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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 577 records · Page 32
All sourcesNASA NTRSDOE OSTI
NASA NTRS2017

Radiation Processing of Polycyclic Aromatic Hydrocarbons (PAHs) in Space: ICEE PoC

Small Polycyclic Aromatic Hydrocarbon molecules or PAHs (<30 carbon atoms) have been identified in comets, meteorites, asteroids, and interplanetary dust particles in our Solar System, while PAHs in the Interstellar Medium (ISM) tend to be much larger, usually between 50 to 100 carbon atoms in size. The cause of the size disparity between PAHs found in the ISM and Solar System as well as their influence on Solar System organics is not yet understood. Two chemical evolutionary paths have been proposed to explain the inventory of solar system organics. In one the prebiotic material was formed from the radiation induced modification of large pre-solar carbon-bearing species (e.g. ISM PAHs). The second path suggests that Solar System prebiotic matter is the result of bottom-up synthesis from small reactive molecules after the Solar System was formed. In this second scenario very few ISM PAHs survived the harsh pre-solar radiation as aromatic structures. ICEE PoC (ICEE Proof of Concept) investigated factors impacting the chemical evolution of large PAHs irradiated under conditions similar to the proto-solar nebula. Likewise ICEE PoC will refine the technical parameters of the proposed ICEE (Institute for Carbon Evolution Experiment) laboratory.

Mattioda, Andrew↗
NASA NTRS1994

A model of galactic cosmic rays for use in calculating linear energy transfer spectra

The Galactic Cosmic Rays (GCR) contain fully stripped nuclei, from Hydrogen to beyond the Iron group, accelerated to high energies and are a major component of the background radiation encountered by satellites and interplanetary spacecraft. This paper presents a GCR model which is based upon our current understanding of the astrophysics of GCR transport through interstellar and interplanetary space. The model can be used to predict the energy spectra for all stable and long-lived radioactive species from H to Ni over an energy range from 50 to 50,000 MeV/nucleon as a function of a single parameter, the solar modulation level phi. The details of this model are summarized, phi is derived for the period 1974 to present, and results from this model during the 1990/1991 Combined Release and Radiation Effects Satellite (CRRES) mission are presented.

Chen, J.↗
NASA NTRS1975

The recognition of extraterrestrial intelligence

The departure from radiative equilibrium - represented by radio, television and radar technology - in the microwave spectrum of the earth is easily detectable over interstellar distances. Even with a technology no more advanced than our own, a civilization on a planet of a nearby star could easily determine, by auto-correlation techniques, the artificiality of these radio signals. Possible message contents for interstellar discourse of a modulated signal at any accessible frequency include (1) m-dimensional imagery represented by the transmission of numbers which are the products of m prime numbers; and (2) the use of a common mathematics, physics or astronomy to convey a range of information on more difficult subjects. The only direct attempts to date to communicate with extraterrestrial intelligence - the plaques aboard the Pioneer 10 and 11 spacecraft - are discussed briefly.

Sagan, C.↗
NASA NTRS1976

The protostellar origin of interstellar grains

A semiquantitative description of the origin of interstellar grains in a protostellar environment is given. Grains grow in the protostellar collapse phase and can be expelled by radiation pressure from stars of at least about 5 solar masses. A convective shell of grains forms in which shattering produces a power-law spectrum of fragments. A bimodal distribution of grain sizes, peaking at approximately 0.1 and 0.01 micron results. The effects of protostellar winds and rotation are considered.

Silk, J.↗
NASA NTRS1980

Laboratory and modeling studies of chemistry in dense molecular clouds

A chemical evolutionary model with a large number of species and a large chemical library is used to examine the principal chemical processes in interstellar clouds. Simple chemical equilibrium arguments show the potential for synthesis of very complex organic species by ion-molecule radiative association reactions.

Huntress, W. T., Jr.↗
NASA NTRS1989

The pre- and post-accretion irradiation history of cometary ices

Comets Halley and Wilson exhibited similar 3.4 micron emission features at approx. 1 AU from the Sun. A simple model of thermal emission from organic grains fits the feature, provides optical depths in good agreement with spacecraft measurements, and explains the absence of longer-wavelength organic features as due to spectral heliocentric evolution (Chyba and Sagan, 1987). The model utilizes transmission spectra of organics synthesized in the laboratory by irradiation of candidate cometary ices; the authors have long noted that related gas-phase syntheses yield polycyclic aromatic hydrocarbons, among other organic residues (Sagan et al., 1967). The authors previously concluded (Chyba and Sagan, 1987) that Halley's loss of several meters' depth with each perihelion passage, combined with the good fit of the Halley 3.4 micron feature to that of comet Wilson (Allen and Wickramasinghe, 1987), argues for the primordial - but not necessarily interstellar - origin of cometary organics. The authors examine the relative importance to the formation of organics of the variety of radiation environments experienced by comets. They conclude that there is at present no compelling reason to choose any of three contributing mechanisms (pre-accretion UV, pre-accretion cosmic ray, and post-accretion radionuclide processing) as the most important.

Chyba, Christopher↗
NASA NTRS1991

Expansion of a superbubble in a uniform magnetic field

The effects of interstellar magnetic fields on the evolution and structure of superbubbles are investigated. First, the governing equations for propagation of a radiative shock are derived and solved analytically in the limit of high expansion velocity. A numerical code able to solve them in the general case and discuss the numerical results is then presented. It is found that magnetic fields of the strength present in the Galactic disk do not significantly modify the overall shape and dimensions of a superbubble. They reduce the volume of the interior cavity by one-third on average during the expansion phase. The shell elongates slightly in the direction of the external field. Magnetic pressure dominates over gas pressure in most of the shell and forces it to thicken substantially. Finally, because of the transverse mass motions which take place from the magnetic poles to the equator, the column density at the poles is reduced by typically a factor of 10.

Ferriere, Katia M.↗
NASA NTRS1992

Scientific objectives and first results from COMPTEL

The imaging Compton telescope (COMPTEL) is the first imaging telescope in space to explore the MeV gamma ray range. At present it is performing a complete sky survey. In later phases of the mission, selected celestial objects will be studied in more detail. Targets of special interest in the COMPTEL energy range are radio pulsars, X-ray binaries, novae, supernova remnants, molecular clouds, and the interstellar medium within the Milky Way, as well as the nuclei of active galaxies, supernovae, and the diffuse cosmic background radiation in extragalactic space. The first four months of operation demonstrated that COMPTEL basically performs as expected. The Crab is clearly seen at its proper position in the first images of the anticenter region of the Galaxy. The Crab pulsar lightcurve was measured with unprecedented accuracy. The quasar 3C273 was seen for the first time at MeV-energies. Several cosmic bursts within the COMPTEL field of view could be located to an accuracy of about 1 degree. On June 9, 11, and 15, 1991 COMPTEL observed gamma ray (continuum and line) emission from three solar flares. Neutrons were also detected from the June 9 flare. At the present state of analysis, COMPTEL achieves the prelaunch predictions of its sensitivity within a factor of 2. Based on the present performance of COMPTEL, the team is confident that COMPTEL will fulfill its primary mission of surveying and exploring the MeV sky.

Schoenfelder, V.↗
NASA NTRS1995

Solar photoionization as a loss mechanism of neutral interstellar hydrogen in interplanetary space

Two primary loss mechanisms of interstellar neutral hydrogen in interplanetary space are resonance charge exchange ionization with solar wind protons and photoionization by solar EUV radiation. The later process has often been neglected since the average photoionization rate has been estimated to be as much as 5 to 10 times smaller than the charge exchange rate. These factors are based on ionization rates from early measurements of solar EUV and solar wind fluxes. Using revised solar EUV and solar wind fluxes measured near the ecliptic plane we have reinvestigated the ionization rates of interplanetary hydrogen. The result of our analysis indicates that indeed the photoionization rate during solar minimum can be smaller than charge exchange by a factor of 5; however, during solar maximum conditions when solar EUV fluxes are high, and solar wind fluxes are low, photoionization can be over 60% of the charge exchange rate at Earth orbit. To obtain an accurate estimate of the importance of photoionization relative to charge exchange, we have included photoionization from both the ground and metastable states of hydrogen. We find, however, that the photoionization from the metastable state does not contribute significantly to the overall photoionization rate.

Ogawa, H. S.↗
NASA NTRS1993

Anatomy of the Photodissociation Region in the Orion Bar

Much of the interstellar gas resides in photodissociation regions whose chemistry and energy balance is controlled by the flux of far-ultraviolet radiation upon them. These photons can ionize and dissociate molecules and heat the gas through the photoelectric effect working on dust grains. These regions have been extensively modeled theoretically, but detailed observational studies are few. Mapping of the prominent Orion Bar photo-dissociation region at wavelengths corresponding to the carbon-hydrogen stretching mode of polycyclic aromatic hydrocarbons, the 1-0 S(l) line of molecular hydrogen, and the J = 1-0 rotational line of carbon monoxide allows the penetration of the far-ultraviolet radiation into the cloud to be traced. The results strongly support the theoretical models and show conclusively that the incident far-ultraviolet radiation field, not shocks as has sometimes been proposed, is responsible for the emission in the Orion Bar.

Tielens, A. G. G. M.↗
NASA NTRS2002

Infrared Emission From Interstellar PAHs, New Probes of the Interstellar Medium

Tremendous strides have been made in the understanding of interstellar material over the past twenty years thanks to significant, parallel developments in two closely related areas: observational IR astronomy and laboratory astrophysics. Twenty years ago the composition of interstellar dust was largely unknown and the notion of abundant, gas phase, polycyclic aromatic hydrocarbons (PAHs) anywhere in the interstellar medium (ISM) considered impossible. Today the dust composition of the diffuse and dense ISM is reasonably well constrained and the spectroscopic case for interstellar PAHs, impossibly large molecules by early interstellar chemistry standards, is very strong. PAH spectral features are now being used as new probes of the ISM. PAH ionization states reflect the ionization balance of the medium while PAH size and structure reflect the energetic and chemical history of the medium. Aromatic carbon-rich materials ranging in size from PAHs and PAH nanoclusters, to sub-micron and micron-sized dust grains represent an important component of the ISM. These species: (1) dominate the heating and cooling of interstellar clouds via energetic photoelectron ejection and infrared (IR) emission; (2) moderate the ionization balance in photodissociation regions and molecular clouds; (3) moderate the composition of the gas phase and play an important role in determining the chemistry of the ISM; (4) contribute to the interstellar extinction in the near IR, visible, and UV spectral regions; and (5) convert UV, visible, and near-IR radiation to mid- and far-IR radiation in the ISM and, as such, are responsible for the well known, widespread family of mid-IR emission features with major components near 3.3, 6.2, 7.7, 8.6, and 11.3 microns.

Hudgins, D. M.↗
NASA NTRS2022

Si5345, SY88422L, LT3482, and MSK130 components Total Ionizing Dose Test Report

The purpose of this test was to characterize the SI-5345, SY88422L, LT3482 and MSK130 parameter degradation for total dose response. The components are associated with REAG ID #21-024. In the test, each device was exposed to high dose rate (HDR) irradiation using gamma radiation. Device parameters such as leakage currents, timing, and overall chip health were investigated. These devices were tested for an interstellar mission on a trajectory between 0.9 AU and 5.3 AU with a 2.5-year mission, yielding an expected overall dose of 29 krad(Si)behind 1 mm of aluminum shielding (at 95% confidence; environment and dose determined through SPENVIS). This test report gives an initial look at candidate parts. Extremely small sample sizes (only 1 or 2 devices tested) do not allow consideration of part-to-part variability. High dose rates were used despite LT3482 and MSK130 having potential enhanced low dose rate sensitivity (ELDRS). Care should be taken in interpreting results given these limitations.

Total Ionizing Dose (TID)↗
NASA NTRS1991

Simulations of the supernova-dominated interstellar medium in disk galaxies

A three-dimensional simulation of the evolution of an SNR-dominated ISM from an initially homogeneous, uniform substrate is presented. Systems with only bremsstrahlung cooling are compared with those cooled by radiation from metals. This distinction translates into a size difference in the resulting SNRs, and thus in the ability of isolated SNRs to affect the evolution of the galactic ecosystem. At a given SN rate, correlating the SNe decreases the global evolution of the galaxy if the SNRs individually can affect the system. It is shown that, when the density of the ISM varies widely, the N(R) test can give the reverse of standard expectations. It is shown that strong SNRs can impede interstellar tunnel production because of their ability to transport material and fill a previously evacuated volume, and that cloud size affects the final picture of the system.

Cioffi, Denis F.↗
NASA NTRS1976

The ionization of cloud and intercloud hydrogen by O and B stars

The ionization and acceleration of neutral clouds in the neighborhood of a single O star located in a low-density intercloud medium (ICM) are analyzed. It is shown that due to ionized cloud debris, the rms electron density may increase to 1 per cu cm in the lifetime of the star. It is suggested that an O star which moves away from its primordial nebula at a velocity of 10 to 20 km/s may continuously surround itself with ionized cloud debris and may never extensively ionize a low-density ICM. The lifetime of a remote cloud exposed to relatively uniform Lyman continuum radiation is derived and evaluated as a function of position in the Galaxy. It is found that extensive O-star ionization of a low-density ICM cannot be maintained for more than a few million years if the cloud debris is not cleared away by, for example, supernovae. Various ICM models are examined in which B stars and possibly nuclei of planetary nebulae play a major role in the ionization of interstellar hydrogen. It is shown that such stars may ionize a larger volume of the Galaxy than O stars, even though the volume emissivity of their Lyman continuum radiation is only 5% to 15% of that of O stars.

Elmergreen, B. G.↗
DOE OSTI2023

Implementation of Chemistry in the Athena++ Code

Chemistry plays a key role in many aspects of astrophysical fluids. Atoms and molecules are agents for heating and cooling, determine the ionization fraction, serve as observational tracers, and build the molecular foundation of life. We present the implementation of a chemistry module in the publicly available magnetohydrodynamic code Athena++ . We implement several chemical networks and heating and cooling processes suitable for simulating the interstellar medium (ISM). A general chemical network framework in the KIDA format is also included, allowing users to easily implement their own chemistry. Radiation transfer and cosmic-ray ionization are coupled with chemistry and solved with the simple six-ray approximation. The chemical and thermal processes are evolved as a system of coupled ordinary differential equations with an implicit solver from the CVODE library. We perform and present a series of tests to ensure the numerical accuracy and convergence of the code. Many tests combine chemistry with gas dynamics, including comparisons with analytic solutions, 1D problems of the photodissociation regions and shocks, and realistic 3D simulations of the turbulent ISM. We release the code with the new public version of Athena++ , aiming to provide a robust and flexible code for the astrochemical simulation community.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1993

Discovery of Jovian dust streams and interstellar grains by the Ulysses spacecraft

Within 1 AU from Jupiter, the dust detector aboard the Ulysses spacecraft during the flyby on February 8, 1992 recorded periodic bursts of submicron dust particles with durations ranging from several hours to two days and occurring at about monthly intervals. These particles arrived at Ulysses in collimate streams radiating from close to the line-of-sight direction to Jupiter, suggesting a Jovian origin for the periodic bursts. Ulysses also detected a flux of micron-sized dust particles moving in high-velocity retrograde orbits. These grains are identified here as being of interstellar origin.

Gruen, E.↗
DOE OSTI2020

CF + excitation in the interstellar medium

The detection of CF + in interstellar clouds potentially allows astronomers to infer the elemental fluorine abundance and the ionization fraction in ultraviolet-illuminated molecular gas. Because local thermodynamic equilibrium (LTE) conditions are hardly fulfilled in the interstellar medium (ISM), the accurate determination of the CF + abundance requires one to model its non-LTE excitation via both radiative and collisional processes. Here, we report quantum calculations of rate coefficients for the rotational excitation of CF + in collisions with para- and ortho-H 2 (for temperatures up to 150 K). As an application, we present non-LTE excitation models that reveal population inversion in physical conditions typical of ISM photodissociation regions (PDRs). We successfully applied these models to fit the CF + emission lines previously observed toward the Orion Bar and Horsehead PDRs. The radiative transfer models achieved with these new rate coefficients allow the use of CF + as a powerful probe to study molecular clouds exposed to strong stellar radiation fields.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1975

On the nature of the intercloud medium

Previous analyses of Copernicus observations of interstellar absorption in the spectra of unreddened stars are reviewed in the framework of steady-state ionization models for the intercloud (low-density) component of the interstellar medium. A simple homogeneous model is described which accounts for the H, C, N, S, and Si interstellar lines in the spectrum of lambda Sco in terms of an extensive low-density medium photoionized by Lyman continuum radiation from the II Sco OB association. Quantitative constraints are placed on the maximum permissible flux of diffuse soft X-rays, and the model is generalized to interpret the observed ion, electron, and hydrogen column densities in the low-density medium. Severe constraints are found to be imposed on ionization models by the ratios Si III/Si, Si IV/Si, and S IV/S. It is concluded that the observations may be explained if 10 to 30% of a representative line of sight passes through a photoionized medium of density 0.1 to 0.2/cu cm, if H I constitutes the bulk of the gas along the line of site, and if the gas is at a similar density or is in the form of shielded clumps or clouds.

Hill, J. K.↗