Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “Interstellar”

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 1,621 records · Page 90
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

Interstellar potassium and argon

Isotopic anomalies are calculated in K and Ar that will be expected to be carried in interstellar grains. Special attention is given to the supernova condensates that precipitate during the expansion of the explosive oxygen-burning zone of the interior, because that is where K-39, K-41 and Ar-36, Ar-38 find their natural origins. Expectations are given for K-Ar ages in excess of the solar age in accumulations of presolar grains, and for K anomalies in Ca-rich mineral within Allende inclusions, because experiments searching for such effects are currently underway. Attention is also given to predictions of excess Ar-36 owing to extinct Cl-36 within presolar grains, and to descriptions of new correlations that can confirm its existence.

Clayton, D. D.↗
NASA NTRS1977

Comments on the origins of the diffuse interstellar bands

Most work in recent years on the unidentified diffuse interstellar bands has been concentrated on the hypothesis that they are produced by some process in solid grains. We review that hypothesis and the arguments which support it, and find that diffuse band formation by certain classes of molecules should not yet be ruled out. Since several relevant observations have been made in the last four years, the observational data on the bands are reviewed. It would appear that these data are as consistent with molecular origins for the bands as with grain origins. Types of molecular transitions which give rise to 'diffuse' bands are described. These transitions can occur in relatively simple molecules composed of cosmically abundant elements and can produce broad, unresolvable profiles without destroying the molecule. New observations are suggested which could produce decisive evidence with which to determine whether the diffuse bands originate in grains, molecules, or both.

Smith, W. H.↗
NASA NTRS1977

Deuterium and hydrogen in the local interstellar medium

Densities of neutral hydrogen and deuterium are found from observation with the Copernicus satellite of the Ly-alpha line toward two nearby stars. The hydrogen density is 0.03 + or - 0.01 per cu cm toward Alpha Aur (Capella) and 0.20 + or - 0.05 per cu cm in the direction of Alpha Cen A, values indicating that the nearby (less than 14 pc) interstellar medium is inhomogeneous and can be of low density in certain directions. The ratios of deuterium to hydrogen - 3.9 (+5.7, -1.7) by 10 to the -5th power and 0.24 (+0.12, -0.07) by 10 to the -5th power for Alpha Aur and Alpha Cen A, respectively - suggest that variations in the deuterium abundance may exist.

Dupree, A. K.↗
NASA NTRS1977

Radio detection of interstellar N2D/+/

Line emission from the J = 1-0 transition of interstellar N2D(+) has been detected toward the cool dust cloud L134 N. The N2H(+)/N2D(+) total-projected-density ratio in this cloud is estimated to be about 2.22. Upper limits are obtained for the fractional ionization as well as the CO and N2 abundances in L134 N. The results are shown to support the previously discovered enhancement of the abundance of deuterated molecules in cool dust clouds.

Snyder, L. E.↗
NASA NTRS1977

Absorption of X-rays in the interstellar medium

The X-ray photoabsorption cross sections (per hydrogen atom) of interstellar clouds and the intercloud medium are calculated separately, taking into account abundance evolution, self-blanketing by grains, molecule formation, and ionization structure. A reference cross section describing a neutral monatomic gas of elements in solar abundances is computed for purposes of comparison, the cross section of the intercloud medium is calculated by assuming that 15% of the hydrogen is ionized, and the cross section of the clouds is computed for the cases where all hydrogen is in either molecular or atomic form. A comparison of the reference cross section with the others shows that a cross section is increased relative to the reference if abundances are enhanced or if molecular hydrogen exists and is decreased if atoms exist in grains or if hydrogen is ionized. It is noted that the total cross section will be a linear combination of the cross sections of the clouds and the intercloud medium and can reflect the characteristics of a particular line of sight.

Ride, S. K.↗
NASA NTRS1977

The interstellar medium in the direction of the Crab Nebula - Reconciling soft X-ray and radio observations

The total soft X-ray photoabsorption cross section of the interstellar medium (ISM) in the direction of the Crab Nebula is computed on the basis of a two-phase model of the ISM. This cross section is used to reanalyze Copernicus data on the X-ray spectrum of the Crab between 0.7 and 1.5 keV. The total hydrogen column density along the line of sight to that nebula is found to be approximately 2.6 by 10 to the 21st power H atoms/sq cm. This result is evaluated in light of the two-phase model of the ISM, and the predictions based on the X-ray data are compared with results of radio and UV observations. A discrepancy between the radio and X-ray measurements of the hydrogen column density is resolved by noting that 21-cm absorption measurements sample only the neutral hydrogen in clouds while X-ray measurements are sensitive to all forms of hydrogen in both cloud and intercloud regions. It is suggested that roughly 50% of the hydrogen in the direction of the Crab Nebula is in clouds and that 85% of this hydrogen is neutral and atomic.

Ride, S. K.↗
NASA NTRS1977

An upper limit for the flux of interstellar dust grains in the solar system

Data from the LEAM (Lunar Ejecta and Meteorite experiment, a micrometeorite detector at the Apollo 17 landing site) have been examined for evidence of interstellar (IS) dust grains traversing the solar system. The analysis technique considers IS grains approaching the solar system from the local solar apex. A model calculates the grains' hyperbolic orbits into the solar system and predicts the impact directions on the moon. The observations are then compared with the predicted impact directions to measure the IS dust flux. No evidence has been found (at the 97.5% confidence level) for a flux greater than about 6 hundred-thousandths per sq m/sec for particles of at least 2 by 10 to the -14th power g.

Tomandl, D. A.↗
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 NTRS1978

O VI gas - Circumstellar or interstellar

Observations in the UV spectra from the Copernicus satellite have revealed absorption lines from five-times ionized oxygen atoms. To explain this abundance, two explanations have been advanced to supplant the traditional model of steady loss from nearby stars. One, ionization from circumstellar bubbles, is said to result from rapidly rotating stars. The other, examined in more detail, suggests a mechanism whereby shock waves from explosive disturbances in nearby supernovae preferentially channel coronal-type O VI regions in the interstellar medium into areas of high temperature (above 100,000 K). An examination of column densities with increasing distribution is proposed to examine the hypotheses, although the results are as yet inconclusive.

Jenkins, E. B.↗
NASA NTRS1978

Vector space methods of photometric analysis - Applications to O stars and interstellar reddening

A multivariate vector-space formulation of photometry is developed which accounts for error propagation. An analysis of uvby and H-beta photometry of O stars is presented, with attention given to observational errors, reddening, general uvby photometry, early stars, and models of O stars. The number of observable parameters in O-star continua is investigated, the way these quantities compare with model-atmosphere predictions is considered, and an interstellar reddening law is derived. It is suggested that photospheric expansion affects the formation of the continuum in at least some O stars.

Massa, D.↗
NASA NTRS1978

An upper limit to the interstellar abundance of the HCN dimer

An unsuccessful search is described for the J = 7-6 transition of HCN ... HCN in several interstellar clouds known to be rich in HCN. An upper limit to the abundance of the dimer relative to the monomer is typically 1 percent. It is suggested that this implies a low abundance for similar weakly bound species.

Storey, J. W. V.↗
NASA NTRS1978

Optical interstellar lines toward 18 stars of low reddening. III

Photoelectric scans of the interstellar D-line of Na I and the K-line of Ca II are compared with those of H I and H sub 2. Three general conclusions are drawn from the observations: (1) normal Na I/H ratios are found in 14 of the 18 cases studied, most likely due to absorption in cool, dense clouds; (2) low Na I/H ratios are observed in the direction of Gamma-Pegasus and Mu-Scorpio, implying that absorption occurs in clouds of warm (over 100 K) gas, or through extended paths of cool gas with low ionization; and (3) the absence of Na I, H I, and H sub 2 lines toward Nu-Scorpio and Sigma-Sagittarius indicates that little neutral gas is present along these light paths.

Hobbs, L. M.↗
NASA NTRS1978

The interstellar wind - Mariner 10 measurements of hydrogen /1216 A/ and helium /584 A/ interplanetary emission

A set of unique observations of interplanetary emission at 1216 A and 584 A is presented which were made from Mariner 10 when that spacecraft was in interplanetary space on its way to Venus. The data provide simultaneous mapping of 1216-A hydrogen emission and 584-A helium emission, which allows a direct cross-correlation of these two aspects of the interstellar wind. The data are examined photometrically and compared with the principles of a simple model which provides a first-order understanding of the expected intensity variation over the celestial sphere. Hydrogen and helium sky maps are given which indicate the minima and maxima in emission intensity, a helium-focusing region in excess of 50 to 60 deg, and asymmetries in both emission profiles.

Broadfoot, A. L.↗
NASA NTRS1978

Collisional excitation of interstellar molecules - Linear molecules CO, CS, OCS, and HC3N

Rates for excitation of CO, CS, OCS, and HC3N by collisions with He atoms and H2 molecules are presented. These have been obtained from extensive theoretical calculations using methods of known reliability. In general, rates necessary for modeling interstellar molecular clouds with kinetic temperatures to 100 K have been included. Methods for fitting these rates in more compact form are discussed.

Green, S.↗
NASA NTRS1978

Extreme ultraviolet spectrum of a hot interstellar plasma

The EUV (100-1000 A) spectrum of an optically thin plasma over the temperature range 100,000-10 million K is calculated, and the effects of free-free, free-bound, and two-photon continuum radiation are considered. The resulting spectra are presented in graphical form, with a separate tabulation of intensities for the emission lines. Results in the 100-200 A region are compared with those of other workers, and the major sources of uncertainty in this type of calculation are discussed. The applicability of the results to EUV observations of the interstellar medium are also mentioned.

Stern, R.↗
NASA NTRS1978

Ionization of interstellar H2 clouds by supernovae

Two destruction mechanisms for molecular hydrogen in the H I regions of the interstellar medium were given by Stecher and Williams (1967). The first is dominant in weak radiation fields and involves photon excitation (at about 1000 A) to the first excited electronic state with a subsequent decay into the vibrational continuum of the ground state. The second destruction mechanism is dominant in the strong photon fields that exist when a supernova explosion occurs. It involves photoionization (at about 1000 A) from metastable high vibrational levels produced by photon excitation (in the same wavelength range) to an excited electronic state and subsequent decay to bound vibrational levels. It is shown that ionization of molecular hydrogen can occur at considerable distances from such supernova events while the atomic hydrogen remains neutral. The effect should be observable in the dispersion measures of new pulsars.

Stecher, T. P.↗
NASA NTRS1978

Interstellar cloud material - Contribution to planetary atmospheres

A statistical analysis of the properties of dense interstellar clouds indicates that the solar system has encountered at least a dozen clouds of sufficient density to cause planets to accumulate nonnegligible amounts of some isotopes. The effect is most pronounced for neon. This mechanism could be responsible for much of the neon in Earth's atmosphere. For Mars, the predicted amount of neon added by cloud encounters greatly exceeds the present abundance.

Butler, D. M.↗
NASA NTRS1978

A survey of interstellar H I from L-alpha absorption measurements. II

This paper presents interstellar H I column densities, mean H I space densities, total neutral hydrogen column densities, and total mean hydrogen space densities toward 100 stars surveyed in the L-alpha region with the U2 detector of the Copernicus satellite. The data for 47 stars are compared with OAO 2 results, and ratios of gas to E(B-V) color excess are computed for 30 'intercloud' stars, 45 'cloud' stars, and the combined sample of 75 stars. A definitive value of 5.8 x 10 to the 21st power atoms/sq cm per mag is obtained for the mean ratio of total neutral hydrogen to color excess, indicating that this ratio seems to be a constant, independent of location in the sky, except possibly toward stars with anomalous extinction. The anomalous ratio determined for Rho Oph is examined, and the galactic distribution of neutral hydrogen is evaluated. The following values are estimated for matter in the galactic plane within 500 pc of the sun: a total mean hydrogen space density of 1.15 atoms/cu cm, a mean H2 space density of 0.143 molecule/cu cm (possibly a lower limit), and a mean H I space density of 0.86 atom/cu cm (a possible overestimate).

Bohlin, R. C.↗