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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 181 records · Page 10
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NASA NTRS1986

Observations of gaseous circumstellar disks. III

Echelle spectra obtained at the Ca II K line and the Na I D lines are reported for seven stars which show excess fluxes in at least one of the IRAS bands at 25, 60, and 100 microns. Detectable circumstellar absorption lines arising from the gaseous components of the presumed circumstellar dust disks are found in none of these cases, in contrast to the previously reported case of Beta Pic. Conversely, six A or F stars which were known before the launch of IRAS to show gaseous circumstellar disks do not show excess fluxes in any of the IRAS bands. Similar echelle spectra of these stars show Ca II circumstellar absorption lines comparable in four cases to that of Beta Pic.

Hobbs, L. M.↗
NASA NTRS1989

Optical polarimetry and thermal imaging of the disc around Beta Pictoris

The dust disk around the main-sequence A5 star Beta Pic is characterized on the basis of recently obtained data. Optical polarimetry indicates that the disk has polarization of about 15 percent and polarization vectors normal to the scattering plane, indicating that the features observed are indeed a dust disk made visible by scattering. Thermal images of the central 13-arcsec-diameter region, obtained at 10 and 20 microns by Telesco et al. (1988) using the NASA Marshall 20-pixel bolometer array, suggest that the dust grains in the disk have a mean diameter of 1 micron or smaller. Also discussed are (1) unsuccessful searches for other nearby stars with disks and (2) the potential value of Hubble Space Telescope images for understanding the structure of Beta Pic.

Wolstencroft, R. D.↗
NASA NTRS1991

A search for circumstellar dust around HR 10, a proposed Beta Pictoris star

Archival IRAS data and recent IUE observations of the A shell star HR 10 (HD 256) are presented. HR 10 is detected in the IRAS Faint Source Survey and ADDSCAN data at 12 and 25 microns and marginally detected at 60 microns. The IUE data indicate a spectral type of A2 with 0.1 mag of UV selective extinction following a galactic average extinction law. HR 10 shows an infrared excess, compared to normal A stars of the same spectral type, beginning at 12 microns and extending to longer wavelengths. The IR excess is consistent with free-free emission and a suggestion of CS dust. The IR colors for this star differ significantly from those of Beta Pic and are closer to those of Vega. The absence of circumstellar selective extinction shortward of 3200 A suggests that the circumstellar grain size distribution does not resemble that of the interstellar medium, but may be more like the grains around Vega and Beta Pic.

Cheng, Kwang-Ping↗
NASA NTRS1991

Monte Carlo simulation of ion-neutral charge exchange collisions and grid erosion in an ion thruster

A combined particle-in-cell (PIC)/Monte Carlo simulation model has been developed in which the PIC method is used to simulate the charge exchange collisions. It is noted that a number of features were reproduced correctly by this code, but that its assumption of two-dimensional axisymmetry for a single set of grid apertures precluded the reproduction of the most characteristic feature of actual test data; namely, the concentrated grid erosion at the geometric center of the hexagonal aperture array. The first results of a three-dimensional code, which takes into account the hexagonal symmetry of the grid, are presented. It is shown that, with this code, the experimentally observed erosion patterns are reproduced correctly, demonstrating explicitly the concentration of sputtering between apertures.

Peng, Xiaohang↗
NASA NTRS1991

The circumstellar disks of Beta Pictoris analogs

A survey using data from the International Ultraviolet Explorer (IUE) of previously known B and A shell stars with IRAS detections has resulted in the identification of three stars, HD 93563, Sigma Her, and 51 Oph, which have spectral signatures of infalling circumstellar plasma similar to Beta Pic. Two of these systems have infrared flux distributions indicating the presence of circumstellar dust disks, while the other, HD 93563, has an infrared excess consistent with free-free emission from the plasma envelope. With the identification of three such systems, it is clear that infalling circumstellar plasma is more common than previously anticipated among late-type B shell stars. The absence of dust in one system, HD 93563, suggests that infalling plasma in these stars, and possibly also in Beta Pic itself, may not be due to either erosion of a dust disk or to high cometary bombardment rates, but may instead be linked to stellar activity.

Grady, Carol A.↗
NASA NTRS1991

Computer simulation of astrophysical plasmas

The role of sophisticated numerical models and simulations in the field of plasma astrophysics is discussed. The need for an iteration between microphysics and macrophysics in order for astrophysical plasma physics to produce quantitative results that can be related to astronomical data is stressed. A discussion on computational requirements for simulations of astrophysical plasmas contrasts microscopic plasma simulations with macroscopic system models. An overview of particle-in-cell simulations (PICS) is given and two examples of PICS of astrophysical plasma are discussed including particle acceleration by collisionless shocks in relativistic plasmas and magnetic field reconnection in astrophysical plasmas.

Max, Claire E.↗
NASA NTRS1993

Density-conserving shape factors for particle simulations in cylindrical and spherical coordinates

It is established that a particle-to-grid weighting which is based on both conventional particle-in-cell (PIC) and cloud-in-cell (CIC) shape factors will yield nonuniform grid densities, even in the case of uniform particle distributions, in both cylindrical and spherical coordinates. In view of this, alternative density-conserving weighting schemes are discussed which encompass modified PIC and CIC weighting. These alternative shape factors are mathematically no more complicated than the conventional nondensity-conserving expressions.

Ruyten, Wilhelmus M.↗
NASA NTRS1992

Physical properties of gas disks around shell stars with and without dust

Analysis of archival IRAS and IUE data has resulted in: (1) identification of 8 new A star proto-planetary candidates; (2) detection of a mass outflow event around Beta Pic (subsequently confirmed by the 1991 July HST observation); and (3) confirmation of the suggestion by Waters et al. (1988) that 51 Oph is a protoplanetary system similar to beta Pic with the detection of high density, high velocity, collisionally ionized accreting gas in the line of sight toward this star.

Grady, Carol A.↗
NASA NTRS1994

A two-dimensional particle-in-cell model of a dusty plasma

Dusty plasmas are present in comets, in the ring systems of the outer planets, and in the interstellar medium. A two-dimensional particle-in-cell (PIC) model of a dusty plasma is presented in this paper. The PIC code is best suited for modeling the plasma-dust interaction for large grains, with diameters of the order of a centimeter. We have modeled the charging process for an individual dust grain and the associated potential pattern in the surrounding plasma. We have also considered the case of a large number of grains in a plasma, with intergrain separations of the order of the Debye length, and have shown that the plasma becomes depleted and the charge on a dust grain is reduced, as other workers in this field have predicted (cf. C. K. Goertz, 1989). We examine the electron and ion distribution functions in the vicinity of a charged grain and demonstrate that the ions near a grain have clearly been accelerated by the electrostatic potential.

Young, B.↗
NASA NTRS1995

Far-infrared observations of main sequence stars surrounded by dust shells

We have used a 20-channel bolometer array on NASA's Kuiper Airborne Observatory to obtain photometry and size information for several main sequence stars surrounded by dust shells. The observations were made at 50 and/or 100 micrometers on flights based in Christchurch, New Zealand, in 1992, 1993. The stars include the 'Vega-like' star, Beta Pic, as well as two stars, HD 135344 and HD 139614, suggested by subsequent studies to belong possibly to the same class. The results of our observations are best interpreted as upper limits to the far-infrared sizes of the dust clouds around these stars. In addition to the basic size and flux measurements, we have fit simple, optically thin models to the Beta Pic data to explore the range of shell parameters consistent with our limits and with previous observations.

Harvey, Paul M.↗
NASA NTRS1996

Transient Accretion Events in Herbig Ae/Be Star Spectra: The Evidence for Infalling Planetesimals in HD 100546 (B9E)

Enhanced gaseous absorption resembling the high velocity circumstellar gas features in the spectrum of Beta Pic have been detected in IUE high dispersion spectra of the 2 Myr-old Herbig Be star, HD 100546, on 1995 March 9. The presence of atomic gas features in transitions of C I and O I implies that the material is produced by the pyrolysis of an unseen, solid body in a star-grazing orbit. The presence of weak Fe II absorption in the same spectrum with heavily saturated magnesium, aluminum, and silicon features is consistent with an origin in the sublimation of the olivenes detected in IRAS LRS and ISO SWS spectra of this star. The C I and O I detections suggest that, compared to Beta Pic, the planetesimals in this system contain substantial amounts of carbonaceous material. The presence of mildly refractory species such as Zn II and S II suggests that the planetesimal had not been substantially heated above 700 K during its previous lifetime.

Grady, C. A.↗
NASA NTRS1996

EUVS Sounding Rocket Payload

During the first half of this year (CY 1996), the EUVS project began preparations of the EUVS payload for the upcoming NASA sounding rocket flight 36.148CL, slated for launch on July 26, 1996 to observe and record a high-resolution (approx. 2 A FWHM) EUV spectrum of the planet Venus. These preparations were designed to improve the spectral resolution and sensitivity performance of the EUVS payload as well as prepare the payload for this upcoming mission. The following is a list of the EUVS project activities that have taken place since the beginning of this CY: (1) Applied a fresh, new SiC optical coating to our existing 2400 groove/mm grating to boost its reflectivity; (2) modified the Ranicon science detector to boost its detective quantum efficiency with the addition of a repeller grid; (3) constructed a new entrance slit plane to achieve 2 A FWHM spectral resolution; (4) prepared and held the Payload Initiation Conference (PIC) with the assigned NASA support team from Wallops Island for the upcoming 36.148CL flight (PIC held on March 8, 1996; see Attachment A); (5) began wavelength calibration activities of EUVS in the laboratory; (6) made arrangements for travel to WSMR to begin integration activities in preparation for the July 1996 launch; (7) paper detailing our previous EUVS Venus mission (NASA flight 36.117CL) published in Icarus (see Attachment B); and (8) continued data analysis of the previous EUVS mission 36.137CL (Spica occultation flight).

Stern, Alan S.↗
NASA NTRS1998

Continuation of the University of Michigan Sounding Rocket Program

This interim report gives the activity on this grant through 31 October 1998, and the planned activities for the next fiscal year. A Project Initiation Conference (PIC) was held at the Wallops Flight Facility (WFF), with planning for the launch of flight 36.174 in Fall 1998. However, following the PIC, the WFF personnel decided that the experiment telemetry system had to be completely rewired, which required substantial design and wiring changes on the experiment side of the payload. These costs had not been anticipated in the FY98 budget, and used up considerable resources of the experimenters. In addition, the telemetry re-design was not completed until Fall 1998, so that there was no opportunity for a launch in 1998 (the Jupiter window closes in late November 1998 and it will reopen in late summer 1999). A Design Review was held by telecon with WFF personnel on 26 October 1998, with plans for the 36.174 launch now in late August 1999 with a launch window which extends into winter 2000. On a more positive note, we have been developing a vacuum UV calibration chamber in Michigan's Space Physics Research Lab, for future measurements of our experiment components, and this facility is now undergoing initial measurements of various calibration lamps in preparation for testing of the telescope UV reflective efficiency. This facility will greatly increase our ability to optimize our experiment for UV efficiency, which has been lacking in the past, and it will be made available for support of other NASA missions at Michigan and for visitors from other institutions. For the next flight 36.174, the following activities will be accomplished: (1) prepare the experiment, including replacement and upgrading of critical components, at University of Michigan; (2) pre-integrate the payload at the Wallops Flight Facility; (3) perform final far-UV calibration of instrument (4) perform final alignment, integration, and electrical checks at WSMR; (5) launch payload from WSMR and check condition of recovered payload; (6) perform post-flight calibration, if necessary and applicable (7) reduce and analyze flight data. The following pages contain information from the experimenter package for the Design Review for the next flight, 36.174, listing the plans, changes, and schedule for this flight.

Clarke, John↗
NASA NTRS1999

Three-Dimensional Simulations of Electron Beams Focused by Periodic Permanent Magnets

A fully three-dimensional (3D) model of an electron beam focused by a periodic permanent magnet (PPM) stack has been developed. First, the simulation code MAFIA was used to model a PPM stack using the magnetostatic solver. The exact geometry of the magnetic focusing structure was modeled; thus, no approximations were made regarding the off-axis fields. The fields from the static solver were loaded into the 3D particle-in-cell (PIC) solver of MAFIA where fully 3D behavior of the beam was simulated in the magnetic focusing field. The PIC solver computes the time-integration of electromagnetic fields simultaneously with the time integration of the equations of motion of charged particles that move under the influence of those fields. Fields caused by those moving charges are also taken into account; thus, effects like space charge and magnetic forces between particles are fully simulated. The electron beam is simulated by a number of macro-particles. These macro-particles represent a given charge Q amounting to that of several million electrons in order to conserve computational time and memory. Particle motion is unrestricted, so particle trajectories can cross paths and move in three dimensions under the influence of 3D electric and magnetic fields. Correspondingly, there is no limit on the initial current density distribution of the electron beam, nor its density distribution at any time during the simulation. Simulation results including beam current density, percent ripple and percent transmission will be presented, and the effects current, magnetic focusing strength and thermal velocities have on beam behavior will be demonstrated using 3D movies showing the evolution of beam characteristics in time and space. Unlike typical beam optics models, this 3D model allows simulation of asymmetric designs such as non- circularly symmetric electrostatic or magnetic focusing as well as the inclusion of input/output couplers.

Kory, Carol L.↗
NASA NTRS2000

Numerical Model of the Plasma Sheath Generated by the Plasma Source Instrument Aboard the Polar Satellite

The plasma sheath generated by the operation of the Plasma Source Instrument (PSI) aboard the POLAR satellite is studied by using a 3-dimensional Particle-In-Cell (PIC) code. When the satellite passes through the region of low density plasma, the satellite charges to positive potentials as high as 4050Volts due to the photoelectrons emission. In such a case, ambient core ions cannot accurately be measured or detected. The goal of the onboard PSI is to reduce the floating potential of the satellite to a sufficiently low value so that the ions in the polar wind become detectable. When the PSI is operated, an ion-rich Xenon plasma is ejected from the satellite, such that the floating potential of the satellite is reduced and is maintained at about 2Volts. Accordingly, in our 3-dimensional PIC simulation, we considered that the potential of the satellite is 2Volts as a fixed bias. Considering the relatively high density of the Xenon plasma in the sheath (approx. 10 - 10(exp 3)/cc), the ambient plasma of low density (less than 1/cc) is neglected. In the simulations, the electric fields and plasma dynamics are calculated self-consistently. We found that an "Apple" shape positive potential sheath forms surrounding the satellite. In the region near the PSI emission, a high positive potential hill develops. Near the Thermal Ion Detection Experiment (TIDE) detector away from the PSI, the potentials are sufficiently low for the ambient polar wind ions to reach it. In the simulations, it takes about a hundred electron gyroperiods for the sheath to reach a quasi-steady state. This time is approximately the time taken by the heavy Xe(+) ions to expand up to about one average Larmor radius of electrons from the satellite surface. Using the steady state sheath, we performed trajectory calculations to characterize the detector response to a highly supersonic polar wind flow. The detected ions' velocity distribution shows significant deviations from a shifted Maxwellian in the ambient polar wind population. The deviations are caused by the effects of electric fields on the ions' motion as they traverse the sheath.

Leung, Wing C.↗
NASA NTRS2001

Numerical Model of the Plasma Sheath Generated by the Plasma Source Instrument Aboard the Polar Satellite

The plasma sheath generated by the operation of the Plasma Source Instrument (PSI) aboard the Polar satellite is studied by using a three-dimensional particle-in-cell (PIC) code. When the satellite passes through the region of low-density plasma, the satellite charges to positive potentials as high as 40-50 V, owing to the photoelectron emission. In such a case, ambient core ions cannot accurately be measured or detected. The goal of the onboard PSI is to reduce the floating potential of the satellite to a sufficiently low value so that the ions in the polar wind become detectable. When the PSI is operated, ion-rich xenon plasma is ejected from the satellite, such that the floating potential of the satellite is reduced and is maintained at approximately 2 V. Accordingly, in our three-dimensional PIC simulation we considered that the potential of the satellite is 2 V as a fixed bias. Considering the relatively high density of the xenon plasma in the sheath (10-10(exp 3)/cc), the ambient plasma of low density (<1/cc) is neglected. In the simulations the electric fields and plasma dynamics are calculated self-consistently. We found that an 'apple'-shape positive potential sheath forms surrounding the satellite. In the region near the PSI emission a high positive potential hill develops. Near the Thermal Ion Dynamics Experiment detector away from the PSI, the potentials are sufficiently low for the ambient polar wind ions to reach it. In the simulations it takes only about a couple of tens of electron gyroperiods for the sheath to reach a quasi steady state. This time is approximately the time taken by the heavy Xe(+) ions to expand up to about one average Larmor radius of electrons from the satellite surface. After this time the expansion of the sheath in directions transverse to the ambient magnetic field slows down because the electrons are magnetized. Using the quasi steady sheath, we performed trajectory calculations to characterize the detector response to a highly supersonic polar wind flow. The detected ions' velocity distribution shows significant deviations from a shifted Maxwellian in the ambient polar wind population. The deviations are caused by the effects of electric fields on the ions' motion as they traverse the sheath.

Singh, N.↗
NASA NTRS2000

Picturing Video

Video Pics is a software program that generates high-quality photos from video. The software was developed under an SBIR contract with Marshall Space Flight Center by Redhawk Vision, Inc.--a subsidiary of Irvine Sensors Corporation. Video Pics takes information content from multiple frames of video and enhances the resolution of a selected frame. The resulting image has enhanced sharpness and clarity like that of a 35 mm photo. The images are generated as digital files and are compatible with image editing software.

Source record↗
NASA NTRS2002

Imaging Forming Planetary Systems: The HST/STIS Legacy and Prospects for Future Missions

The first indication that debris and protoplanetary disks associated with other, young planetary systems were sufficiently nearby to image came with the IRAS detection of infrared excesses around $\beta$ Pic, Vega, Fomalhaut, and $\epsilon$ Eri. Moving beyond analysis of the infrared excess to optical and near-IR imaging requires access to high Strehl ratio and high contrast imaging techniques, with the ability to efficiently reject the residual scattered and diffracted light from the star to reveal the fainter scattered light and circumstellar emission originating from the vicinity of the star. HST/STIS imaging studies have made use of incomplete Lyot coronagraphic imaging modes to reveal the warped, inner disk of $\beta$ Pic, provide the highest spatial resolution images of young debris disk systems such as HR 4796A, have revealed the presence of azimuthally symmetric structure in HD 141569 and HD 163296, and have demonstrated that currently active, collimated outflows survive to higher stellar masses than previously expected, and through more of the star's pre-main sequence lifetime than anticipated. The HST/STIS coronagraphic imaging legacy will be discussed, together with the implications for future NIR and optical high contrast imaging capabilities.

Grady, Carol↗