Engineering topics

Ahearn, M. F.

Publications and source records attributed to Ahearn, M. F..

At least 37 records · Page 2

NASA NTRS1984

Temporal and spatial behavior of the ultraviolet emissions of comet IRAS-Araki-Alcock 1983d

The spatial distribution and temporal variation of the ultraviolet emissions from comet IRAS-Araki-Alcock 1983d, observed by IUE near closest approach of the comet to the earth, suggest a model for this comet of a rotating cometary nucleus with a nonuniform distribution of volatile ices. The short lifetime of S2 (about 500 s), a species whose presence was first identified in this comet and is presumed to be present generally in the cometary ice, makes S2 a valuable indicator of short-term nuclear activity. In the case of comet 1983d, the S2 production rate decreased by a factor of 10 over a period of 28 hr.

Feldman, P. D.↗

NASA NTRS1984

Infrared photometry of Comet Bowell and other comets

Broad-band and intermediate-band infrared photometry of Comets Bowell, Elias, and Gunn is presented. The data for Comet Bowell show that the strongly depressed albedo near 3 microns observed by Campins and colleagues (1982) is very broad and extends at least to 4 microns. Scattering calculations indicate that existing infrared data do not provide direct, observational evidence for the presence of H2O ice. However, the material responsible for the 3-micron absorption observed by Campins and colleagues remains therefore unidentified.

Dwek, E.↗

NASA NTRS1984

Comet 1983d - A contrast between data from IRAS and data from the ground

Optical photometry of comet IRAS-Araki-Alcock 1983d is presented. The data provide values for the product of albedo and average optical depth in the coma. These turn out to be at least a factor of 20 lower than expected from the IRAS measurements reported by Walker et al. (1984).

Ahearn, M. F.↗

NASA NTRS1984

Comet Bowell (1980b) - Measurement of the optical thickness of the coma and particle albedo from a stellar occultation

Comet Bowell (1980b) was observed to pass within 0.25 + or 0.09 arc sec of a star (about 540 km at the comet), where the absorption of starlight by the dust coma was found to be 3 percent (+ or - 1 percent). The implied optical thickness of 0.03 differs greatly from other determinations and gives a mass of 3 x 10 to the 13th g for the coma within 10,000 km of the nucleus. Coupled with absolute continuum filter photometry, these results indicate a very low particle albedo consistent with fluffy carbonaceous material. This experiment indicates the need to observe nearly central occultations by several observers to measure the optical thickness profile of a comet. The advantages of using a charge-coupled device area photometer for such observations are discussed.

Larson, S. M.↗

NASA NTRS1984

Comet Bowell 1980b

Optical filter photometry, and optical and ultraviolet spectrophotometry data collected between November 1980 and June 1982 of Comet Bowell are presented. It was determined that Comet Bowell began producing significant amounts of OH, though not other species, before perihelion at a heliocentric distance near 4.6 AU. As the comet approached perihelion at 3.4 AU, OH production decreased and CN and C2 species were detected at normal concentrations. An outburst in April 1982 was dominated by OH. The grains in 1982 near perihelion showed increases in albedo between 3150 and 4500 A and 1.2 and 1.6 microns. The presence of two grains populations was suggested, together with the conclusion that Comet Bowell was physically a typical new comet entering the inner solar system from the Oort cloud for the first time.

Feldman, P. D.↗

NASA NTRS1984

Evolution of the ultraviolet coma of comet Austin (1982g.)

Ultraviolet spectra of comet Austin (1982) obtained in July 1982 at heliocentric distances ranging from 1.10 to 0.81 AU preperihelion with the IUE reveal that the comet is very similar in chemical composition and appearance to comet Bradfield (1979X). In addition, the derived H2O production rate is found to vary with heliocentric distance as r to the -3.6, similar to the r to the -3.7 behavior found for comet Bradfield. It is pointed out, however, that the limited sample rate precludes the observation of the short-term variations which could be produced by a rotating nonuniform cometary nucleus.

Feldman, P. D.↗

NASA NTRS1984

Infrared photometry of the dust in comets

Infrared observations of six comets ranging in heliocentric distance from 1 to 5 AU are reported. Data consist of wide and intermediate bandpass filter photometry, 1-20 microns, for periodic comets Stephan-Oterma, Swift-Gehrels, Kearns-Kwee, Gunn, and Grigg-Skjellerup and for parabolic Comet Elias (1981 XV). All of the comets show infrared temperatures hotter than a theoretical black body, indicating small absorbing grains. No emission feature at 10 microns was observed within the accuracy of the filter photometry. The photometric system is discussed in detail, and temperature-dependent corrections to monochromatic magnitudes are presented.

Tokunaga, A. T.↗

NASA NTRS1983

The discovery of S2 in comet IRAS-Araki-Alcock 1983d

Ultraviolet spectra of comet IRAS-Araki-Alcock 1983d, obtained with the International Ultraviolet Explorer spacecraft when the comet was only 0.032 AU from earth, show strong emission bands due to S2, the first detection of this species in an astronomical object. The spatial profiles imply that the S2 is released directly from the nucleus and has a lifetime of the order of 450 s. The derived production rate of S2 was 2 x 10 to the 25th per s. It is suggested that the S2 can be formed by cosmic-ray irradiation of other sulfurous compounds in the ices and that it need not be primordial. It has been also determined that the scale length of the parent of CS (presumably CS2) is 300 km and that the CS production is approximately equal to that of S2, both near 5 x 10 to the -4th that of OH and comparable to that of other trace species observed in comets.

Ahearn, M. F.↗

NASA NTRS1983

The evolution of the infrared emission from the Type II supernova 1980k in NGC 6946 - The dust formation model

The paper presents 1-4 micron photometry of supernova 1980 k in NGC 6946 obtained over a period of 1 year following the outburst. During the period between 1980 November 1 and December 19, the infrared emission probably originated from the extended atmosphere of the expanding star. The JHKL colors and a 1.3-2.6-micron spectrum observed during this period correspond to those of a blackbody with an average temperature of about 5000 K. Observations around 1981 May 31 showed that the supernova developed an infrared excess after 1980 December. This infrared excess persisted through 1981 October and is consistent with the appearance of thermal emission from about 700 to 900 K dust in addition to a hotter photosphere. The similarity of this behavior to that of the infrared evolution of some novae suggests that dust formation may be occurring in the supernova ejecta. The hypothesis, that the emission arises from preexisting grains in a circumstellar shell which are heated by the supernova outburst, is also consistent with the data.

Dwek, E.↗

NASA NTRS1983

The disappearance of OH from Comet P/Encke

Photoelectric observations of Comet P/Encke during its 1980 apparition are combined with other published data to relate molecular production rates to the visual lightcurve. In addition to a substantial asymmetry about perihelion which is already well known, there are shorter-term variations in specific molecules which have not been duplicated by models. The most dramatic of these fluctuations is a rapid decrease by more than a factor of 3 in the production of OH at 0.75 AU preperihelion.

Ahearn, M. F.↗

NASA NTRS1983

Studies of proton-irradiated cometary-type ice mixtures

Cometary ice mixtures are studied in a laboratory experiment designed to simulate the temperature, pressure and radiation environments of the interstellar Oort cloud region, in order to test the hypothesized radiation synthesis mechanism for changing the characteristics of the outer few meters of a comet stored in the Oort cloud for 4.6 billion years. All experiments conducted confirm the synthesis of new molecular species in solid phase mixtures at 20 K. When CH4 is present in the irradiated ice mixture, long chained, voltaile hydrocarbon and CO2 are synthesized together with high molecular weight C compounds present in the room temperature residue. Due to radiation synthesis, about 1 percent of the ice was converted into a nonvolatile residue containing complicated C compounds not present in the blank samples. These results suggest that initial molecular abundances can be altered, and new species created, as a result of radiation synthesis. Irradiated mixtures exhibited thermoluminescence and pressure enhancements during warming, showing the synthesis of reactive species. Outbursts in new comets resulting from similar irradiation-induced exothermic activity would be expected to begin occurring at distances of the order of 100 AU.

Moore, M. H.↗

NASA NTRS1983

Ultraviolet cometary spectrophotometry

During the 13 shifts dedicated to observations of Comet Bradfield (including the two European shifts), five high dispersion exposures were obtained with the LWR camera, 27 low dispersion images with the LWR camera, and 36 low dispersion images with the SWP camera of which 5 were observations of the geocoronal background and 4 were taken in a serendipity mode while the nucleus of the comet was centered on the large aperture of the LWR camera.

Ahearn, M. F.↗

NASA NTRS1983

The grains and gas in Comet Bowell 1980b

The importance of coordinating different types of observations of a comet is illustrated in terms of spectral data on Comet Bowell. Photometric data in 1980 indicated that OH emissions were occurring when the comet was still more than 4 AU from the sun. Further measurements were arranged with the IUE satellite, the photoelectric scanner at Cerro Tololo and the IR telescope on Mauna Kea. Several sizes of diaphragms were used in order to estimate variations with respect to aperture. Calculations were made to quantify the wavelengths observed as a function of the grains' cross-sections. The OH emission was confirmed by the spectrographic and photometric data. It is suggested that sufficient dark materials are present on the cometary surface to have enhanced the absorption of solar radiation to levels sufficient for heating and dissociating surface ice particles.

Dwek, E.↗

NASA NTRS1982

Narrowband photometry of Comet P/Stephan-Oterma and the backscattering properties of cometary grains

The results of narrowband filter photometry of Comet P/Stephan-Oterma are presented. For heliocentric distances between 1.95 and 1.58 AU prior to perihelion, OH, CN, and C2 production rates varied approximately as r to the -5th power. Following perihelion, an even steeper dependence of production rate on heliocentric distance was observed for these species. A pronounced brightening of the comet's continuum near opposition is interpreted in terms of preferential backscattering by the cometary dust. While Comet P/Stephan-Oterma is one of the dustier comets observed, it shows no indication of compositional anomalies in either the gas or the dust.

Millis, R. L.↗

NASA NTRS1982

OH fluorescence in comets - Fluorescence efficiency of the ultraviolet bands

The present investigation is concerned with the calculation of the fluorescence efficiency of OH over a wide range of heliocentric radial velocities. Three vibrational states in each of the two lowest electronic states are included in these calculations, producing a total of nine vibrational bands in the ultraviolet. The attempt is made to reproduce observed, ultraviolet, cometary spectra of OH by utilizing the Swings fluorescence mechanism. The inclusion of four rotational states within each of the three vibrational states and a detailed solar atlas make it possible to match observations very well, although there remain some discrepancies. This ability to reproduce observations provides confidence that the more general results obtained for the fluorescence efficiency as a function of heliocentric radial velocity can be used in interpreting both previous and future observations of OH in comets.

Schleicher, D. G.↗

NASA NTRS1982

Vaporization in Comets; outbursts from Comet Schwassmann-Wachmann 1

Revised estimates are presented which show that the total mass and kinetic energy in a typical Comet P/Schassmann-Wachmann 1 outburst are lower than previously estimated, and that this mass is comparable to the mass of gas vaporized, as inferred from recent observations of CO(+) in this comet. The storage of energy suggested by many authors is therefore not neccessary. A simple equilibrium vaporization of CO2 or CO which is suddenly exposed, on a nucleus that is otherwise primarily composed of H2O, is proposed. Under these conditions, calculations of the variation of vaporization with rotational phase indicate that the mechanism can quantitatively produce outbursts of the size observed.

Cowan, J. J.↗

NASA NTRS1982

Spectrophotometry of comets at optical wavelengths

Techniques for spectrophotometry of comets are discussed, and results are reviewed for line and continuum spectrophotometry of comets at optical wavelengths. The techniques considered include photographic spectroscopy and spectrophotometry, photoelectric spectrophotometry, and methods based on the use of image dissector scanners, Fourier-transform instruments, and Fabry-Perot spectrometers. Results are summarized for the study of cometary emission features due to C2, CN, CH, OH, NH, C3, NH2, O, H, CO(plus), and H2O(plus). Relative abundances of various species in comets are examined, along with continuum spectrophotometry of the nuclei, comas, and tails of comets.

Ahearn, M. F.↗

NASA NTRS1981

Water production models for comet Bradfield /1979 X/

Whipple (1950, 1951) has proposed that water might be the dominant volatile constituent of the cometary nucleus. The considered investigation is concerned with the clues which observations of the cometary coma provide regarding the parents of the dominant observed coma species. In the early part of 1980, comet Bradfield (1979 X) was observed with the aid of the International Ultraviolet Explorer (IUE). These measurements are the first which provide a basis for a comprehensive study of all three water dissociation products, H, O, and OH, simultaneously. The IUE observations of comet Bradfield are compared with the predictions derived from an H2O model. The obtained results indicate that the observed brightnesses of H, O, and OH are certainly consistent with a common water source for all three species.

Weaver, H. A.↗