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NASA NTRS1987

Observations of the brightness temperature distribution of the quiet solar corona at decametric wavelengths

The brightness temperature distribution of the quiet solar corona at a wavelength of 8.9 meters is measured by two types of radio telescope: (1) a 'T' type array with a resolution of 26'X38', and (2) a fan beam interferometer with an E-W resolution of 3'. It is found that the persistent bright regions do not have any angular structure on scales of 6' or less. The daily variations of the brightness temperature of different regions are studied and the possible interpretation discussed.

Sastry, Ch. V.↗

NASA NTRS1987

Search for molecules in Comet Halley at millimeter wavelengths

Data collected with the Five College Radio Astronomy Observatory 13.7-m radio telescope are used to search for the possible CN parent molecules HNC, HC3N, and CH3CN in Comet Halley at millimeter wavelengths. Maximum relative abundances for HNC/HCN of 0.3, for HC3N/HCN of 0.4, and for CH3CN/HCN of 0.8 are obtained, showing that these three molecules are not a major source of the CN radical observed in optical and UV spectroscopy. Upper limits to the beam averaged column densities and production rates of these molecules are determined (in addition to an upper limit for the beam averaged column density for the formyl ion of less than 10 to the 11th/sq cm), providing important constraints for chemical models of the coma.

Swade, Daryl A.↗

NASA NTRS1988

Neutral stellar winds that drive bipolar outflows in low-mass protostars

The Arecibo radio telescope at the 21-cm line of atomic hydrogen has been used to detect a neutral atomic wind in the bipolar flow source HH 7-11. An atomic mass of about 0.015 solar associated with the rapidly flowing gas is deduced. The stellar mass-loss rate is roughly 3 x 10 to the -6th solar mass/yr if the crossing time of the decelerating wind is 5000 yr. The excess emission in the H I line core gives a total duration of the outflow of about 70,000 yr. A detailed analysis of the H I line shape yields a reasonable deceleration rate for the atomic wind if the stellar wind continuously entrains ambient molecular gas as it propagates from the protostar. A stellar wind with the described characteristics and a terminal velocity of 170 km/s would be more than sufficient to drive the known extended CO bipolar outflow in HH 7-11.

Lizano, Susana↗

NASA NTRS1987

A 1.6 MHz survey of the galactic background radio emission

Observations of the galactic radio emission at 1.6 MHz have been made during the current solar activity minimum using a radio telescope with a beamwidth of 25 deg. The radiation intensity was mapped for six declinations between -12 and -72 degrees and from 1000 to 0500 hours R.A.

Ellis, G. R. A.↗

NASA NTRS1988

Phasing the antennas of the Very Large Array (VLA) for reception of telemetry from Voyager 2 at Neptune encounter

The Very Large Array (VLA) radio telescope is being instrumented at 8.4 GHz to receive telemetry from Voyager 2 during its encounter with Neptune in 1989. The procedure in which the 27 antennas have their phases adjusted in near real time so that the signals from the individual elements of the array can be added coherently is examined. Calculations of the expected signal to noise ratio, tests of the autophasing process at the VLA, and off-line simulations of that process are all presented. Various possible procedures for adjusting the phases are considered. It is shown that the signal to noise ratio at the VLA is adequate for summing the signals from the individual antennas with less than 0.1 dB of loss caused by imperfect coherence among the antennas. Tropospheric variations during the summer of 1989 could cause enough loss of coherence to make the losses higher than 0.1 dB. Experiments show that the losses caused by the troposphere can probably be kept below 0.2 dB if the time delay inherent in the phase adjustment process is no longer than approx. 5 secs. This relatively small combining loss meets the goal estabished to minimize the bit error rate in the Voyager telemetry and implies adequate autophasing of the VLA.

Ulvestad, J. S.↗

NASA NTRS1988

A very low frequency radio astronomy observatory on the Moon

Because of terrestrial ionospheric absorption, very little is known of the radio sky beyond 10 m wavelength. An extremely simple, low cost very low frequency radio telescope is proposed, consisting of a large array of short wires laid on the lunar surface, each wire equipped with an amplifier and a digitizer, and connected to a common computer. The telescope could do simultaneous multifrequency observations of much of the visible sky with high resolution in the 10 to 100 m wavelength range, and with lower resolution in the 100 to 1000 m range. It would explore structure and spectra of galactic and extragalactic point sources, objects, and clouds, and would produce detailed quasi-three-dimensional mapping of interstellar matter within several thousand parsecs of the Sun.

Douglas, James N.↗

NASA NTRS1988

Global fringe fitting for space interferometry

Global fringe fitting for VLBI has been incorporated into the NRAO Astronomical Image Processing System as the VBFIT program. A generalized program, called VBGFF, has been developed for use with VLBI arrays that include orbiting radio telescopes. Various aspects of these programs are discussed.

Jones, Dayton L.↗

NASA NTRS1989

Arecibo emission-absorption observations of Galactic OH

Observations of OH emission/absorption toward 19 low-Galactic-latitude background-continuum sources are reported. Data obtained at 1.665 and 1.667 GHz using the 12-m radio telescope at Arecibo Observatory during February-March 1984, are presented in extensive tables and graphs and characterized in detail, with reference to theoretical models. A total of 11 OH absorption features were detected toward nine of the sources, with an average of one feature of optical depth 0.02 or greater per 5 kpc of path length and an average column density of 1.6 x 10 to the 12th/sq cm (corrected to b = 90 deg).

Colgan, Sean W. J.↗

NASA NTRS1987

Microwave emission from steady and moving sunspots

Force-free extrapolations of photospheric magnetic field observations from Marshall Space Flight Center have been used to compute the total intensity and circular polarizaton of sunspot associated emission from active region 2502 in the period June 13 to 15, 1980. The computed maps were compared to high resolution observations of the same active region obtained with the Westerbork Synthesis Radio Telescope. The most interesting aspect of the active region was the development of a new spot between the preceding and the following spots on June 14, which subsequently merged with the preceding with the preceding spot. The new spot was associated with enhanced microwave emission with a peak brightness temperature in excess of 4 x 10 to the 6th K. It is shown that unrealistic values of the conductive flux are required for the interpretation of the emission of the new sunspot in terms of thermal processes. It is suggested that this source is due to gyrosynchrotron radiation from mildly relativistic electrons accelerated by resistive instabilities in the evolving magnetic field.

Alissandrakis, C. E.↗

NASA NTRS1989

Tracking requirements to support space very long baseline interferometry

The use of VLBI with a radio telescope in space is exmained. The concept of space VLBI and Soviet, Japanese, and U.S./European studies for developing space VLBI observatories are reviewed. The proposed NASA/ESA Quasat mission is described, including the potential ground tracking sites for high earth orbiters.

Jordan, J. Frank↗

NASA NTRS1989

Structures for remotely deployable precision antennas

Future space missions such as the Earth Science Geostationary Platform (ESGP) will require highly accurate antennas with apertures that cannot be launched fully formed. The operational orbits are often inaccessible to manned flight and will involve expendable launch vehicles such as the Delta or Titan. There is therefore a need for completely deployable antenna reflectors of large size capable of efficiently handling millimeter wave electromagnetic radiation. The parameters for the type of mission are illustrated. The logarithmic plot of frequency versus aperture diameter shows the regions of interest for a large variety of space antenna applications, ranging from a 1500-meter-diameter radio telescope for low frequencies to a 20-meter-diameter infrared telescope. For the ESGP, a major application is the microwave radiometry at high frequencies for atmospheric sounding. Almost all existing large antenna reflectors for space employ a mesh-type reflecting surface. Examples are shown and discussed which deal with the various structural concepts for mesh antennas. Fortunately, those concepts are appropriate for creating the very large apertures required at the lower frequencies for good resolution. The emphasis is on the structural concepts and technologies that are appropriate to fully automated deployment of dish-type antennas with solid reflector surfaces. First the structural requirements are discussed. Existing concepts for fully deployable antennas are then described and assessed relative to the requirements. Finally, several analyses are presented that evaluate the effects of beam steering and segmented reflector design on the accuracy of the antenna.

Hedgepeth, J. M.↗

NASA NTRS1990

Extraterrestrial life in the universe

The possibility that life exists elsewhere in the universe, even in our own planetary system, has intrigued scientists, philosophers, and theologians for centuries. The spaceflight programs of NASA have provided much new information about our planetary neighbors and have put to rest some speculations about the existence of life on those planets or their satellites. However, there are still undetermined questions about the possibility of some form of life existing in the far distant past in our planetary system. Beyond our planetary system, the astronomical quest for scientific clues about life continues, largely via the radio telescope. Thus far there is no conclusive evidence. Here, some of the recent findings about our planetary neighbors are reviewed and the question about life elsewhere in the universe is addressed.

Graham, Robert W.↗

NASA NTRS1990

A further study of the molecular cloud associated with the supernova remnant G109.1-1.0

The region of the semicircular SNR G109.1-1.0 is studied on the basis of CO observations with a 45-m radio telescope and X-ray data from the archive of the Einstein Observatory. By observing the J = 1-0 transition of CO at 115 GHz, the distribution of the molecular cloud associated with the remnant is investigated in detail. The resolution of the CO mapping observations is 0.6-1.2 pc and the number of the CO spectra obtained is about 2000. The molecular ridge (CO arm), which was known to show an apparent anticorrelation with the curled x-ray jetlike feature of the remnant, is resolved into two CO filaments. The hardness of the X-ray spectrum toward the CO arm is consistent with the column densities of the two CO filaments. However, the overall appearance of the remnant will not be affected very much by the absorption, because the X-ray absorption is found to be a minor effect.

Tatematsu, Ken'ichi↗

NASA NTRS1990

The discrete nature of circumstellar OH maser emission

The Arecibo radio telescope was used to obtain high-resolution, high-SNR 1612-MHz observations of seven circumstellar OH maser sources. Each spectrum displays the double-peaked emission line profile characteristics of an expanding shell. The jaggedness of the spectra, which varies from source to source, is consistent with statistical fluctuations in the number of discrete emitting elements contributing at a given velocity. In particular, it is found that the spectra of WX Psc and OH 53.6-0.2 contain narrow, weak, isolated emission features which may arise from individual emitting elements.

Zell, Philip J.↗

NASA NTRS1990

Identifying galaxies in the zone of avoidance

A set of IR color criteria was applied to sources in the IRAS Point Source Catalog at low Galactic latitudes in order to select candidate galaxies. A total of 371 of the selected sources within a Galactic latitude range (b) = 2-16 deg were searched for H I with the Arecibo 305 m radio telescope. Twenty-six percent (97) were detected, including 36 galaxies previously known only as IRAS sources. The H I spectra of the detected galaxies are presented together with their observed redshifts and velocity widths.

Lu, N. Y.↗

NASA NTRS1990

High velocity gas in external galaxies

Two nearby, nearly face-on spiral galaxies, M 101 and NGC 6946, observed in the HI with the Westerbork Synthesis Radio Telescope (WSRT) as part of a program to search for high velocity gas in other galaxies, are used to illustrate the range of properties of high velocity gas in other galaxies found thusfar.

Kamphuis, J.↗

NASA NTRS1990

Spiral structure of M51: Streaming motions across the spiral arms

The atomic hydrogen (HI) and the H alpha emission line in the grand-design spiral galaxy M51 have been observed with the Westerbork Synthesis Radio Telescope and the Taurus Fabry-Perot imaging spectrometer, respectively. Across the inner spiral arms significant tangential and radial velocity gradients are detected in the H alpha emission after subtraction of the axi-symmetric component of the velocity field. The shift is positive on the inside and negative on the outside of the northern arm. Across the southern arm this situation is reversed. The direction of the shifts is such that the material is moving inward and faster compared to circular rotation in both arms, consistent with the velocity perturbations predicted by spiral density wave models for gas downstream of a spiral shock. The observed shifts amount to 20 to 30 km (s-1), corresponding to streaming motions of 60 to 90 km (s-1) in the plane of the disk (inclination angle 20 degrees). Comparable velocity gradients have also been observed by Vogel et al. in the CO emission from the inner northern arm of M51. The streaming motions in M51 are about 2 to 3 times as large as the ones found in HI by Rots in M81, and successfully modelled by Visser with a self-consistent density wave model. Researchers have not been able to detect conclusively streaming motions in the HI emission from the arms, perhaps due to the relatively poor angular resolution (approx. 15 seconds) of the HI observations.

Tilanus, R. P. J.↗

NASA NTRS1989

H I and dust in the high latitude dark cloud L1642

The high latitude dark cloud L1642 in the 21 cm H I region was mapped using a 100 m radio telescope. A remarkable H I line broadening from 2.5 to 2.9 km/s is observed over a small area on the bright side of L1642, i.e., the side facing the galactic plane. Results are presented concerning the effects of the asymmetrical UV radiation field of OB stars on the H I gas and the very small dust grains associated with L1642.

Liljestroem, Tarja↗