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At least 19 records
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NASA NTRS1980

Dust-to-gas ratio in the outer regions of spiral galaxies

Some spiral galaxies are surrounded by extended clouds of neutral hydrogen; if dust is associated with this gas, the dust should appear as a faint extended reflection nebula illuminated by the light from the entire galaxy. It is suggested that observations of the surface brightness of the outer regions of M83 and M101 could be worthwhile, since these two galaxies are bright and have extended surrounding gas clouds. Such studies would be of interest with regard to the possible presence of faint extended halos of stars around galaxies, the extent of the metallicity gradients in galaxies, and the value of the dust-to-gas ratio in quasar absorption-line systems.

Jura, M.↗
NASA NTRS1981

Observing chemical abundances in comets

The atomic resonance lines of the major elements were observed in the atmospheres of a few comets, by using vacuum ultraviolet spectrographs on board rockets or orbiting observatories. Dust-to-gas ratios were also deduced for two comets through a Finson-Probstein's analysis of their dust-tail isophotes. The geometric albedo of the dust for the phase angle alpha of the observations is not accurately known but, the dust-to-gas ratio is not overly sensitive to the actual value of this albedo. Infrared observations of the dust head of some comets show that the bulk of cometary dust must be silicates, although a minor component (5-10 percent) of carbon compounds is rather likely, because of poor dielectric properties of the grains. This interpretation is confirmed by the fact that interplanetary dust probably of cometary origin, that was collected in the stratosphere by NASA-U2 Spacecraft, is chondritic in nature. Metal abundances in the head of a sungrazing comet support the chondritic hypothesis.

Delsemme, A. H.↗
NASA NTRS1990

Anomalous molecular abundances and the depletion of NH2 in Comet P/Giacobini-Zinner

Observations of the CN, C2, C3, and NH2 molecules in spectra of the Comet P/Giacobini-Zinner (P/G-Z) obtained during the 1985 apparition are presented. Haser-model production rates are derived, and it is found that Q(C2)/Q(CN) and Q(C3)/Q(CN) are low compared to those in normal comets. This indicates that C2 and C3 were underabundance in P/G-Z, in agreement with previous results. Q(NH2)/Q(CN) is determined for five comets for the first time to determine a 'normal' value for this ratio. The ratio for P/G-Z is low compared to the value for the ensemble, suggesting that NH2 was underabundant. For the five-comet sample, Q(C3)/Q(CN) exhibits a strong dependence with heliocentric distance, in contrast to Q(C2)/Q(CN) and Q(C3)/Q(CN). The dust-to-gas ratio is characterized by calculating the product of the dust-grain albedo, the filling factor, and the projected radius of the aperture and by computing the dust-to-gas mass ratio.

Beaver, John E.↗
NASA NTRS1974

Are quasars dusty

Two separate techniques have been applied to estimate the amount of dust in the line of sight toward a number of quasars. No evidence for dust was found. Estimates of the dust-to-gas ratio in the emission-line region of quasars suggest that it is much less than the value in our Galaxy. These results provide strong evidence against dust theories of the infrared emission from quasars. The possible relationship between quasars and galaxies is discussed. Finally, a new upper limit on the dust content of the intergalactic medium is obtained.

Mckee, C. F.↗
NASA NTRS1977

The pristine nature of comets

Abundance considerations suggest that comets are likely to be the most pristine minor bodies in the solar system. In proportion to solar abundances, the present scanty data suggest that cometary oxygen is not depleted, whereas carbon is by a factor of 4 and hydrogen, by a factor of 2000. This implies that comets are less depleted in H, C, N, O than CI chondrites, namely 10:1 in hydrogen, 4:1 in carbon and 3:1 in oxygen. These results have been obtained by using dust-to-gas ratios in comets to measure the relative abundance of silicon and metals to volatile material, and the spectra of atomic lines, mainly from the vacuum ultraviolet, to determine the H/O and C/O ratios of the mixture of volatile molecules.

Delsemme, A. H.↗
NASA NTRS1979

Observed properties of interstellar dust

Recent information on the observed properties of interstellar dust is reviewed, with an attempt made to clarify some of the observational uncertainties associated with obtaining dust parameters. Attention is given to interstellar extinction, the interstellar dust distribution, the dust-to-gas ratio, and light scattering by dust grains. Interstellar polarization is also examined, along with heavy-element depletion in the interstellar medium, thermal emission from interstellar dust grains, diffuse interstellar features, and the composition and origin of interstellar grains.

Savage, B. D.↗
NASA NTRS1982

Models for far-infrared emission from normal galaxies

It is shown that the presence of grains distributed throughout either disk or spherical galaxies can lead to detectable far-infrared sources. In particular, an elliptical such as NGC 4278 with an appreciable amount of neutral gas should be easily detectable with the Infrared Astronomical Satellite (IRAS) if it has a dust-to-gas ratio similar to the value in the solar neighborhood. A disk should be detectable at 100 Mpc with IRAS. Also, disks may emit most of their energy longward of 100 microns; this could be important for estimating the mass-to-light ratios of these galaxies.

Jura, M.↗
NASA NTRS1982

Ultraviolet images of M101 - Observations of dust and inferences on the metallicity

Since extreme Population I dominates three rocketborne telescope UV images of the Sc I galaxy NGC 5457, the delineation of the spiral structure is sharper than that of ground photographs. The UV pictures, in which the H II regions appear as reflection nebulae and illumine the nearby dust, reveal faint arms which, although correlated with the 21 cm H I radiograph, are only marginally detectable on deep visual photographs. The surface brightness of the arms is explainable in terms of UV light scattering off dust from hot stars and bright H II regions, in which case the dust would have extinction properties and a dust-to-gas ratio similar to that in the solar neighborhood and would in addition suggest that the NGC 5457 outer arm metal content is similar to that of the galaxy.

Stecher, T. P.↗
NASA NTRS1983

Numerical models for cometary dust environments

The development of a numerical model for the dust environment near a comet is described. The major components of the model are a simple treatment of the acceleration of the dust by the gas outflow from an isothermal icy nucleus, a detailed treatment of the dust particle trajectories controlled by solar gravity and radiation pressure, including the formation of the dust envelope and tail, and a simple treatment of the radiation from the dust in the visible and thermal infrared. The results of recent comet observations are used to estimate parameter values for the model, including dust-to-gas ratios by light and by mass, the distribution of dust and gas emission on the nuclear surface, the distributions of dust particle size, density, mass, and radiative properties, and other quantities. Gas and dust production rates as functions of time are derived from observed light curves using the simple photometric theory. Sample results are presented for the dust concentration, flux and fluence along the trajectory of a spacecraft which flies by Comet P/Halley in 1986.

Divine, N.↗
NASA NTRS1984

A far-infrared survey of the galactic center

The region within + or - 1.5 deg of the galactic center has been surveyed at a resolution of 1 arcmin between 40 and 250 microns. Forty-seven FIR sources were detected in the field, 38 of them spatially coincident with prominent radio-emitting regions. The 27 sources associated with H II regions have luminosities ranging from 500,000 to ten million solar luminosities, average infrared excesses of 16, and average dust-to-gas ratios of 0.002. They are located in a narrow band centered on Sgr A. Each FIR source appears to be powered by small numbers of early type O stars. The initial luminosity functions for these sources is best fitted by an exponential form with an e-folding luminosity of 2.3 million solar luminosities. The extended thermal emission of the region is consistent with a population of 30,000-50,000 ZAMS stars embedded in an ISM with a Lyman-continuum optical depth of 1-1.5.

Odenwald, S. F.↗
NASA NTRS1985

Far-infrared and submillimeter photometry of evolved stars

Broad-band photometric observations at far infrared and submillimeter wavelengths (lambda is greater than 30 microns and less than 1) provide a unique probe of circumstellar shells in evolved objects and of the mass loss processes which produce them. The problems which these observations explore include: dust properties and dust-to-gas ratios in the outflowing material, the mass and structure of the circumstellar shell, and the pumping mechanism for maser emission. This contribution reviews the early work in this still largely unexploited field and reports results in each of the areas listed above. The dramatic advances forthcoming in this area from the IRAS satellite are previewed along with further progress which will come from the next generation of infrared and submillimeter telescopes.

Werner, M. W.↗
NASA NTRS1987

Conditions for the formation of massive stars

Upper limits on the masses of stars that can form are reexamined and models for the inflow of matter through cocoons around stars of 60, 100, and 200 solar masses are calculated. Consideration is given to the general conditions that must hold at the inner and outer boundaries of a protostellar cocoon; limits on the dust-to-gas ratios and mass inflow rates that will permit inflow onto very massive stars are determined. It is found that inflow can occur if intermediate-sized grains (0.05-0.25 micron) are missing from the initial gas/dust mixture. The existence of massive stars in certain locations in galaxies indicates that preconditioning of the interstellar medium by shocks or turbulence is necessary for massive star formation.

Wolfire, Mark G.↗
NASA NTRS1988

Mass loss from S stars

The mass-loss process in S stars is studied using 65 S stars from the listing of Wing and Yorka (1977). The role of pulsations in the mass-loss process is examined. It is detected that stars with larger mass-loss rates have a greater amplitude of pulsations. The dust-to-gas ratio for the S stars is estimated as 0.002 and the average mass-loss rate is about 6 x 10 to the -8th solar masses/yr. Some of the properties of the S stars, such as scale height, surface density, and lifetime, are measured. It is determined that scale height is 200 pc; the total duration of the S star phase is greater than or equal to 30,000 yr; and the stars inject 3 x 10 to the -6th solar masses/sq kpc yr into the interstellar medium.

Jura, M.↗
NASA NTRS1988

An all-sky search for molecular cirrus clouds

An all-sky map of IR-excess gas has been produced by subtracting the H I component from the IR brightness. The large IR-excess residuals that are spatially connected are used to define IR-excess clouds. Also, nearby molecular clouds at high Galactic latitude and clouds with local heating sources or a larger dust-to-gas ratio than average are identified. The map and a catalog of 516 significant clouds are presented and statistically analyzed.

Desert, F. X.↗
NASA NTRS1989

Element abundances at high redshift

Abundances of Si(+), S(+), Cr(+), Mn(+), Fe(_), and Zn(+) are considered for two absorption-line systems in the spectrum of the QSO PKS 0528 - 250. Zinc and sulfur are underabundant, relative to H, by a factor of 10 compared to their solar and Galactic interstellar abundances. The silicon-, chromium-, iron-, and nickel-to-hydrogen ratios are less than the solar values and comparable to the local interstellar ratios. A straightforward interpretation is that nucleosynthesis in these high-redshift systems has led to only about one-tenth as much heavy production as in the gas clouds around the sun, and that the amount of the observed underabundances attributable to grain depletion is small. The dust-to-gas ratio in these clouds is less than 8 percent of the Galactic value.

Meyer, David M.↗
NASA NTRS1990

The probable dust formation episode around Rho Cassiopeiae

By comparing IRAS photometry with previous 10-micron data, it appears that dust formed in the circumstellar envelope around the supergiant G star, Rho Cas, sometime between 1973 and 1983. This dust formation may have been the consequence of the gas outflowing and cooling from the intense mass outburst of 1946. In 1983, the dust was detected at a temperature between 600 and 800 K at a distance between 10 to the 15th cm and 4 x 10 to the 15th cm from the star. While the dust-to-gas ratio in the circumstellar envelope may be as low as 0.00001 and thus much lower than in other mass-losing stars, the luminosity of Rho Cas is sufficiently large that radiation pressure on this dust might be dynamically important in helping to drive mass loss.

Jura, M.↗
NASA NTRS1990

A model for the infrared emission from an OB star cluster environment

Researchers developed an interactive radiative transfer code that predicts the infrared emission from an HII region containing diffuse ionized and atomic gas and dense molecular clouds. This model complements the investigation of the redistribution of OB star luminosity in the interstellar medium (Leisawitz and Hauser 1988, Ap. J., 332, 954). The model can be used as a diagnostic tool to probe the radiation field and matter density in an HII region, place constraints on the proximity and orientation of an illuminated molecular cloud with respect to the ionizing stars, test for the presence of small, transiently heated dust grains, and determine whether the dust-to-gas ratio is normal. Predictions of the model agree qualitatively and quantitatively with observations of blister-type HII regions ionized by well-studied OB clusters in which the distribution of dense neutral material is known. This is illustrated by a model for Infrared Astronomy Satellite (IRAS) observations of the region around NGC 7380 (S142). Researchers plan to use the model in a survey of regions of massive star formation in the outer Galaxy to study OB stars embedded to various degrees in their parental molecular clouds.

Leisawitz, D.↗
NASA NTRS1989

Dust in regions of massive star formation

It is suggested that protostars increase mass by accreting the surrounding gas and dust. Grains are destroyed as they near the central protostar creating a dust shell or cocoon. Radiation pressure acting on the grains can halt the inflow of material thereby limiting the amount of mass accumulated by the protostar. General constraints were considered on the initial dust-to-gas ratio and mass accretion rates that permit inflow. These results were constrained further by constructing a numerical model, including radiative deceleration on grains and grain destruction processes. Also the constraints on dust properties were investigated which allow the formation of massive stars. The obtained results seem to suggest that massive star formation requires rather extreme preconditioning of the grain and gas environment.

Wolfire, Mark G.↗