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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 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

A near-infrared imaging survey of interacting galaxies - The disk-disk merger candidates subset

Near-infrared imaging obtained for systems believed to be advanced disk-disk mergers are presented and discussed. These systems were chosen from a sample of approximately 170 objects from the Arp Atlas of Peculiar Galaxies which have been imaged in the JHK bands as part of an investigation into the stellar component of interacting galaxies. Of the eight remnants which show optical signs of a disk-disk merger, the near-infrared surface brightness profiles are well-fitted by an r exp 1/4 law over all measured radii in four systems, and out to radii of about 3 kpc in three systems. These K band profiles indicate that most of the remnants in the sample either have finished or are in the process of relaxing into a mass distribution like that of normal elliptical galaxies.

Stanford, S. A.↗
NASA NTRS2007

The Role of Evolutionary Age and Metallicity in the Formation of Classical BE Circumstellar Disks II. Assessing the True Nature of Candidate Disk Systems

Photometric 2-color diagram (2-CD) surveys of young cluster populations have been used to identify populations of B-type stars exhibiting excess Ha emission. The prevalence of these excess emitters, assumed to be "Be stars". has led to the establishment of links between the onset of disk formation in classical Be stars and cluster age and/or metallicity. We have obtained imaging polarization observations of six SMC and six LMC clusters whose candidate Be populations had been previously identified via 2-CDs. The interstellar polarization (ISP) associated with these data has been identified to facilitate an examination of the circumstellar environments of these candidate Be stars via their intrinsic polarization signatures, hence determine the true nature of these objects. We determined that the ISP associated with the SMC cluster NGC 330 was characterized by a modified Serkowski law with a lambda(sub max) of approx. 4500Angstroms, indicating the presence of smaller than average dust grains. The morphology of the ISP associated with the LMC cluster NGC 2100 suggests that its interstellar environment is characterized by a complex magnetic field. Our intrinsic polarization results confirm the suggestion of Wisniewski et al. that a substantial number of bona-fide classical Be stars are present in clusters of age 5-8 Myr. Hence, our data contradict recent assertions that the Be phenomenon develops in the second half of a B star's main sequence lifetime, i.e. no earlier than 10 Myr. These data imply that a significant number of B-type stars must emerge onto the zero-age-main-sequence rotating at near-critical rotation rates, although we can not rule out the possibility that these data instead reveal the presence of a sub-group of the Be phenomenon characterized by sub-critically rotating objects. Comparing the polarimetric properties of our dataset to a similar survey of Galactic classical Be stars, we find that the prevalence of polarimetric Balmer jump signatures decreases with metallicity. We speculate that these results might indicate that either it is more difficult to form large disk systems in low metallicity environments, or that the average disk temperature is higher in these low metallicity environments. We have characterized the polarimetric signatures of all candidate Be stars in our data sample and find approx. 25% are unlikely to arise from true classical Be star-disk systems. This detection of such a substantial number "contaminants" suggests one should proceed with caution when attempting to determine the role of evolutionary age and/or metallicity in the Be phenomenon purely via 2-CD results.

Wisniewski, J. P.↗
NASA NTRS2007

The Chemistry of Multiply Deuterated Molecules in Protoplanetary Disks: I. The Outer Disk

We present new models of the deuterium chemistry in protoplanetary disks, including, for the first time, multiply deuterated species. We use these models to explore whether observations in combination with models can give us clues as to which desorption processes occur in disks.We find, in common with other authors, that photodesorption can allow strongly bound molecules such as HDO to exist in the gas phase in a layer above the midplane. Models including this process give the best agreement with the observations. In the midplane, cosmic-ray heating can desorb weakly bound molecules such as CO and N2. We find the observations suggest that N2 is gaseous in this region, but that CO must be retained on the grains to account for the observed DCO+/HCO+. This could be achieved by CO having a higher binding energy than N2 (as may be the case when these molecules are accreted onto water ice) or by a smaller cosmic-ray desorption rate for CO than assumed here, as suggested by recent theoretical work. For gaseous molecules the calculated deuteration can be greatly changed by chemical processing in the disk from the input molecular cloud values. On the grains singly deuterated species tend to retain the D/H ratio set in the molecular cloud, whereas multiply deuterated species are more affected by the disk chemistry. Consequently, the D/H ratios observed in comets may be partly set in the parent cloud and partly in the disk, depending on the molecule.

circumstellar matter↗
NASA NTRS2012

Discovery of Small-Scale Spiral Structures in the Disk of SAO 206462 (HD 135344B)(exp 1): Implications for the Physical State of the Disk from Spiral Density Wave Theory

We present high-resolution, H-band, imaging observations, collected with Subaru /HiCIAO, of the scattered light from the transitional disk around SAO 206462 (HD 1353448). Although previous sub-mm imagery suggested the existence of the dust-depleted cavity at r <= 46 AU, our observations reveal the presence of scattered light components as close as O".2 (approx 28 AU) from the star. Moreover , we have discovered two small-scale spiral structures lying within 0".5 (approx 70 AU). We present models for the spiral structures using the spiral density wave theory, and derive a disk aspect ratio of h approx. 0.1, which is consistent with previous sub-mm observations. This model can potentially give estimates of the temperature and rotation profiles of the disk based on dynamical processes. independently from sub-nun observations. It also predicts the evolution of the spiral structures, which can be observable on timescales of 10-20 years, providing conclusive tests of the model. While we cannot uniquely identify the origin of these spirals, planets embedded in the disk may be capable of exciting the observed morphology. Assuming that this is the case, we can make predictions on the locations and, possibly, the masses of the unseen planets. Such planets may be detected by future multi-wavelengths observations,

Muto, T.↗
NASA NTRS2012

Discovery of Small-Scale Spiral Structures in the Disk of SAO 206462 (HD 135344B): Implications for the Physical State of the Disk from Spiral Density Wave Theory

We present high-resolution, H-band, imaging observations, collected with Subaru/HiCIAO, of the scattered light from the transitional disk around SAO 206462 (HD 135344B). Although previous sub-mm imagery suggested the existence of the dust-depleted cavity at r approximates 46 AU, our observations reveal the presence of scattered light components as close as 0".2 (approx 28 AU) from the star. Moreover, we have discovered two small-scale spiral structures lying within 0".5 (approx 70 AU). We present models for the spiral structures using the spiral density wave theory, and derive a disk aspect ratio of h approx 0.1, which is consistent with previous sub-mm observations. This model can potentially give estimates of the temperature and rotation profiles of the disk based on dynamical processes, independently from sub-mm observations. It also predicts the evolution of the spiral structures, which can be observable on timescales of 10-20 years, providing conclusive tests of the model. While we cannot uniquely identify the origin of these spirals, planets embedded in the disk may be capable of exciting the observed morphology. Assuming that this is the case, we can make predictions on the locations and, possibly, the masses of the unseen planets. Such planets may be detected by future multi-wavelengths observations.

Grady, C. A.↗
NASA NTRS2018

Variability of Disk Emission in Pre-Main-Sequence and Related Stars. IV. Investigating the Structural Changes in the Inner Disk Region of MWC 480

We present five epochs of near-IR observations of the protoplanetary disk around MWC 480 (HD 31648) obtained with the SpeX spectrograph on NASA’s Infrared Telescope Facility between 2007 and 2013, inclusive. Using the measured line fluxes in the Pa β and Br γ lines, we found the mass accretion rates to be (1.26–2.30) x 10(exp −7) Solar Mass/yr and (1.4–2.01) x 10(exp −7) Solar Mass/yr, respectively, but which varied by more than 50% from epoch to epoch. The spectral energy distribution reveals a variability of about 30% between 1.5 and 10 μm during this same period of time. We investigated the variability using of the continuum emission of the disk in using the Monte- Carlo Radiative Transfer Code HOCHUNK3D. We find that varying the height of the inner rim successfully produces a change in the NIR flux but lowers the far-IR emission to levels below all measured fluxes. Because the star exhibits bipolar flows, we utilized a structure that simulates an inner disk wind to model the variability in the near-IR, without producing flux levels in the far-IR that are inconsistent with existing data. For this object, variable near-IR emission due to such an outflow is more consistent with the data than changing the scale height of the inner rim of the disk.

Fernandes, Rachel B.↗
DOE OSTI2022

Deterministic Linear Time for Maximal Poisson‐Disk Sampling using Chocks without Rejection or Approximation

Abstract We show how to sample uniformly within the three‐sided region bounded by a circle, a radial ray, and a tangent, called a “chock.” By dividing a 2D planar rectangle into a background grid, and subtracting Poisson disks from grid squares, we are able to represent the available region for samples exactly using triangles and chocks. Uniform random samples are generated from chock areas precisely without rejection sampling. This provides the first implemented algorithm for precise maximal Poisson‐disk sampling in deterministic linear time. We prove O(n · M(b) log b), where n is the number of samples, b is the bits of numerical precision and M is the cost of multiplication. Prior methods have higher time complexity, take expected time, are non‐maximal, and/or are not Poisson‐disk distributions in the most precise mathematical sense. We fill this theoretical lacuna.

Mitchell, Scott A.↗
DOE OSTI2022

Hot Circumsingle Disks Drive Binary Black Hole Mergers in Active Galactic Nucleus Disks

Abstract Binary black hole (BBH) mergers, particularly those with component masses in the pair-instability gap, may be produced by hierarchical mergers in the disks surrounding Active Galactic Nuclei (AGNs). While the interaction of an embedded BBH with an AGN disk is typically assumed to facilitate a merger, recent high-resolution hydrodynamical simulations challenge this assumption. However, these simulations often have simplified treatments for gas thermodynamics. In this work, we model the possible consequence of various feedback from an embedded BBH with a simple model that maintains an enhanced temperature profile around each binary component. We show that when the minidisks around each BH become hotter than the background by a factor of three, the BBH orbital evolution switches from expansion to contraction. By analyzing the gravitational torque profile, we find that this change in direction is driven by a weakening of the minidisk spirals and their positive torque on the binary. Our results highlight the important role of thermodynamics around BBHs and its effect on their orbital evolution, suggesting that AGN disks could be efficient factories for BBH mergers.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1991

Hot gas in the disk, halo, and disk-halo interaction

A critical review of some of the evidence for hot gas in the Galactic disk, and the corresponding disk-halo connection mechanisms is presented. The soft X-ray background indicates that the solar system environment is 10 exp 6 K gas, but that such gas may be unusual. X-ray observations of other galaxies are reported. High ionization stages in UV absorption studies of the ISM are discussed. The mean densities are estimated. High ion stages in the halo and implications of the thick disk are also addressed.

Cox, D. P.↗
NASA NTRS2005

Techniques for writing and reading data on an optical disk which include formation of holographic optical gratings in plural locations on the optical disk

An optical memory for storing and/or reading data on an optical disk. The optical disk incorporates a material in which holographic gratings can be created, and subsequently detected, at plural locations within the disk by an electro-optical head. Creation and detection of holographic gratings with variable diffraction efficiency is possible with the electro-optical head. Multiple holographic gratings can also be created at each one of the plural locations via a beam of light which has a different wavelength or point of focus. These data elements can be read by the electro-optical head using a beam of light sequentially varied in wavelength or point of focus to correspond to the multiple holographic gratings to be recorded.

Liu, Tsuen-Hsi↗
NASA NTRS2021

Variability of Disk Emission in Pre-main Sequence and Related Stars. V. OccultationEvents from the Innermost Disk Region of the Herbig Ae Star HD 163296

HD 163296 is a Herbig Ae star that underwent a dramatic∼0.8 magnitude drop in brightness in the V photometric band in 2001 and a brightening in the near-IR in 2002. Because the star possesses Herbig–Haro objects traveling in out flowing bipolar jets, it was suggested that the drop in brightness was due to a clump of dust entrained in a disk wind, blocking the line of sight toward the star. In order to quantify this hypothesis, we investigated the brightness drop at visible wavelengths and the brightening at near-IR wavelengths of HD 163296 using the Monte Carlo Radiative Transfer Code,HOCHUNK3D. We created three models to understand the events. Model 1 describes the quiescent state of the system. Model 2 describes the change in structure that led to the drop in brightness in 2001.Model 3 describes the structure needed to produce the observed 2002 brightening of the near-IR wavelengths. Models 2 and 3 utilize a combination of a disk wind and central bipolar flow. By introducing a filled bipolar cavity in Models 2 and 3, we were able to successfully simulate a jet-like structure for the star with a disk wind and created the drop and subsequent increase in brightness of the system. On the other hand, when the bipolar cavity is not filled, Model 1 replicates the quiescent state of the system

Monika Pikhartova↗
DOE OSTI2020

Synthesis and magnetic performance of gadolinium powder produced with rotating disk atomization

Spherical powder is key for achieving good heat transfer and maintaining low pressure drop for heat transfer fluid in magnetocaloric regenerator. Obtaining relatively large spherical powder of rare earth metals that are extremely sensitive to oxygen is challenging. Comparing to gas atomization and plasma rotating electrode methods, rotating disk atomization (RDA) method is more robust for obtaining a small amount of powder with a short turn-around time, thereby, more suitable for research and development of new magnetocaloric materials. In this work, commercially bulk gadolinium was successfully atomized to produce spherical powder in the targeted 150–250 μm range. The obtained powder exhibited the expected magnetocaloric performance with a maximum change of magnetic entropy of 12.1 J/kg K and a maximum adiabatic temperature change of 12.8 K with an applied magnetic field of 7 T.

36 MATERIALS SCIENCE↗
NASA NTRS1987

The wind-disk interaction in X-ray burst sources. II - Collimation by thick accretion disks

The wind-disk interaction model in type I cosmic X-ray bursts is here examined in the case where the accretion disk surrounding the neutron star is geometrically thick. The funnel structure along the rotation axis of such a system may collimate the radiatively driven wind following a thermonuclear flash in the star's surface layers, and could thereby enhance the radiative flux (diffusive plus advective) along the beam direction. It is shown that for plausible conditions, this mechanism can account for the super-Eddington fluxes observed in some bursts. The overall time scale for variability, the color temperature, and the spectral shape in this picture are consistent with the observed properties of type I X-ray bursts.

Melia, Fulvio↗
NASA NTRS1992

Optical spectroscopy of Z Canis Majoris, V1057 Cygni, and FU Orionis - Accretion disks and signatures of disk winds

High resolution, high SNR optical spectra have been used to investigate the hypothesis that in outburst, FU Ori objects are self-luminous accretion disks whose light dominates at optical and near-IR wavelengths. Strong evidence has been found for linewidth versus wavelength correlation in good agreement with model predictions for Z CMa and V1057 Cyg, but not for FU Ori itself. Linewidth varies continuously with wavelength at optical wavelengths in the former two objects, In the case of FU Ori, it is argued that a combination of strong wind components to spectral lines, and surface gravity possibly being lower than that of supergiants, conceals the underlying linewidth versus wavelength relationship. A marginal correlation is found between linewidth and lower excitation potential in all three objects. Synthetic disk spectra are subtracted from observed spectral, and remarkably good fits are found for all three objects for wavelengths longer than about 5000 A.

Welty, Alan D.↗
NASA NTRS1996

RAID-S Technical Overview: Raid 4 and 5-Compliant Hardware and Software Functionality Improves Data Availability Through Use of XOR-Capable Disks in an Integrated Cached Disk Array

The purpose of this paper is to provide a technical description of redundant array of independent disks - Symmetrix (RAID-S). It is intended to give the reader an understanding of how RAID-S is architected and implemented in the EMC Symmetrix 3000/5000 series integrated cached disk array. Topics include RAID-S taxonomy, configuration considerations, operational characteristics, performance, and implementation guidelines.

Quinn, Brett↗
NASA NTRS1989

Ly-alpha emission from disk absorption systems at high redshift - Star formation in young galaxy disks

Narrow-band imaging observations are reported which were made in an attempt to detect Ly-alpha emission from high-redshift candidate galaxy disk systems discovered as high column density absorbers of background QSOs. For four systems with z = 2.3-2.8, no emission is detected to a limit of about 10 to the -16th ergs/sq cm/s, corresponding to luminosity limits of about 10 exp 42-43 ergs/s for the material producing the absorption. The inferred Ly-alpha luminosities lie one to two orders of magnitude below estimates of the Ly-alpha luminosities for active star-forming epochs of many prescriptions for galaxy formation and also considerably below measured Ly-alpha luminosities for other candidate young galaxies detected in radio surveys. A limiting star-formation rate in these systems of about 2-7 solar masses/yr is set; the limit may be about 10 times larger with small but observationally allowable amounts of dust.

Smith, Harding E.↗