion disks. Condensation from the accretion disk favors the formation of small rocky planets in the hot inner region, and the formation of gas giants in the cool outer region. Consequently, terrestrial-type planet in inner orbits should be commonplace. From geometrical considerations , Borucki and Summers have shown that 1% of planetary systems resembling our solar system should show transits for Earth-sized (or larger) planets. Thus a photometric satellite that uses a wide field of view telescope and a large detector array to simultaneously monitor 5000 target stars should detect 50 planetary systems. To differentiate regularly recurring transits from statistical fluctuations of the stellar flux, one must observe over several orbital periods so that the false positive rate can be reduced to one event or less. A one-meter aperture telescope placed in a halo orbit about either the L1 or L2 Lagrange points and viewing perpendicular to both the orbital and ecliptic planes can view continuously for the required period because neither the Sun, Earth, or Moon would enter the field of view. Model calculations show that the observations should provide statistically significant estimates of the distributions of planetary size, orbital radius, coplanarity, and the frequency of planetary systems that have Earth-sized planets in inner orbits. Because approximately one half of the star systems observed will be binary systems, the frequency of planetary systems orbiting either one or both of the stars can also be determined.

Borucki, William↗
NASA NTRS1989

Nonaxisymmetric instabilities in thin self-gravitating rings and disks

The stability of geometrically thin self-gravitating rings and disks is studied by computing the eigenvalues and eigenfunctions of linearized normal mode oscillations in these systems. For the vertically averaged incompressible models and models of gaseous and stellar disks with softened gravity, analytic treatment is possible. The existence of oscillatory normal modes is established using a variational principle and infer instability using perturbation theory. Effects due to the Lindblad and corotation resonances are analyzed in detail. The distribution of the ratio of vorticity to surface density is important for determining the stability of the systems.

Papaloizou, John C. B.↗
NASA NTRS1973

UV photometry of eclipsing variables

UV photometric observations of the eclipsing variable CW Cephei by orbiting OAO-2 instrument package reveal the binary to be a system in an active stage of evolution, with evidence for stellar mass loss or mass exchange.

Sobieski, S.↗
NASA NTRS1975

Ultraviolet observations of alpha Aurigae from Copernicus

Emission lines of L-alpha (1215.67 A) and O VI (1031.94 A) were detected in the spectroscopic binary alpha Aur (Capella) with the Princeton experiment on Copernicus. Temperatures of the emitting regions are inferred to be in excess of 300,000 K. The temperature and emission measure are consistent with a variable source of soft X-rays. If the emission is attributed to the primary star (G5 III), the atmosphere is expanding with velocities of about 20-100 km/s. Such expansion can lead to material within the binary system. The density of interstellar hydrogen inferred from absorption of stellar L-alpha appears to be approximately 0.01 hydrogen atoms per cu cm.

Dupree, A. K.↗
NASA NTRS1982

Cool luminous stars

The consequences of magnetic fields for the atmospheres of cool stars are addressed based on IUE and Einstein Observatory observations. Gross atmospheric structure data for cool luminous stars are discussed, and the relevance of the observations for the energy balance and nonradiative heating rates is considered. Data from X-ray fluxes, low dispersion UV spectra, and high dispersion spectra in the region of the Mg II resonance lines are presented. New results on cool star atmospheres provided by high dispersion UV spectra are considered, including those pertaining to spectral line identification, identification of emission components in close binary systems, densities and atmospheric extension, emission line widths, properties of stellar winds as derived from IUE data, and systematic flows of transition region plasma. An explanation for the observed spectra of cool giants and supergiants is proposed in terms of active, quiet, and hybrid stars.

Linsky, J. L.↗
NASA NTRS1989

Stellar Inertial Navigation Workstation

Software and hardware assembled to support specific engineering activities. Stellar Inertial Navigation Workstation (SINW) is integrated computer workstation providing systems and engineering support functions for Space Shuttle guidance and navigation-system logistics, repair, and procurement activities. Consists of personal-computer hardware, packaged software, and custom software integrated together into user-friendly, menu-driven system. Designed to operate on IBM PC XT. Applied in business and industry to develop similar workstations.

Johnson, W.↗
NASA NTRS1993

Controllable Optical Delay Line For Stellar Interferometry

Prototype optical delay line for use in Big Optical Array stellar interferometer developed by Naval Research Laboratory, is advanced version of Mark III stellar interferometer at Mt. Wilson. Several delay lines used in system; their purpose to equalize optical pathlengths from target star to beam combiner in interferometer via each of arms of interferometer. Features passive mechanical suppression of vibrations plus active control for tracking, slewing, and suppression of vibrations.

Colavita, Mark↗
NASA NTRS1993

A formulation of convection for stellar structure and evolution calculations without the mixing-length theory approximations. II - Application to Alpha Centauri A and B

We have constructed a series of models of Alpha Centauri A and Alpha Centauri B for the purposes of testing the effects of convection modeling both by means of the mixing-length theory (MLT), and by means of parameterization of energy fluxes based upon numerical simulations of turbulent compressible convection. We demonstrate that while MLT, through its adjustable parameter alpha, can be used to match any given values of luminosities and radii, our treatment of convection, which lacks any adjustable parameters, makes specific predictions of stellar radii. Since the predicted radii of the Alpha Centauri system fall within the errors of the observed radii, our treatment of convection is applicable to other stars in the H-R diagram in addition to the sun. A second set of models is constructed using MLT, adjusting alpha to yield not the 'measured' radii but, instead, the radii predictions of our revised treatment of convection. We conclude by assessing the appropriateness of using a single value of alpha to model a wide variety of stars.

Lydon, Thomas J.↗
NASA NTRS1996

An IRU for Cassini

The JPL Inertial Reference Unit (IRU) is the single most sophisticated assembly on the Cassini Spacecraft. At the core of the IRU is the state-of-the-art, Litton (formerly Delco) Hemispherical Resonator Gyroscope (HRG). Launched in October 1997, Cassini's trajectory utilizes gravity assist manuevers around Venus (twice), Earth, and Jupiter over a seven year period, arriving at Saturn in June 2004.

Cassini IRU Inertial Reference Unit attitude stabi↗
NASA NTRS2021

The Far Ultraviolet M-dwarf Evolution Survey. I. The Rotational Evolution of High-energy Emissions

M-dwarf stars are prime targets for exoplanet searches because of their close proximity and favorable properties for both planet detection and characterization. However, the potential habitability and atmospheric characterization of these exoplanetary systems depends critically on the history of high-energy stellar radiation from X-rays to NUV, which drive atmospheric mass loss and photochemistry in the planetary atmospheres. With the Far Ultraviolet M-dwarf Evolution Survey, we have assessed the evolution of the FUV radiation, specifically eight prominent emission lines, including Lyα, of M-dwarf stars with stellar rotation period and age. We demonstrate tight power-law correlations between the spectroscopic FUV features, and measure the intrinsic scatter of the quiescent FUV emissions. The luminosity evolution with rotation of these spectroscopic features is well-described by a broken power law, saturated for fast rotators, and decays with increasing Rossby number, with a typical power-law slope of −2, although likely shallower for Lyα. Our regression fits enable FUV emission-line luminosity estimates relative to bolometric from known rotation periods to within ∼0.3 dex, across eight distinct UV emission lines, with possible trends in the fit parameters as a function of source layer in the stellar atmosphere. Our detailed analysis of the UV luminosity evolution with age further shows that habitable-zone planets orbiting lower-mass stars experience much greater high-energy radiative exposure relative the same planets orbiting more massive hosts. Around early- to mid-M dwarfs, these exoplanets, at field ages, accumulate up to 10–20× more EUV energy, relative to modern Earth. Moreover, the bulk of this UV exposure likely takes place within the first Gyr of the stellar lifetime.

J Sebastian Pineda↗