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Clark, F. O.

Publications and source records attributed to Clark, F. O..

12 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1996

Observations of Two Non-Standard Grain Components

We discuss evidence for two non-standard grain components: small conglomerates and astronomical iron. The small conglomerates are small, temperature fluctuating grains made up of loosely bound 12 micron emitters (either PAH's or dielectrics). When these grains are exposed to strong UV radiation fields they break apart into smaller pieces. For high UV radiation regimes, such as H II regions, conglomerate grains best explain the reduction of 25 micron emission relative to 12 micron. Astronomical iron was originally proposed for isolated cirrus clouds in order to account for the strength of the 60 micron emission. Within the diffuse ISM, these grains emit at a relatively high equilibrium temperature of 50 K. Our analysis of IRAS data shows the presence of a strong 60 micron emitter in the Lambda Orionis H II region and a component within the Rosette Nebula emitting primarily at 25 micron. Both of these observations can be accommodated by the presence of iron grains within the H II regions.

Shipman, R. F.↗
NASA NTRS1996

IRAS Colors of the Pleiades

We present large scale images of the infrared emission of the region around the Pleiades using the ISSA data product from the IRAS mission. Residual Zodiacal background and a discontinuity in the image due to the scanning strategy of the satellite necessitated special background subtraction methods. The 60/100 color image clearly shows the heating of the ambient interstellar medium by the cluster. The 12/100 and 25/100 images peak on the cluster as expected for exposure of small dust grains to an enhanced UV radiation field; however, the 25/100 color declines to below the average interstellar value at the periphery of the cluster. Potential causes of the color deficit are discussed. A new method of identifying dense molecular material through infrared emission properties is presented. The difference between the 100 micron flux density and the 60 micron flux density scaled by the average interstellar 60/100 color ratio (Delta I(sub 100) is a sensitive diagnostic of material with embedded heating sources (Delta I(sub 100) less than 0) and cold, dense cores (Delta I(sub 100) greater than 0). The dense cores of the Taurus cloud complex as well as Lynds 1457 are clearly identified by this method, while the IR bright but diffuse Pleiades molecular cloud is virtually indistinguishable from the nearby infrared cirrus.

Carey, Sean J.↗
NASA NTRS1993

The out-of-plane distribution of zodiacal dust near the earth

Analysis of IRAS data of the thermal zodiacal emission reveals an out-of-plane dust distribution near the Earth which is well-represented by a Lorentzian function. We suggest a possible explanation for the similarity as near-Earth gravitational perturbations of dust grains. Such perturbations are expected to be chaotic and will randomize any residual band structure near the Earth.

Clark, F. O.↗
NASA NTRS1992

An IRAS study of pre-main sequence stars in B209

We have constructed high resolution IRAS maps of the B209 star forming region in the Taurus complex. By combining survey scans with pointed observations using advanced techniques specially developed to handle IRAS data in crowded fields, we have constructed images that reveal two new premain-sequence (PMS) objects. One of these, IRAS 04114+2757G, has FIR colors close to those of typical T Tauri stars and is probably associated with a faint stellar object visible on the POSS prints. The other new PMS source, IRAS 04111+2800G, is a deeply embedded object not detected at 12 microns and not optically visible. The object is projected close to the center of an ammonia core. Our detection of compact blue and red shifted (C-12)O emission peaks around the position of IRAS 04111+2800G confirms the PMS nature and youth of this object. We have examined the reasons for the exclusion of IRAS 04111+2800G and IRAS 04114+2757G from the IRAS Point Source Catalog and estimated the incompleteness of the currently known sample of embedded PMS stars in Taurus. The implications for the determination of evolutionary time scales in low mass star formation are discussed.

Prusti, T.↗
NASA NTRS1991

IRAS detection of very cold dust in the Lynds 134 cloud complex

IRAS maps at 60 and 100 microns of the complex of dark clouds containing L1345, L183, and L1780 are presented and discussed. Extended regions are apparent where the 60 micron emission is very low compared to the 100 micron emission. The 60-100 micron flux ratio has a constant value of 0.2 in the outer diffuse parts of the clouds but suddenly drops to less than 0.03 in the opaque cloud centers. A geometrical model is used to show that the drop occurs in a narrow transition layer that has not been resolved by IRAS. The observations provide direct evidence for dust temperatures colder than 15 K in dark clouds. A number of 60 micron-deficient regions are defined using the quantity Delta I(100) = I(60)/Theta, where Theta is the ratio I(60)/I(100) in the diffuse parts of the complex. Delta I(100) is highly proportional to extinction and (C-13)O column density. The relationships indicate that the dust temperature must be constant over a large fraction of the volume sampled by Delta I(100). An upper limit of 4.8 g/sq cm is obtained for the mass absorption coefficient of dust at 100 microns.

Laureijs, R. J.↗
NASA NTRS1991

The temperature of large dust grains in molecular clouds

The temperature of the large dust grains is calculated from three molecular clouds ranging in visual extinction from 2.5 to 8 mag, by comparing maps of either extinction derived from star counts or gas column density derived from molecular observations to I(100). Both techniques show the dust temperature declining into clouds. The two techniques do not agree in absolute scale.

Clark, F. O.↗
NASA NTRS1989

Dust emission from high latitude cirrus clouds

In order to study dust emission from grains in the interstellar medium, the infrared properties were analyzed in a number of isolated high latitude dust clouds which contain no dominant internal heating sources. The clouds are spatially resolved, have a simple geometry, and are mapped in the IRAS bands at 12, 25, 60, and 100 microns. For a number of these clouds, extinction data (A sub B) were obtained from starcounts. A large part (30 to 50 percent) of the infrared radiation of the clouds in the IRAS wavelength range of 8 to 130 micron is emitted in the short wavelength bands at 12 and 25 micron. The 60/100 micron ratios for the integrated fluxes of the clouds have a typical value of 0.19 + or - 0.05.

Laureijs, Rene J.↗
NASA NTRS1987

Shock heated dust in L1551: L(IR) greater than 20 solar luminosities

The infrared bolometric luminosity of the extended emission from the L1551 flow exceeds 20 solar luminosities. Ultraviolet radiation from the shock associated with the flow appears to heat the dust requiring shock temperatures from 10,000 to 90,000 K in L1551, velocities of approximately 50 km/s near the end of the flow, and a minimum mechanical luminosity of approximately 40 solar luminosities. The total energy requirement of the infrared emission over a 10,000 year lifetime is 10 to the 46th to 47th ergs, two orders of magnitude higher than previous estimates for L1551. Infrared radiation offers a method of probing interstellar shocks, by sampling the untraviolet halo surrounding the shock. At least one current model for bipolar flows is capable of meeting the energetic requirements.

Clark, F. O.↗
NASA NTRS1981

A turbulent origin for the rotation of molecular clouds

The observed rotational properties of dark clouds are shown to be consistent with the turbulent character of the interstellar medium. The turbulent flow is easily maintained by the shearing action of differential galactic rotation. The size distribution of molecular clouds inferred from observations is found to closely resemble a Kolmogorov turbulent spectrum. These findings suggest that the processes leading to the formation and evolution of such clouds may depend strongly on the properties and characteristics of interstellar turbulence.

Fleck, R. C., Jr.↗
NASA NTRS1980

Vibrationally excited silicon monoxide masers

Published data on a select group of SiO maser sources have been analyzed for velocity variations as a function of phase. No apparent correlation was found to a level of about 2 km/s. This places constraints on the location of the maser molecules. Such a correlation should be present at some level. The implications for future high resolution infrared measurements are discussed.

Buhl, D.↗
NASA NTRS1974

Radio detection of interstellar dimethyl ether

The detection of interstellar dimethyl ether, in emission from the direction of the Orion Nebula molecular cloud, is reported. The largest molecule detected in space, dimethyl ether has a large collisional cross section and C (sub 2V) symmetry. Hence, it should be useful for future studies of molecular pumping models.

Snyder, L. E.↗