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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 235 records · Page 13
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NASA NTRS1989

The brightness and lightcurve of Triton in 1987

At the 8900 A wavelength of the CCD and photoelectric photometry obtained for Triton in 1987, where than moon's methane absorption might be reflected as a strong contrast, no rotational lightcurve has been noted to a level of below 0.02 mag, in contradiction of Franz's (1981) apparent lightcurve of 0.06 mag observed in 1977. Triton's photoelectrically determined 1987 V magnitude is consistent with the most reliable of previous measurements, exhibiting no clear indication of long-term change.

Lark, Neil L.↗
NASA NTRS1989

Background studies in gas ionizing x ray detectors

The background response of a gas ionizing proportional x ray detector is estimated by solving the one dimensional photon transport equation for two regions using Monte Carlo techniques. The solution was effected using the SSL VAX 780 and the CRAY XMP computers at Marshall Space Flight Center. The isotropic photon energy spectrum encompassing the range from 1 to 1000 KeV incident onto the first region, the shield, is taken so as to represent the measured spectrum at an altitude of 3 mb over Palastine, Texas. The differential energy spectrum deposited in the gas region, xenon, over the range of 0 to 100 KeV is written to an output file. In addition, the photon flux emerging from the shield region, tin, over the range of 1 to 1000 KeV is also tabulated and written to a separate file. Published tabular cross sections for photoelectric, elastic and inelastic Compton scattering as well as the total absorption coefficient are used. Histories of each incident photon as well as secondary photons from Compton and photoelectric interactions are followed until the photon either is absorbed or exits from the regions under consideration. The effect of shielding thickness upon the energy spectrum deposited in the xenon region for this background spectrum incident upon the tin shield was studied.

Eldridge, Hudson B.↗
NASA NTRS1990

Hydrodynamic simulations of stellar wind disruption by a compact X-ray source

This paper presents two-dimensional numerical simulations of the gas flow in the orbital plane of a massive X-ray binary system, in which the mass accretion is fueled by a radiation-driven wind from an early-type companion star. These simulations are used to examine the role of the compact object (either a neutron star or a black hole) in disturbing the radiatively accelerating wind of the OB companion, with an emphasis on understanding the origin of the observed soft X-ray photoelectric absorption seen at late orbital phases in these systems. On the basis of these simulations, it is suggested that the phase-dependent photoelectric absorption seen in several of these systems can be explained by dense filaments of compressend gas formed in the nonsteady accreation bow shock and wake of the compact object.

Blondin, John M.↗
NASA NTRS1992

A new investigation of photometric changes in RW Persei

New photoelectric observations of RW Per were obtained, and photometric solutions were carried out for these data, for earlier photoelectric observations, and for archival photographic observations. It is concluded that no third light exists in this binary, and that the original nodal variation explanation for historical changes in the depth of primary eclipse is therefore incorrect. A revised nodal variation hypothesis is suggested. It is also noted that these changes could have been produced by a gradual increase in the polar radius of the hot star, perhaps related to a change in that star's rotational velocity.

Olson, Edward C.↗
NASA NTRS1992

Atlas of the spectrum of a platinum/neon hollow-cathode reference lamp in the region 1130-4330 A

The spectrum of a platinum hollow-cathode lamp containing neon carrier gas was recorded photographically and photoelectrically with a 10.7 m normal-incidence vacuum spectrograph. Wavelengths and intensities were determined for about 5600 lines in the region 1130-4330 A. An atlas of the spectrum is given, with the spectral lines marked and their intensities, wavelengths, and classifications listed. Lines of impurity species are also identified. The uncertainty of the photographically measured wavelengths is estimated to be +/- 0.0020 A. The uncertainty of lines measured in the photoelectric scans is 0.01 A for wavelengths shorter than 2030 A and 0.02 A for longer wavelengths. Ritz-type wavelengths are given for many of the classified lines of Pt II with uncertainties varying from +/- 0.0004 to +/- 0.0025 A. The uncertainty of the relative intensities is estimated to be about 20 percent.

Sansonetti, Jean E.↗
NASA NTRS1991

The variability of the solar diameter

It is argued here that most of the variation in observations of the solar radius of over three centuries measure properties of the sun's limb darkening function and are affected by many sources of degradation of the solar image. They do not directly measure a true solar radius. From a long series of visual observations made by a single observer as well as recent photoelectric observations, evidence is found for periodicities in the apparent radius that occur in both modern and historical records. The magnitude of the observed variations is quite different in visual and photoelectric observations, suggesting that the process responsible for the periodicities is either one that modifies the solar limb darkening function or one that causes systematic variations in image blurring by the earth's atmosphere. A connection between solar magnetic activity an apparent radius seems likely, with evidence for such a relation dating back as fast as the Maunder minimum.

Ribes, E.↗
NASA NTRS1993

Standardization work for the calibration of intensities of celestial objects

Observations of celestial phenomena need to be calibrated, to be related, to some measurable quantity. There continues to be a long-term need for accurate photometric standard stars, those with known intensities and colors, so that brightness and color measurements made of various celestial phenomena by different observers can be integrated and compared with one another. Toward this end, the author has been emphasizing data collection in recent years for stars of 'intermediate' brightness, those approximately in the magnitude range 11.5 is less than V is less than 16.0. Photoelectric data were obtained at the Cerro Tololo InterAmerican Observatory's (CTIO) telescopes for stars in certain selected areas near the celestial equator. Stars of extreme color outside the selected areas, but near the equator, also were selected to provide a more broad and complete range in color index. It was proposed to complete the photoelectric phase of the program. The author proposed developing extremely faint sequences of photometric standard stars useful for both large space-based detectors and for land-based detectors. These data were to be collected via charge-coupled devices (CCD's) at telescopes located at CTIO and at the Las Campanas Observatory (LCO), both observatories being located in Chile. It was hoped that accurate data could be collected down to the 21st or 22nd magnitude.

Landolt, Arlo U.↗
NASA NTRS1994

Energy deposition and K-shell ionization of supernovae

We present explicit evaluation of K-shell ionization caused by photoelectric absorption, by Compton scattering, and by electron collisions in supernovae envelopes. Each process derives from the radioactivity that dominates the bolometric luminosity of the model supernovae. We include the ionization by bremsstrahlung. We find that electron-collision ionization with decelerating Compton electrons is the dominant process of K-shell ionization for light elements (lighter than Si) whereas photoelectric K-shell absorption of Comptonized gammas is the dominant process of K-shell ionization for heavy elements (like Fe). The relative importance of the ionization processes also depends upon composition and upon time, as well as upon supernova type. For Type Ia all four ionization sources are of importance. We present fitting formulas for the deposition of radioactive power in each class of models studied. The energy deposition of all mechanisms decreases with time, including the rate of K-shell ionizations of all elements throughout the supernovae model evaluated herein. We compare the relative contributions to the energy deposition and confirm with greater numerical detail results already established.

The, Lih-Sin↗
NASA NTRS1994

The hard X-ray emission spectra from accretion columns in intermediate polars

We consider the hard (greater than 2 keV) X-ray emission from accretion columns in an intermediate polar system, GK Per, using a simple settling solution. The rate of photon emission per logarithmic energy interval can be fitted with a power law, E(exp -gamma), with gamma approximately 2.0, in agreement with observations. This index is only weakly dependent on the mass accretion rate, dot-M, for dot-M in the range of a few times 10(exp 16-18) g/s. The peak energy of the photon spectra (after photoelectric absorption) is expected to be E(sub p) approximately (5 keV) gamma(exp -1/3) (N(sub H)/10(exp 23)/sq cm)(exp 1/3) where N(sub H) is the hydrogen column density along the line of sight. The observed spectra of GK Per and possibly of V1223 Sgr suggest N(sub H) approximately 10(exp 23)/sq cm. This large N(sub H) may be due to partially ionized preshock column material. Alternatively, we also consider absorption by the cool outer parts of an accretion disk. In this case the photoelectric absorption depth in the disk is a sensitive function of inclination. For GK Per the required inclination is approximately 83 deg. For mass accretion rates larger than a critical rate of approximately 10(exp 18) g/s, X-ray emission from the column accretion is significantly affected by radiation drag. Although the mass accretion rate increases dramatically during outbursts, the observed hard (greater than 2 keV) X-ray luminosity will not rise proportionately. The slope and peak energy of the outburst spectra are only weakly affected. We conclude that the observed X-ray spectra can be explained by this simple analytic solution and that the production of hard X-rays from the accretion shock at the magnetic poles in the intermediate polars is in general agreement with the observations. However, since the X-ray emission and absorption depend on the mass accretion rate in a complicated manner, observed hard X-ray luminosities (greater than 2 keV) are not a good indicator of the mass accretion rate in the X-ray emitting regions.

Yi, Insu↗
NASA NTRS1995

The neutral atomic phases of the interstellar medium

We calculate the thermal equilibrium gas temperature of the diffuse interstellar medium. Our method incorporates a new photoelectric heating rate from small grains and polycyclic aromatic hydrocarbons (PAHs) that accounts for a size distribution of particles extending from 100 to 3 A radius. We also include a detailed treatment of the ionization rates and heating due to the soft X-ray background and due to cosmic rays. Phase diagrams (thermal pressure P versus hydrogen density n) are presented for gas that is illuminated by local interstellar far-ultraviolet (FUV) and X-ray radiation fields. A stable two-phase medium is produced with thermal pressure in the range P/k approximately = to 10(exp 3-4) K/cc. We demonstrate that photoelectric heating from PAHs dominates in the warm neutral phase (WNM) and cold neutral phase (CNM). If the C II (158 micrometers cooling per hydrogen nucleus in the solar neighborhood represents an average value for the Galaxy, we predict L(sub CII) approximately = to 7 x 10(exp 7) solar luminosities from the CNM in the Galaxy, comparable to that observed by the Cosmic Background Explorer (COBE). We discuss the dependence of the results on absorbing column density, gas phase abundances, dust abundances and metallicity, FUV field, and the X-ray radiation field. These results will be useful in modeling the multiphase structure of high-velocity clouds in the halo, the interstellar matter (ISM) at other galactocentric radii, and the ISM in external galaxies and galactic nuclei.

Wolfire, M. G.↗
NASA NTRS2001

The New BeppoSAX Observation of the Brightest X-Ray Quasar at Redshift

This grant was to support the reduction and analysis of our approved SAX observation of the high redshift (z=3.2) blazar PKS 2126-158. This is the brightest quasar at z greater than 3 and has been intensively studied in X-ray, since the first Einstein detection. In 1994 Elvis et al., discovered a strong low energy cutoff in this object, which could imply either quasar frame photoelectric absorption by a column of 0.8-2.7 x 1e22 cm-2 cold gas, or a lower column of cold gas at z=0. Subsequent ASCA observations of this object, could not definitely address this issue, nor could establish whether the curvature of the low energy portion of the spectrum was due to pure photoelectric absorption (considerably exceeding the Galactic value along the line of sight) or to an intrinsic continuum curvature. We proposed to observe PKS 2126-158 with BeppoSAX, to try to solve this puzzle (thanks to the broadband of BeppoSAX: 0.1-250 keV). PKS 2126 was observed by BeppoSAX on May 1999, with a MECS exposure of 100 ks. We have reduced and analyzed the BeppoSAX data, and compared them with a Chandra ACIS observation of the same object, taken only 6 months apart (Nov. 1999). We have recently finished to write a paper on the BeppoSAX data only, that concentrate on the properties of the X-ray absorber, which is highly requested by our SAX data, independently on the continuum model adopted. The paper (P.I.F. Fiore) will be submitted to APJ in the next few days. A second paper on the combined BeppoSAX and Chandra data, and based on the broad band spectral energy distribution of this quasar, is currently in preparation. Our main results, on the X-ray absorber, are: (a) the presence of an X-ray absorber is confirmed, indipendently on the continuum adopted (simple power law, or curved continuum); (b) if the absorber is not significantly ionized, then the BeppoSAX data do prefer a low redshift absorber; (c) if the gas is ionized, then it can be located in the quasar environment, but its metal abundances must be lower than 0.2 times solar (because of the absence of a strong FeK absorption edges that would follow at approx. 2 keV, where the MECS response is the highest).

Nicastro, Fabrizio↗
NASA NTRS2001

Photoemission Experiments for Charge Characteristics of Individual Dust Grains

Photoemission experiments with UV radiation have been performed to investigate the microphysics and charge characteristics of individual isolated dust grains of various compositions and sizes by using the electrodynamic balance facility at NASA Marshall Space Flight Center. Dust particles of 2-10 gm diameter are levitated in a vacuum chamber at pressures approximately 10(exp-5) torr and exposed to a collimated beam of UV radiation in the 120-200 nm spectral range from a deuterium lamp source with a MgF2 window. A monochromator is used to select the UV wavelength with a spectral resolution of 8 nm. The electrodynamic facility permits measurements of the charge and diameters of particles of known composition, and monitoring of photoemission rates with the incident UV radiation. Experiments have been conducted on test particles of silica and polystyrene to determine the photoelectric yields and surface equilibrium potentials when exposed to UV radiation. A brief description of an experimental procedure for photoemission studies is given and some preliminary laboratory measurements of the photoelectric yields of individual dust particles are presented.

Abbas, M. M.↗
NASA NTRS2002

Laboratory Studies of the Optical Properties and Condensation Processes of Cosmic Dust Grains

A laboratory facility for conducting a variety of experiments on single isolated dust particles of astrophysical interest levitated in an electrodynamics balance has been developed at NASA/Marshall Space Flight Center. The objective of the research is to employ this experimental technique for studies of the physical and optical properties of individual cosmic dust grains of 0.1-100 micron size in controlled pressure/temperatures environments simulating astrophysical conditions. The physical and optical properties of the analogs of interstellar and interplanetary dust grains of known composition and size distribution will be investigated by this facility. In particular, we will carry out three classes of experiments to study the micro-physics of cosmic dust grains. (1) Charge characteristics of micron size single dust grains to determine the photoelectric efficiencies, yields, and equilibrium potentials when exposed to UV radiation. (2) Infrared optical properties of dust particles (extinction coefficients and scattering phase functions) in the 1-30 micron region using infrared diode lasers and measuring the scattered radiation. (3) Condensation experiments to investigate the condensation of volatile gases on colder nucleated particles in dense interstellar clouds and lower planetary atmospheres. The condensation experiments will involve levitated nucleus dust grains of known composition and initial mass (or m/q ratio), cooled to a temperature and pressure (or scaled pressure) simulating the astrophysical conditions, and injection of a volatile gas at a higher temperature from a controlled port. The increase in the mass due to condensation on the particle will be monitored as a function of the dust particle temperature and the partial pressure of the injected volatile gas. The measured data will permit determination of the sticking coefficients of volatile gases and growth rates of dust particles of astrophysical interest. Some preliminary results based on measurements of photoelectric emission and radiation pressure on single isolated 0.2 to 6.6 micron size silica particles exposed to UV radiation at 120-200 nm and green laser light at 532 nm are presented.

Abbas, M. M.↗
NASA NTRS2003

High-Sensitivity X-ray Polarimetry with Amorphous Silicon Active-Matrix Pixel Proportional Counters

Photoelectric X-ray polarimeters based on pixel micropattern gas detectors (MPGDs) offer order-of-magnitude improvement in sensitivity over more traditional techniques based on X-ray scattering. This new technique places some of the most interesting astronomical observations within reach of even a small, dedicated mission. The most sensitive instrument would be a photoelectric polarimeter at the focus of 2 a very large mirror, such as the planned XEUS. Our efforts are focused on a smaller pathfinder mission, which would achieve its greatest sensitivity with large-area, low-background, collimated polarimeters. We have recently demonstrated a MPGD polarimeter using amorphous silicon thin-film transistor (TFT) readout suitable for the focal plane of an X-ray telescope. All the technologies used in the demonstration polarimeter are scalable to the areas required for a high-sensitivity collimated polarimeter. Leywords: X-ray polarimetry, particle tracking, proportional counter, GEM, pixel readout

Black, J. K.↗
NASA NTRS2004

Experiments on Dust Grain Charging

Dust particles in various astrophysical environments are charged by a variety of mechanisms generally involving collisional processes with other charged particles and photoelectric emission with UV radiation from nearby sources. The sign and the magnitude of the particle charge are determined by the competition between the charging processes by UV radiation and collisions with charged particles. Knowledge of the particle charges and equilibrium potentials is important for understanding of a number of physical processes. The charge of a dust grain is thus a fundamental parameter that influences the physics of dusty plasmas, processes in the interplanetary medium and interstellar medium, interstellar dust clouds, planetary rings, cometary and outer atmospheres of planets etc. In this paper we present some results of experiments on charging of dust grains carried out on a laboratory facility capable levitating micron size dust grains in an electrodynamic balance in simulated space environments. The charging/discharging experiments were carried out by exposing the dust grains to energetic electron beams and UV radiation. Photoelectric efficiencies and yields of micron size dust grains of SiO2, and lunar simulates obtained from NASA-JSC will be presented.

Abbas, M. N.↗
NASA NTRS2003

Carbon-Nitrogen-Oxygen Line Radiation and the X-ray Bowen Fluorescence Mechanism in Optically Thick, Highly Ionized Media

Radiative transfer effects due to overlapping X-ray lines in a high-temperature, optically thick, highly ionized medium are investigated. One particular example, in which the O VIII Lyalpha doublet (2(sup 2) P(sub 1/2,3/2)-1(sup 2) S(sub 1/2) coincides in frequency with the N VII Lyzeta lines (7(sup 2) P(sub 1/2,3/2)-1(sup 2) S(sub 1/2) is studied in detail to illustrate the effects on the properties of the emergent line spectrum. We solve the radiative transfer equation to study the energy transport of resonance-line radiation in a static, infinite, plane-parallel geometry, which is used to compute the destruction/escape probabilities for each of the lines for various total optical thicknesses of the medium, as well as destruction probabilities by sources of underlying photoelectric opacity. It is found that a large fraction of the O vIII Lyalpha line radiation can be destroyed by N VII, which can result in a reversal of the O VIII Lyalpha/N VII Lyalpha line intensity ratio similar to what may be seen under nonsolar abundances. Photoelectric absorption by ionized carbon and nitrogen can also subsequently increase the emission-line intensities of these ions. We show that line ratios, which are directly proportional to the abundance ratios in optically thin plasmas, are not good indicators of the true CNO abundances. Conversely, global spectral modeling that assumes optically thin conditions may yield incorrect abundance estimates when compared with observations, especially if the optical depth is large. Other potentially important overlapping lines and continua in the X-ray band are also identified, and their possible relevance to recent high-resolution spectroscopic observations with Chandra and XMM-Newton are briefly discussed.

Sako, Masao↗
NASA NTRS2005

Laboratory Investigations of the Physical and Optical Properties of the Analogs of Individual Cosmic Dust Grains

Microdsub-micron size cosmic dust grains play an important role in the physical and dynamical process in the galaxy, the interstellar medium, and the interplanetary and planetary environments. The dust grains in various astrophysical environments are generally charged by a variety of mechanisms that include collisional process with electrons and ions, and photoelectric emissions with UV radiation. The photoelectric emission process is believed to be the dominant process in many astrophysical environments with nearby UV sources, such as the interstellar medium, diffuse clouds, the outer regions of the dense molecular clouds, interplanetary medium, dust in planetary environments and rings, cometary tails, etc. Also, the processes and mechanisms involved in the rotation and alignment of interstellar dust grains are of great interest in view of the polarization of observed starlight as a probe for evaluation of the galactic magnetic field.

Abbas, M. M.↗
NASA NTRS2008

Lunar Dust Charging by Secondary Electron Emission and its Complex Role in the Lunar Environment

The lunar surface is covered with a thick layer of micron/sub-micron size dust grains formed by billions of years of meteoritic impact. With virtually no atmosphere and exposed to the solar wind plasma and solar electromagnetic radiation, the lunar surface and the dust grains are electrostatically charged. The dominant charging processes include: photoelectric emissions (UV, X-rays), impact of solar wind electrons and ions, and secondary electron emissions (SEE) induced by energetic solar wind electrons. During the Apollo missions, the astronauts found the lunar dust to be extraordinarily high in its adhesive characteristics, sticking to the suits and the mechanical equipment. Electrostatically charged lunar dust is believed to be transported over long distances by the induced electric fields, as indicated by the observed dust streamers and the horizon glow [e.g., 1-3]. The hazardous effects of dust in the lunar environment are recognized to be one of the major issues that must be addressed in planning the forthcoming missions for robotic and human exploration of the Moon. Theoretical studies are being performed along with the development of analytical models and a variety of experimental investigations, to better understand the lunar dust phenomena. [e.g., 4-6]. The lunar dust is believed to be charged negatively on the lunar night-side by interaction With solar wind electrons. However, rigorous theoretical expressions for calculation of SEE yields and the sticking efficiencies of individual micron size dust grains are not yet available, and the information has to be obtained by experiment. On theoretical considerations, however, it is well recognized that SEE yields, similar to the photoelectric yields for small-size grains, would be totally different from the corresponding bulk values [e.g., 7-9]. Some theoretical models for charging of individual small spherical particles have been developed [e.g., 10], and some limited measurements on individual metallic dust grains at keV electron energies have been made [e.g., i 1]. In this paper, we present the first measurements of the secondary electron emission yields of individual micron/sub-micron size dust grains selected from sample returns of Apollo 11 and Apollo 17 missions.

Abbas, M. M.↗