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At least 847 records · Page 47

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NASA NTRS1979

O VI absorption in interstellar cloud surfaces

The velocity profiles of O VI absorption lines of 24 stars, observed in early Copernicus surveys, have been compared with the line profiles of Si III (1206.51 A) and N II (1083.99 A). The velocity structures of the O VI lines appear to be correlated with those of the material in the lower ionization stages. It is argued that the O VI absorption arises in the coronal gas of the conductive interface between hot gas, responsible for extended, soft X-ray emission, and cooler interstellar clouds. The velocity broadening of both sets of lines is attributed to motions of the cloud surfaces induced by pressure fluctuations in the interstellar medium.

Cowie, L. L.↗

NASA NTRS1980

Ion chemistry in interstellar space

The temperature dependence of the bimolecular reactions of Cl(+), HCl(+), CH(+), CH2(+), N(+), NH(+), and NH2(+) reacting with H2 have been investigated. For Cl(+) and HCl(+) rate constants have been determined over the temperature range of 150 to 400 K. Preliminary data have been obtained for the other systems. The Cl(+)/H2 system shows a weak, but significant positive temperature dependence that could be important in interstellar modeling studies. The HCl(+)/H2 system shows a substantial negative temperature dependence with the rate constant approaching the collision rate at low temperatures. The association reaction CH3(+) + HCN yields CH3 HCN(+) has been theoretically modeled using statistical phase space theory. Both radiative and collisional stabilization have been included. The results are compared with experiment with good agreement obtained over wide variations in T and p. This system is potentially important in the mechanism of large molecule synthesis in interstellar space.

Bowers, M. T.↗

NASA NTRS1979

Interstellar synthesis of the cyanopolyynes and related molecules

The cyanopolyynes HC2CN, HC4CN, HC6CN, and HC8CN, and the molecules CH3CHCN and CH3CH2CN, have recently been detected in the interstellar medium. It is shown that the observed abundances of these molecules can be obtained by gas-phase formation pathways if the reaction of H2CN(+) with C2H2 is rapid at low interstellar temperatures. The molecules CH2CHCN and C3N may be formed also by the reactions of H2CN(+) with C2H2, and CH3CH2CN may be formed by reaction of H2CN(+) with C2H4.

Mitchell, G. F.↗

NASA NTRS1979

Copernicus observations of interstellar matter toward the Orion OB1 association. I - Epsilon and Pi-5 Orionis

Copernicus UV data on interstellar lines toward Epsilon Ori and Pi-5 Ori are analyzed to study abundances and physical conditions in both low- and intermediate-velocity components. Clouds at -8 and +5 km/s (LSR) toward Epsilon Ori show typical depletions of Fe, Ti, Mg, and Si in dense (H number density about 100 per cu cm) gas. Low-column-density intermediate-velocity clouds toward both stars, with low densities (hydrogen number density less than 1 per cu cm) and near-cosmic Si abundances, are consistent with a widespread pattern of high-velocity gas over a 15-deg area surrounding the Orion region. Such activity may be attributed to the repeated action of supernovae in a patchy low-density region of interstellar gas.

Shull, J. M.↗

NASA NTRS1979

Interstellar absorption of the extreme ultraviolet flux from two hot white dwarfs

Photometric upper limits on the 300 A flux from the hot white dwarfs Feige 24 and G191-B2B are presented. The limits, which were obtained with a rocket-borne extreme ultraviolet imaging telescope, are interpreted as lower limits on the density of the intervening interstellar matter. The limits are used to investigate the state of interstellar gas within 100 pc. A local clumpiness factor, which is of value in planning future extreme ultraviolet observations, is derived.

Cash, W.↗

NASA NTRS1979

Interstellar lines in the spectra of four stars

Observations of optical interstellar absorption lines arising from Na I, K I, and/or Ca II are reported in the spectra of HD 72127, Iota(1) Sco, 102 Her, and 6 Cas. Line components showing strikingly large Ca II/Na I ratios are found toward HD 72127 and are verified for 102 Her. The absorption toward Iota(1) Sco and 6 Cas illustrates features of the local galactic distribution of interstellar gas.

Hobbs, L. M.↗

NASA NTRS1979

Studies of interstellar absorption lines with IUE

The paper deals with a survey of interstellar absorption lines in the wavelength region from 1150 to 3200 A toward 25 stars. The spectra of 10 of these stars show very strong narrow absorption lines of Si IV and C IV. Lines of N V appear in eight directions. The N V lines can arise in the same gas that is responsible for the ubiquitous interstellar lines of O VI, observed previously with a Copernicus spectrometer, but the lines of Si IV and C IV must be formed in a cooler gas. The measured lines of Si IV and C IV can be produced by normal H II regions photoionized by the observed stars.

Black, J. H.↗

NASA NTRS1979

New observations of interstellar abundances and depletions of boron, vanadium, chromium, and cobalt

New observations of interstellar lines of boron, vanadium, chromium, and cobalt in the spectra of Zeta Oph and Xi Per have been obtained with the Copernicus satellite. Chromium has been detected for the first time toward a reddened star, and cobalt has been seen for the first time in any interstellar line of sight. New limits have been obtained for boron and vanadium. These new data, along with limits on scandium and other species from the literature, have been compared with models for the depletion process. No fully conclusive test of depletion models is yet possible, but the new data on boron appear to favor the hypothesis that the depletions are dominated by accretion of gas-phase particles onto grains, rather than being due to grain condensation under pressure equilibrium. The impact of these new data on the study of grain surface properties is described.

Snow, T. P., Jr.↗

NASA NTRS1980

A prelude to interstellar flight

A 20 to 50 year interstellar precursor mission extending 400 to 1000 AU from the solar system is outlined as a means of bringing out and solving engineering problems inherent in a star mission, and of studying the heliopause, the interstellar medium, and cosmic rays outside the heliosphere. Solar or laser sailing combined with a 500 kWe nuclear-electric propulsion system using fission would achieve a heliocentric excess velocity of 100km/s for the 32,000 kg spacecraft having a Shuttle derivative as a launch vehicle, and containing a Pluto flyby or separate orbiter powered by radioiosotope thermoelectric generators. X-band transmission using 40 w of power, a 15 m diameter spacecraft antenna and a 100 m receiving antenna on earth and providing 100 b/s is proposed, but a rate of 2 to 4 kb/s via 500 to 1000 w of power using the K-band and a 300 m diameter receiving antenna located on an Orbiting Deep Space Relay Station is also considered.

Jaffe, L. D.↗

NASA NTRS1979

Ultraviolet observations of interstellar molecules and grains from Spacelab

The paper presents observational results obtained on interstellar grains and molecules and suggests promising areas for Spacelab research. Regarding grains, useful data can be expected on the shape of the ultraviolet extinction curve for new interstellar regions, on the nature of UV extinction at short wavelengths, the presence of structure in the UV extinction curve comparable to the visible-wavelength diffuse bands, and the polarization properties of grains in UV wavelengths. The advances in observations of molecules will include the ability to prove more heavily-obscured regions, coverage of wavelength regions inadequately studied with previous instruments, and the capability of observing in optical absorption species detected in the same line of sight in radio emission.

Snow, T. P., Jr.↗

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 NTRS1980

A laboratory study of magnesium-tetrabenz-porphyrin - Lack of agreement with diffuse interstellar bands

The visible and infrared spectra and thermal behavior of the bis-pyridal-magnesium-tetrabenz-porphyrin molecule proposed as the carrier of the diffuse interstellar bands were measured. Of the six band coincidences reported by Johnson (1977), only one, 4430 A, occurs in these experiments. This coincidence requires a special environment, not likely to occur in interstellar space but the infrared spectrum does not support Johnson's vibrational scheme. These spectroscopic and thermal measurements contradict the hypothesis that this molecule causes the diffuse bands.

Donn, B.↗

NASA NTRS1980

Molecular diagnostics of interstellar shocks

The chemistry of molecules in shocked regions of the interstellar gas is considered and calculations are carried out for a region subjected to a shock at a velocity of 8 km/sec. Substantial enhancements are predicted in the concentrations of the molecules H2S, SO, and SiO compared to those anticipated in cold interstellar clouds.

Hartquist, T. W.↗

NASA NTRS1980

Visible and ultraviolet /800-130 nm/ extinction of vapor-condensed silicate, carbon, and silicon carbide smokes and the interstellar extinction curve

The extinction curves from 800 to 130 nm (1.25-7.7/micron) of amorphous silicate smokes nominally of olivine and pyroxene composition, carbon smokes, and crystalline SiC smokes are presented. The SiC smoke occurred in the low-temperature (beta) cubic structural form. The SiC smoke showed an absorption edge which occurred at significantly longer wavelengths than the calculated extinction profile of the hexagonal SiC form previously used to calculate the interstellar extinction profile. Neither SiC nor amorphous silicates show an extinction band similar to the observed 6.6/micron astronomical extinction band. The infrared absorption peaks for the silicate and SiC samples near 10 microns and 11-13 microns, respectively, were also measured. The ultraviolet to infrared extinction ratio for the amorphous silicate samples is similar to the observed astronomical extinction ratio. The measured extinction ratios for SiC smokes are significantly below the interstellar extinction ratio. The extinction peak of the carbon smokes occurred at 4.0 and 4.25/micron, for samples of mean radii 13 and 6 nm, respectively. The extinction profile is distinctly different from that predicted for graphite grains of the same size, and is similar to that predicted for glassy carbon grains.

Stephens, J. R.↗

NASA NTRS1979

Interstellar C2 molecules toward Zeta Persei

A search for absorption by interstellar C2 molecules, conducted at the (2-0) Phillips band near 8760 A, is reported for three stars. No interstellar absorption is seen in the spectra of 20 Tau and Epsilon Aur, at upper limits of approximately 5 mA and 1 mA, respectively. Toward Zeta Per, however, the R(0), Q(2), and Q(4) lines are detected, and a useful upper limit on the Q(6) line is obtained. The resulting column density, 1.4 x 10 to the 13th to 2.0 x 10 to the 13th per sq cm, and the excitation temperature, T = (78 plus or minus 25) K, show acceptable agreement with an existing theoretical model of the absorbing cloud.

Hobbs, L. M.↗

NASA NTRS1980

A search for interstellar scandium

The resonance line of Sc II at 3642.785 A has been sought in the interstellar spectrum of Zeta Oph, resulting in an upper limit N(Sc II) not greater than 8.6 billion/sq cm and a depletion of this element at least by a factor of 200, if Sc III contributes a negligible column density. The present upper limit may be somewhat below the level expected under the grain-formation model for interstellar depletions, and is consistent with the abundance expected if the depletions are due to gas-grain collisions. The new limit on this element helps to support the hypothesis that species within a specific range of ionization potential are preferentially depleted.

Snow, T. P., Jr.↗

NASA NTRS1980

Protostellar formation in rotating interstellar clouds. III - Nonaxisymmetric collapse

The paper discusses a full three spatial-dimension gravitational hydrodynamic code used to follow the collapse of isothermal rotating clouds subjected to various nonaxially symmetric perturbations (NAP). An initially axially symmetric cloud collapsed to form a ring which then fragmented into a binary protostellar system; a low thermal energy cloud with a large bar-shaped NAP collapsed and fragmented into a binary, and higher thermal energy clouds damp out such NAPs while higher rotational energy clouds produce binaries with wider separations. The three-dimensional calculations indicate that isothermal interstellar clouds may fragment into protostellar objects while still in the isothermal regime. Interstellar clouds and their fragments may pass through collapse phases with fragmentation and reduction of spin angular momentum terminating in the formation of pre-main-sequence stars with the observed pre-main-sequence rotation rates.

Boss, A. P.↗

NASA NTRS1980

Detection of the 610 micron /492 GHz/ line of interstellar atomic carbon

The ground-state transition of neutral atomic carbon, 3P1-3P0, has been detected in the interstellar medium at the frequency of 492.162 GHz determined in the laboratory by Saykally and Evenson (1980). The observations were made from the NASA Kuiper Airborne Observatory using an InSb heterodyne bolometer receiver. The line was detected as strong emission from eight molecular clouds and apparently provides a widely useful probe of the interstellar medium.

Phillips, T. G.↗