Engineering topics

Rickard, L. J.

Publications and source records attributed to Rickard, L. J..

11 records

NASA NTRS1995

A Fizeau optical interferometer astrometric satellite

The proposed design and construction of the Fizeau astrometric mapping explorer (FAME), a small astrometric instrument for use on an artificial satellite, is reported on. The instrument and spacecraft are designed to slowly spin and will repeatedly scan great circles on the sky so that, over a period of time, it will cover the complete sky and repeat in a manner similar to that of the Hipparcos satellite. The instrument will use the two fixed dilute aperture telescopes to measure a fixed angle between stars and detect the positions, magnitude and color of all stars crossing its field of view to a visual magnitude of approximately 15 mag. The aim of the instrument is to obtain a catalog of positions, proper motions and parallaxes of all stars down to about 15 mag, with a magnitude dependent accuracy of positions of 20 to 800 micro arcsec, proper motions of 20 to 800 micro arcsec per year and parallaxes of 20 to 800 micro arcsec.

Seidelmann, P. Kenneth↗

NASA NTRS1992

The separation between gas and dust filaments at the edge of the expanding shell in Eridanus

Observations obtained from IRAS for a 120 sq deg region at the boundary of a large expanding shell in Eridanus are presented. It is found that the separation between the structures in the H I and IR emission is not due to a transition of the gas into molecular hydrogen at the location of the IR filaments. The molecular hydrogen column density of these filaments is at least two orders of magnitude below what is typically seen in high-latitude molecular clouds. As the H I and dust in this region are associated with a large expanding shell (or superbubble), the action of shocks and radiation fields may be responsible for the separations of gas and dust. Unless the gas and dust structures are resolved and lie in a favorable viewing geometry, the effect tends to be hidden. The results indicate that it is not always evident, from observations with spatial resolutions coarser than a few arcminutes, how interstellar gas and dust are related.

Verschuur, G. L.↗

NASA NTRS1990

Observations of the galactic plane by the zodiacal infrared project

The two rocket flights of the Zodiacal Infrared Project (ZIP), flown 18 August 1980 and 31 July 1981, were intended to provide data on the near-infrared thermal emission of the interplanetary dust cloud over a broad range of ecliptic coordinates (latitudes -60 to +85 degrees, solar elongation angles 22 to 90 degrees and 140 to 180 degrees). In addition, their multiple crossings of the Galactic plane provided low resolution spectral data (delta lambda/lambda ranging from 1. to 0.1, for effective wavelengths from 3 to 30 microns) for most of the first quadrant (longitudes 30 to 100 degrees). Examples are displayed. Having made a thorough reanalysis of the calibration of the ZIP database, researchers present the salient features of the Galactic plane as observed by ZIP. The binned, in-plane data, corrected for zodiacal emission, generally show an exponential decrease with increasing longitude. The fitted exponential scale-length is 0.038/degree, and can be inverted to derive a radial density profile. Channel ratios are converted to temperatures by using model spectra in which thermal emitters with emissivity approx. 1/lambda are convolved with the filter responses. The results for channels 5 (11 microns) and 12 (21 microns) are shown, along with similarly derived temperatures from Infrared Astronomy Satellite (IRAS) 12 microns and 25 microns data. The ZIP data show little variation with longitude, consistent with IRAS results. A narrow spectral feature at 13 microns appears consistently in data for the plane (uncorrected for zodiacal emission). However, this is strongly contaminated by calibration problems for channel 8. Researchers suggest that residual emission at 13 microns arises from the (NeII) line at 12.8 microns.

Rickard, L. J.↗

NASA NTRS1989

Optical depth of molecular gas in starburst galaxies - Is M82 the prototype?

An attempt is made to survey the CO(2-1) emission toward the centers of 17 IR-luminous galaxies which have previously been detected in CO(1-0). These galaxies span a wide range of size and L(FIR)/L(B) ratio, many have multiple-wavelength studies establishing them as starbursts, and some bear a morphological resemblance to M 82. Nine galaxies are detected and useful upper limits are placed on the remaining eight. Using the CO(2-1)/CO(1-0) ratio of antenna temperature as a diagnostic of optical depth, it is found that all of the galaxies contain predominantly optically thick molecular gas. This implies that the phase of starburst during which the molecular gas is optically thin, currently witnessed in M 82, is either uncommon or short-lived.

Verter, F.↗

NASA NTRS1985

More extragalactic carbon monoxide

CO emission has been detected toward the centers of six spiral galaxies - NGC 660, 3504, 3628, 4303, 4631, and 4826 - which had been predicted to have molecular sources similar to that in the center of the Galaxy on the basis of OH and IR studies. Analysis of the entire sample of galaxies with strong central CO peaks supports the interpretation of the central peaks as components distinct from the disk molecular clouds. A new method for estimating the mass of associated H2 is suggested that emphasizes the different kinds of clouds in these different regions. There appears to be no correlation of CO and H I linewidths. However, there are good correlations between CO emission, far-infrared emission, and nonthermal radio continuum emission. The correlation with infrared emissions shows that more molecular gas implies more star formation in galactic centers.

Rickard, L. J.↗

NASA NTRS1985

Carbon monoxide isotope ratios in galactic centers and disks

For each of the six galaxies whose mm-wavelength emission of CO and C-13O were observed from both the center and the disk, and the ratio of integrated intensities P(CO)/P(C-13O) is noted to vary markedly from galactic center to galactic center, from galactic disk to galactic disk, and from point to point within a given galaxy. The observed integrated intensity ratios vary from 3 to 16, indicating that the use of a constant P(CO) to sigma(H2) conversion to derive H2 surface densities from extragalactic CO observations can produce major errors.

Rickard, L. J.↗

NASA NTRS1984

Starbursts in Galaxies: Implications of Molecular and Far-infrared Observations

Starburst galaxies are defined in several ways (colors, optical spectroscopic signatures, and excess radio flux), and observational evidence indicating that episodes of rapid star formation occur in many galaxies with active nuclei is presented. There is a good correlation of 100 micron luminosity with CO emission, and of both quantities with excess nonthermal radio flux. This fact requires some linkage between central and global star formation rates. In addition, the presence of starbusts distorts the appearance of the molecular gas in which they occur. Using far infrared color temperatures and comparisons of CO isotopes, it is shown that the strong (12)CO emission in these galaxies does not accurately trace the H2 distribution, probably because the starburst raises the avearage temperature of the cloud ensemble.

Rickard, L. J.↗

NASA NTRS1984

Far-infrared observations of galactic nuclei

Measurements of far-infrared continuum emission from the central regions of 30 late-type galaxies, 24 of which were previously unreported, are given. Photometry covering wavelengths from 40 to 160 microns has been obtained, yielding information on luminosities, temperatures, and dust masses for most. Good correlations of the far-infrared luminosity with blue colors, reddening, and molecular line radiation are found, supporting the identification of the sources with the molecular cloud component in these galaxies, and strengthening the identification of the energy sources as the young massive stars formed from those clouds. Comparisons with 10-micron data indicate that such data are useful for statistical studies, but have too much scatter to use directly in the analysis of individual sources. Strong correlation with nonthermal radio continuum emission and radio-to-optical flux ratios suggest that massive star formation underlies a wide range of galactic activity.

Rickard, L. J.↗

NASA NTRS1983

The variation of dust temperatures in Maffei 2

We report far-infrared photometry for the central regions of Maffei 2. The fitted dust temperatures show a definite decline away from the nucleus. This supports the hypothesis that the molecular clouds close to the nucleus represent a different population from the disk clouds.Extragalactic studies that assume constant temperatures to convert CO integrated intensity to molecular hydrogen surface mass density may be in serious error.

Rickard, L. J.↗

NASA NTRS1980

Star formation in IC 1848 A

Recent far-infrared photometric and near-infrared spectroscopic observations of IC 1848 A/W5 East are reported. The source appears to be excited by a star of spectral type near B0, which can explain the far-infrared luminosity, 2 micron spectrum, radio continuum, and radio molecular observations. Although this star appears surrounded by a local very compact H II region, the density over a large scale is relatively low. This puts constraints on models for the formation of this star.

Thronson, H. A., Jr.↗

NASA NTRS1977

Observations of formaldehyde absorption in the region of NGC 2264 and other Bok globules

The paper reports observations of 6-cm H2CO absorption over an extended region surrounding the globule in NGC 2264 and toward the globules B68, B72, B133, B134, B227, B238, and B335. The lines observed from the globules are similar to those seen in more extended dark clouds. It is inferred that physical conditions are similar in the two types of region. Negative results for several other globules are reported.

Rickard, L. J.↗