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NASA NTRS1984

The dust around R Coronae Borealis type stars

Measurements taken by the International Ultraviolet Explorer spacecraft of the stars RY Sgr and R CrB have been analyzed using Mie theory. The extinction data, which show a 2400-2500 A peak, are consistent with a distribution of 5-60 nm glassy or amorphous carbon particles obscuring the stellar flux. The data are also fairly consistent with a cloud ejection model. Since the extinction data lack the commonly observed peak at 2170 A, it is proposed that this difference is due to the conditions present when the dust condenses. Interstellar carbon grains appear to originate in normal carbon stars which are carbon and hydrogen rich. In contrast, the grains around R CrB type stars seem to condense from a carbon-rich and hydrogen-poor vapor.

Hecht, J. H.↗
NASA NTRS1994

Voyager observations of dust scattering near the Coalsack nebula

We present the results of four observations of the sky in the direction of the Coalsack nebula. These observations were made using the ultraviolet spectrometers aboard the two Voyager spacecraft in the spectral range between 912 and 1600 A. Intense diffuse emission with a spectrum characteristic of an early B star was observed in all four targets, which we interpret as starlight forward scattered by interstellar dust in the foreground of the main mass of the Coalsack. While more detailed modeling is necessary to derive values for the optical constants of the dust grains, our data indicate that there is no decrease in the albedo toward shorter wavelengths, arguing that the far-ultraviolet rise in the interstellar extinction curve is due to an increasing number density of small particles rather than to a new population of low albedo grains.

Murthy, Jayant↗
NASA NTRS1972

Spectroscopic and photometric observations of M supergiants in Carina.

Spectroscopic study of 30 Southern-Hemisphere M supergiants mostly in Carina in the blue and near-infrared, and photometrical study of these stars from 0.4 to 18 microns. The uncertainties in the determinations of interstellar extinction are discussed, and the spatial distribution of the M supergiants in the Carina arm is shown. The presence of the 11-micron excess attributed to silicate dust is a common feature. Stars of the same spectral type and luminosity class are remarkably homogeneous in their long-wave behavior. The silicate feature becomes more prominent in the more luminous stars and in stars of later spectral type. Four composite systems show little long-wave excess. The two VV Cephei objects have excesses probably produced by gas emission, and the other two have little or no excess - supporting the suggestion that the presence of the early star prohibits the formation of a dust envelope. Three stars - VY CMa, VX Sgr, and HD 9767 - appear to be extreme examples of stars with large excesses over the entire long-wave region. It is suggested that these objects are surrounded by large amounts of particulate material over a great range of distances from the stars.

Humphreys, R. M.↗
NASA NTRS1983

Observation of ice mantles toward HD 29647

The water ice absorption band at 3.1 microns has been observed in HD 29647, which is a late B type star with an anomalous ultraviolet extinction curve. The interpretation is that ice mantles are present on the dust grains in the intervening cloud and that these mantles are responsible for suppressing the 2200 A interstellar absorption feature. This is the first observation of both the 2200 A region and the 3.1 micron interstellar absorption feature toward the same star. A comparison of recent UV laboratory data of water ice with the UV spectrum of HD 29647 further supports the presence of water ice mantles in the same column density as that observed in the infrared. The diffuse interstellar features have been reported to be weakened in this star, consistent with their origin being on grain core surfaces, rather than in mantles or gaseous molecules.

Goebel, J. H.↗
NASA NTRS2006

Grain Growth and Silicates in Dense Clouds

Interstellar silicates are likely to be a part of all grains responsible for visual extinction (Av) in the diffuse interstellar medium (ISM) and dense clouds. A correlation between Av and the depth of the 9.7 micron silicate feature (measured as optical depth, tau(9.7)) is expected if the dust species are well 'mixed. In the di&se ISM, such a correlation is observed for lines of sight in the solar neighborhood. A previous study of the silicate absorption feature in the Taurus dark cloud showed a tendency for the correlation to break down at high Av (Whittet et al. 1988, MNRAS, 233,321), but the scatter was large. We have acquired Spitzer Infrared Spectrograph data of several lines of sight in the IC 5 146, Barnard 68, Chameleon I and Serpens dense clouds. Our data set spans an Av range between 2 and 35 magnitudes. All lines of sight show the 9.7 micron silicate feature. The Serpens data appear to follow the diffuse ISM correlation line whereas the data for the other clouds show a non-linear correlation between the depth of the silicate feature relative to Av, much like the trend observed in the Taurus data. In fact, it appears that for visual extinctions greater than about 10 mag, tau(9.7) begins to level off. This decrease in the growth of the depth of the 9.7 micron feature with increasing Av could indicate the effects of grain growth in dense clouds. In this poster, we explore the possibility that grain growth causes an increase in opacity (Av) without causing a corresponding increase in tau(9.7).

Pendeleton, Yvonne J.↗
NASA NTRS1975

Ultraviolet photometry from the Orbiting Astronomical Observatory. XX - The ultraviolet extinction bump

Ultraviolet extinction bumps are investigated in the interstellar extinction curves between 1800 and 3600 A for 36 stars which have (B-V) excesses ranging from 0.03 to 0.55 and are mostly confined to the brighter OB associations distributed along the galactic plane. Each extinction curve is found to have a broad bump which peaks near 2175 A and whose position and profile appear to be constant among all the stars. It is shown that the bump is probably interstellar in origin and that the constancy of its position and shape places such severe restrictions on grain geometrical parameters that classical scattering theory cannot be used to explain the feature unless the dust grains in widely separated regions of space and with very different physical conditions are assumed to have nearly identical size and shape distributions. Three extinction curves which extend to 1100 A are examined and found to have the same general characteristics as the others. Several extinction curves are analyzed for fine structure, but no convincing evidence is found in the present interval. Some processes are discussed which may be responsible for the bumps.

Savage, B. D.↗
NASA NTRS1994

UV extinction and IR emission in diffuse H2 regions

HII regions occupy a unique position in our understanding of the physical relationships between stars, the interstellar medium, and galactic structure. Observations show a complex interaction between a newly formed hot star and its surroundings. In particular, the ultraviolet radiation from the stars modifies the pre-existing dust, which again affects both the amount of ionizing radiation absorbed by the gas, and the infrared spectrum emitted by the heated dust. The aim of this project was to use UV and far-UV observations to gain information on the nebular dust, and to use this dust to model the far-IR emission, for a consistent picture of a few selected diffuse HII regions. Using archival data from the IUE and Voyager data banks and computed model atmospheres, we have deduced extinction curves for early-types stars. The requisite spectral resolution turned out to be a major task. We have successfully modelled these curves in terms of a multi-component, multi-size distribution of dust grains, and interpret the differences in the curves as primarily due to the presence or non-presence of intermediate size grains (0.01 to 0.04 micron). Much smaller (0.005 micron) grains must also be present. Finally, we have made calculations of the temperature fluctuations and the corresponding infra-red emission in such small grains.

Aannestad, Per A.↗
NASA NTRS1987

Infrared properties of dust grains derived from IRAS observations

The analysis of several diffuse interstellar clouds observed by the Infrared Astronomy Satellite (IRAS) is presented. The 60/100 micron flux ratios appear to be nearly constant in clouds with up to 1 sup m visual extinction at the center. Observations of a highly regular cloud in Chamaeleon show that the 12/100 micron ratio peaks at an intermediate radial distance and declines towards the center of the cloud. These observations indicate that nonequilibrium emission accounts only for the 12 and 25 micron bands; strong emission observed at the 60 micron band is probably due to equilibrium thermal radiation. The correlation of the 12 micron emission with a red excess observed for a high latitude cloud, L1780, is shown to be consistent with the assumption that both features are due to fluorescence by the same molecular species.

Chlewicki, G.↗
NASA NTRS1995

The galactic distribution of aliphatic hydrocarbons in the diffuse interstellar medium

The infrared absorption feature near 2950(exp -1) (3.4 micron), characteristic of dust in the diffuse interstellar medium (ISM), is attributed to C-H stretching vibrations of aliphatic hydrocarbons. We show here that the strength of the band does not scale linearly with visual extinction everywhere, but instead increases more rapidly for objects near the center of the Galaxy, a behavior that parallels that of the Si-O stretching band due to silicate materials in the diffuse ISM. This implies that the grains responsible for the diffuse medium aliphatic C-H and silicate Si-O stretching bands are different from those responsible for much of the observed visual extinction. It also suggests that the distribution of the carbonaceous component of the diffuse ISM is not uniform throughout the Galaxy, but instead may increase in density toward the center of the Galaxy. The similar behavior of the C-H and Si-O stretching bands suggests that these two components may be coupled, perhaps in the form of silicate-core, organic-mantle grains. Several possible models of the distribution of this material are presented and it is demonstrated that the inner parts of the Galaxy has a carrier density that is 5 to 35 times higher than in the local ISM. Depending on the model used, the density of aliphatic material in the local ISM is found to be about 1 to 2 -CH3 groups m(exp -3) and about 2 to 5 -CH2- groups m(exp -3). These densities are consistent with the strengths of the 2955 and 2925 cm(exp -1) (3.4 micron) band being described by the relations A(sub nu)/tau(sub 2955 cm(exp -1)) = 270 +/- 40 and A(sub nu)/tau(sub 2925 cm(exp -1)) = 250 +/- 40 in the local diffuse ISM.

Sandford, Scott A.↗
NASA NTRS1995

The Galactic Distribution of Aliphatic Hydrocarbons in the Diffuse Interstellar Medium

The infrared absorption feature near 2950/ cm (3.4 microns), characteristic of dust in the diffuse interstellar medium (ISM), is attributed to C-H stretching vibrations of aliphatic hydrocarbons. We show here that the strength of the band does not scale linearly with visual extinction everywhere, but instead increases more rapidly for objects near the center of the Galaxy, a behavior that parallels that of the Si-O stretching band due to silicate materials in the diffuse ISM. This implies that the grains responsible for the diffuse medium aliphatic C-H and silicate Si-O stretching bands are different from those responsible for much of the observed visual extinction. It also suggests that the distribution of the carbonaceous component of the diffuse ISM is not uniform throughout the Galaxy, but instead may increase in density toward the center of the Galaxy. The similar behavior of the C-H and Si-O stretching bands suggests that these two components may be coupled, perhaps in the form of silicate-core, organic-mantle grains. Several possible models of the distribution of this material are presented and it is demonstrated that the inner parts of the Galaxy has a carrier density that is 5 to 35 times higher than in the local ISM. Depending on the model used, the density of aliphatic material in the local ISM is found to be about 1 to 2 -CH3 groups /cc and about 2 to 5 -CH2- groups/cc. These densities are consistent with the strengths of the 2955 and 2925/ cm (3.38 and 3.42 microns) subfeatures (due to -CH3 and -CH2- groups, respectively) within the overall 2950/ cm (3.4 microns) band being described by the relations A(sub upsilon)/tau(sub 2925/cm) = 270 +/- 40 and A(sub upsilon)/tau(sub 2925/cm) = 250 +/- 40 in the local diffuse ISM.

Sandford, Scott A.↗
NASA NTRS1989

Correlation properties of interstellar dust: Diffuse interstellar bands

Results are presented from a research program in which an attempt was made to establish the physical nature of the interstellar grains, and the carriers of the diffuse interstellar bands, by comparing relations between different observed properties; the properties used include the extinction in the optical and ultraviolet (including wavelength 2200 and the far-UV rise), cloud density, atomic depletions, and strengths of the diffuse bands. Observations and also data from literature were used, selecting particularly sight-lines where some observed property was found to have anomalous behavior.

Somerville, W. B.↗
NASA NTRS1974

The local complex of O and B stars. I - Distribution of stars and interstellar dust

The O-B5 stars, supergiants, young clusters, and associations within 1 kpc of the sun populate two flat systems inclined to each other by 19 to 22 deg. The historical background, statistical significance, composition, spatial arrangement of the contents, and interstellar extinction in the two belts are discussed. A more or less random distribution in space and in age characterizes the O-B5 stars of the 'galactic belt', which is aligned nearly along the Milky Way. The 'Gould belt' is inclined to the Milky Way (north in Sco-Oph and south in Orion), and exhibits a projected distribution of O-B5 stars in its mean plane that resembles a 'dragonfly', with five major features defining it. A crude 'diameter' of the system is 750 to 1000 pc, and the sun's position is eccentric, lying toward Ophiuchus. The nuclear age of the system, while not unique, may be characterized as 30 m.y. from the spectral type of the broad main-sequence turnup near B2-5. Most of the O-B2 stars and youngest stellar groups near the sun belong to the Gould belt, but both belts have approximately equal space densities of B3-B5 stars and similar average values of interstellar extinction.-

Stothers, R.↗
NASA NTRS1995

Discovery and imaging of a Galactic cirrus cloud with the far ultraviolet space telescope

We present new far-ultraviolet (1400-1800 A) data concerning a Galactic cirrus cloud G251.2+73.3 near the north Galactic pole obtained with the space-borne imaging telescope FAUST (Far Ultraviolet Space Telescope). We obtain a good correlation between the far-ultraviolet (FUV) and IRAS 100 micrometers surface brightnesses, their relation being I(sub FUV) = (128 +/- 3) I(sub 100 micrometers) - (264 +/- 9), where the I(sub FUV) flux is given in units of photon/s/sq cm/A/sr and I(sub 100 micrometers) in MJy/sr. Using uvbyH-beta photometry, we get a distance of 120 pc and a visual extinction in the center of the cloud of 0.39 mag corresponding to an extinction of 1.0 mag at 1565 A. We have performed a multiple scattering calculation for the scattered light using the Monte Carlo method. These calculations provide restrictions on the FUV scattering properties of the interstellar dust.

Haikala, Lauri K.↗
NASA NTRS1986

Measurements of X-ray scattering from interstellar grains

The results of an Einstein Observatory imaging proportional counter investigation of the halos produced by the scattering of X-rays from interstellar grains of four compact Galactic (low-latitude) and two extragalactic (high-latitude) X-ray sources are reported. It is found that the intensity of these halos correlates well with the amount of visual extinction and the distance through the Galaxy's dust layer: quantities which measure the column density of grains to a given source. From this result, and from the size and shape of the halos from the Galactic sources, a number of grain parameters are derived in the context of two alternate grain size distributions: the Mathis-Rumpl-Nordsieck (MRN) and the Oort-van de Hulst distributions, either of which is capable of producing the observed halos. Though no single grain size is capable of producing the shapes observed for these halos, a mean size of about 0.1 micron and a number density of about 10 to the -12th grains/cu cm produce the correct halo intensities. From the additional parameters determined from the size and shape of the halos, results concerning the amount of material in grains which are in general agreement with the observed depletion of the elements from the gas phase of the interstellar medium, as well as with the detailed predictions of the MRN size distribution, are found.

Mauche, C. W.↗
NASA NTRS1989

Properties of grains derived from IRAS observations of dust

The authors used the results of Infrared Astronomy Satellite (IRAS) observations of diffuse medium dust to develop a theoretical model of the infrared properties of grains. Recent models based entirely on traditional observations of extinction and polarization include only particles whose equilibrium temperatures do not exceed 20 K in the diffuse interstellar medium. These classical grains, for which the authors have adopted the multipopulation model developed by Hong and Greenberg (1980), can explain only the emission in the IRAS 100 micron band. The measurements at shorter wavelengths (12, 25 and 60 microns) require two new particle populations. Vibrational fluorescence from aromatic molecules provides the most likely explanation for the emission observed at 12 microns, with polycyclic aeromatic hydrocarbons (PAHs) containing about 10 percent of cosmic carbon. A simplified model of the emission process shows that PAH molecules can also explain most of the emission measured by IRAS at 25 microns. The authors identified the warm particles responsible for the excess 60 microns emission with small (a approx. equals 0.01 microns) iron grains. A compilation of the available data on the optical properties of iron indicates that the diffuse medium temperature of small iron particles should be close to 50 K and implies that a large, possibly dominant, fraction of cosmic iron must be locked up in metallic particles in order to match the observed 60 microns intensities. The model matches the infrared fluxes typically observed by IRAS in the diffuse medium and can also reproduce the infrared surface brightness distribution in individual clouds. In particular, the combination of iron and classical cool grains can explain the surprising observations of the 60/100 microns flux ratio in clouds, which is either constant or increases slightly towards higher opacities. The presence of metallic grains has significant implications for the physics of the interstellar medium, including catalytic H2 formation, for which iron grains could be the main site; differences in depletion patterns between iron and other refractory elements (Mg, Si); and superparamagnetic behavior of large grains with embedded iron clusters giving rise to the observed high degree of alignment by the galactic magnetic field.

Wesselius, P. R.↗
NASA NTRS1981

Far-infrared observations of globules

Observations of far-infrared emission from nine globules are presented. The intensity and uniformity of the emission confirm that the heat source is the interstellar radiation field. Spectra of B133 and B335 are presented; they are consistent with optically thin thermal emission from dust with temperature 13-16 K. The temperature and intensity of B335 are used to calculate the ratio of visual extinction to far-infrared emission frequency.

Keene, J.↗
NASA NTRS1989

On the size distribution of newly formed grains in red supergiant atmospheres

Theoretical ultraviolet extinction curves have been calculated for comparison with observed curves for circumstellar dust in M supergiants. The theoretical curves assume a silicate grain composition, because silicate grains are expected in the oxygen-rich environments that are observed. Calculations were performed with and without the inclusion of scattering into the beam, with largely similar results. A comparison of the computed curves with the observed ultraviolet extinction curve for circumstellar dust in Alpha Scorpii indicates that the size distribution of the circumstellar grains must cut off near 800 A; that is, there are few or no grains smaller than this. The conclusion is that smaller interstellar silicate grains, where they exist, must come from other sources such as grain fragmentation in shocks.

Seab, C. Gregory↗
NASA NTRS1989

Observational constraints on interstellar dust models

No single model has been able to account for all of the observed spectroscopic properties of interstellar or circumstellar dust. The reason for this is that, despite the agreement that the grains are composed of silicaceous/metal oxide and carbonaceous material, there is strong disagreement as to their exact structure and composition. This led Draine and Lee (1984) to use interstellar extinction data to define an interstellar graphitic material; new observational findings have made even that identification uncertain. But the great advantage of their approach is that they used observations at all of the wavelengths available to define the material. Here, the authors attempt a variation of that approach. They examine recent UV and IR data and attempt to put constraints on the possible types of interstellar grain composition, and to connect these constraints with grain models. A summary of some of the important constraints imposed by the observations is given.

Hecht, James H.↗