Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “INTERSTELLAR GRAINS”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Why do interstellar grains exist?

There exists a discrepancy between calculated destruction rates of grains in the interstellar medium and postulated sources of new grains. This problem was examined by modelling the global life cycle of grains in the galaxy. The model includes: grain destruction due to supernovae shock waves; grain injection from cool stars, planetary nebulae, star formation, novae, and supernovae; grain growth by accretion in dark clouds; and a mixing scheme between phases of the interstellar medium. Grain growth in molecular clouds is considered as a mechanism or increasing the formation rate. To decrease the shock destruction rate, several new physical processes, such as partial vaporization effects in grain-grain collisions, breakdown of the small Larmor radius approximation for betatron acceleration, and relaxation of the steady-state shock assumption are included.

Seab, C. G.↗
NASA NTRS1985

Photochemical reactions in interstellar grains photolysis of CO, NH3, and H2O

The interstellar grains are currently considered to be the basic building blocks of comets and, possibly, meteorites. To test this theory, a simulation of the organic layer accreted onto interstellar dust particles was prepared by slow deposition of a CO:NH3:H2O gas mixture on an Al block at 10 K, with concomitant irradiation with vacuum UV. The results of the HPLC and IR analyses of the nonvolatile residue formed by photolysis at 10 K are compared with those observed at 77 K and 298 K. Some of the compounds that may be present on the surfaces of interstellar dust particles have been identified, and some specific predictions concerning the types of molecular species present in comets could be drawn. The results also suggest that photochemical reactions may have been important for the formation of meteorite components. The implication of the findings to the questions of the source of organic matter on earth and the origin of life are discussed.

Agarwal, V. K.↗
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

Origin of large interstellar grains toward Rho Ophiuchi

The paper considers why the interstellar grains toward Rho Oph are unusually large. Because the visual opacity per gram is lower than average toward Rho Oph, it is concluded that the simplest interpretation of the observations is that grain coagulation has occurred. This result may be important for understanding the evolution of interstellar clouds and may provide insight into the initial stages of the formation of large solid bodies such as planets.

Jura, M.↗
NASA NTRS1975

On the presence of phyllosilicate minerals in the interstellar grains

The composition of the interstellar silicate dust is investigated. Condensation or alteration of silicate grains at temperatures of a few hundred degrees, in the presence of H2O, would result in hydrous or phyllosilicates, the silicate type most abundant in the type I carbonaceous chondrites. Infrared spectra of small particles (about 0.1 microns) of the high temperature condensates, olivine and pyroxene, at 300 K and 4 K do not give a good match to the interstellar absorption band near 9.8 microns. Laboratory spectra of several phyllosilicates give better agreement as does the spectrum of a carbonaceous chondrite. We propose that the silicates in the interstellar grains are predominantly phyllosilicates and suggest additional spectral tests for this hypothesis.

Zaikowski, A.↗
NASA NTRS1977

Gamma-ray lines from interstellar grains

The letter points out the existence of a hitherto unknown component of gamma-ray line emission: very narrow (FWHM less than 5 keV) lines from interstellar grains. The prime candidate for detection is the line at 6.129 MeV from O-16, but other very narrow lines could also be detected at 0.847, 1.369, 1.634, 1.779, and 2.313 MeV from Fe-56, Mg-24, Ne-20, Si-28, and N-14. Measurements of this line emission can provide information on the composition, size, and spatial distribution of interstellar grains.

Lingenfelter, R. E.↗
NASA NTRS1986

Shock processing of interstellar grains

Shock processing plays an important role in the life of a typical interstellar grain. Shocks of 100 km/s-l or greater can destroy about 50% of the grain material under appropriate preshock conditions of density and magnetic field. The destruction occurs by grain-grain collisions and nonthermal sputtering for steady state radiative shocks and by thermal sputtering for fast adiabatic shocks. The evaluation of the lifetime of grains against shock destruction depends on models of the interstellar medium (ISM) structure and on supernova remnants (SNR) evolution. Results from various authors give lifetimes between 10 to the 8th and 10 to the 9th power years, compared to typical injection times for new grains of a few times 10 to the 9th power years. These numbers require that a major portion of the interstellar silicon bearing grain material must be formed by grain growth in the ISM. At the same time, the presence of isotopic anomalies in some meteorites implies that at least some grains must survive from their formation in SNRs or red giant winds through incorporation into the solar system.

Seab, C. G.↗
DOE OSTI2020

Revisiting the reactivity between HCO and CH 3 on interstellar grain surfaces

The formation of interstellar complex organic molecules is currently thought to be dominated by the barrierless coupling between radicals on the interstellar icy grain surfaces. Previous standard density functional theory (DFT) results on the reactivity between CH 3 and HCO on amorphous water surfaces showed that the formation of CH 4 + CO by H transfer from HCO to CH 3 assisted by water molecules of the ice was the dominant channel. In contrast, the adopted description of the electronic structure of the biradical (i.e. CH 3 /HCO) system was inadequate [without the broken-symmetry (BS) approach]. In this work, we revisit the original results by means of BS-DFT both in gas phase and with one water molecule simulating the role of the ice. Results indicate that the adoption of BS-DFT is mandatory to describe properly biradical systems. In the presence of the single water molecule, the water-assisted H transfer exhibits a high energy barrier. In contrast, CH 3 CHO formation is found to be barrierless. Yet, direct H transfer from HCO to CH 3 to give CO and CH 4 presents a very low energy barrier, hence being a potential competitive channel to the radical coupling and indicating, moreover, that the physical insights of the original work remain valid.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1975

Clumping of interstellar grains during formation of the primitive solar nebula

The author has previously shown that a considerable amount of clumping of interstellar grains is likely to take place during the free-fall collapse phase of an interstellar cloud which is forming the primitive solar nebula, with the assumption of sonic turbulence in the gas. The original estimate involved the crude assumption of hierarchal amalgamation of the grains upon collision. A Monte Carlo simulation of this process confirmed the general features of the results, but it was further found that the introduction of a low sticking probability reduced the size of the lumps quite significantly. A more realistic calculation was therefore carried out in which it was assumed that clumps of grains would tend to stick together if their collisions were approximately head-on, but that they would tend to fragment into smaller pieces if the collisions were more tangential. For typical values of the amalgamation parameter, this tends to spread the mass of the interstellar grains over a wide range of clump sizes, ranging from individual grains to objects in the millimeter or centimeter size.

Cameron, A. G. W.↗
NASA NTRS1986

The alignment of interstellar grains

The magnetic-field alignment of interstellar silicate grains containing superparamagnetic (SPM) inclusions is investigated theoretically. It is shown that the observed dependence of polarization on wavelength can be fully explained in terms of the alignment mechanisms and the size distribution of the SPM grains, at least for interstellar polarizations peaking at wavelength 550 nm or higher. The implications of this finding for models of grain coagulation and shattering are considered.

Mathis, J. S.↗
NASA NTRS1989

Optical properties of irregular interstellar grains

In order to study the interaction of light with interstellar grains, the authors represent an irregular particle by a network of interacting dipoles whose polarizability is determined in a first approach by the Clausius-Mossoti relationship. Typically, 10,000 dipoles are considered. In the case of spherical particles, the results from Mie theory are fully recovered. The main interest of this method is to study with good accuracy the implications of surface roughness and/or inhomogeneities on optical properties in the infrared spectral range, particularly of the silicate emission features.

Perrin, J. M.↗
DOE OSTI2015

IMPULSIVE SPOT HEATING AND THERMAL EXPLOSION OF INTERSTELLAR GRAINS REVISITED

The problem of the impulsive heating of dust grains in cold, dense interstellar clouds is revisited theoretically with the aim of better understanding the leading mechanisms of the explosive desorption of icy mantles. We rigorously show that if the heating of a reactive medium occurs within a sufficiently localized spot (e.g., the heating of mantles by cosmic rays (CRs)), then the subsequent thermal evolution is characterized by a single dimensionless number λ. This number identifies a bifurcation between two distinct regimes: when λ exceeds a critical value (threshold), the heat equation exhibits the explosive solution, i.e., the thermal (chemical) explosion is triggered. Otherwise, thermal diffusion causes the deposited heat to spread over the entire grain—this regime is commonly known as whole-grain heating. The theory allows us to find a critical combination of physical parameters that govern the explosion of icy mantles due to impulsive spot heating. In particular, our calculations suggest that heavy CR species (e.g., iron ions) colliding with dust are able to trigger the explosion. Based on recently calculated local CR spectra, we estimate the expected rate of explosive desorption. The efficiency of the desorption, which in principle affects all solid species independent of their binding energy, is shown to be comparable to other CR desorption mechanisms typically considered in the literature. Also, the theory allows us to estimate the maximum abundances of reactive species that may be stored in the mantles, which provides important constraints on the available astrochemical models.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

THE COMPOSITION OF INTERSTELLAR GRAINS TOWARD ζ OPHIUCHI: CONSTRAINING THE ELEMENTAL BUDGET NEAR THE DIFFUSE-DENSE CLOUD TRANSITION

We investigate the composition of interstellar grains along the line of sight toward ζ Ophiuchi, a well-studied environment near the diffuse-dense cloud transition. A spectral decomposition analysis of the solid-state absorbers is performed using archival spectroscopic observations from the Spitzer Space Telescope and Infrared Space Observatory. We find strong evidence for the presence of sub-micron-sized amorphous silicate grains, principally comprised of olivine-like composition, with no convincing evidence of H{sub 2}O ice mantles. However, tentative evidence for thick H{sub 2}O ice mantles on large (a ≈ 2.8 μm) grains is presented. Solid-state abundances of elemental Mg, Si, Fe, and O are inferred from our analysis and compared to standard reference abundances. We find that nearly all of the Mg and Si atoms along the line of sight reside in amorphous silicate grains, while a substantial fraction of the elemental Fe resides in compounds other than silicates. Moreover, we find that the total abundance of elemental O is largely inconsistent with the adopted reference abundances, indicating that as much as ∼156 ppm of interstellar O is missing along the line of sight. After taking into account additional limits on the abundance of elemental O in other O-bearing solids, we conclude that any missing reservoir of elemental O must reside on large grains that are nearly opaque to infrared radiation.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1995

The Composition of Interstellar Grains and Ices

Immense progress has been made in our understanding of the composition of interstellar dust over the past 15 years. This progress is largely due to the synergism of improved telescopic observations (especially in the infrared), laboratory studies of dust analogs, and the study of the interstellar grains found in meteorites. This paper reviews our current understanding of the composition of interstellar dust in light of results obtained by these three disciplines.

Sandford, Scott A.↗
NASA NTRS1994

Properties of a B0 I stellar wind and interstellar grains derived from Ginga observations of the binary X-ray pulsar 4U 1538-52

From measurements of the X-ray eclipse phenomena of the binary X-ray pulsar 4U 1538-52, we derive properties of the stellar wind of its B0 I companion, QV Nor, and a constraint on models of interstellar grains. Estimates of the wind density as a function of the distance from the center of QV Nor are obtained from an analysis of the variation of X-ray attenuation during an eclipse egress. A Monte Carlo computation of the absorption and scattering of X-rays in the X-ray-ionized wind accounts for approximately two-thirds of the spectrum of X-rays with energies above 4.5 keV observed during the eclipse. Our upper limit on R(sub XV) is 0.06/mag, which implies that the X-ray scattering efficiency of interstellar dust is less than expected for solid grains with a size distribution of the form n(sub g)(a) approximately a(exp -3.5) in the range from 0.005 to 0.25 microns and composed of silicate (R(sub XV) = 0.22/mag) or a silicate-graphite mixture (R(sub XV) = 0.11/mag) as derived from the calculations of Martin & Rouleau (1991). This lends support to the idea (Mathis & Whiffen 1989) that interstellar grains are 'fluffy' aggregates with an average bulk density less than that of their constitutent particles. Such aggregates would have a smaller ratio of X-ray scattering efficiency to optical extinction efficiency compared with solid grains of the same material.

Clark, George W.↗