Engineering topics

Schleicher, D. G.

Publications and source records attributed to Schleicher, D. G..

12 records

NASA NTRS1987

Photometric observations of comet P/Giacobini-Zinner

Narrow-band filter photometry of comet P/Giacobini-Zinner was obtained on 15 nights at Lowell Observatory in the interval from June 15 to Nov. 20, 1985. The emission bands of OH, NH, CN, C3, and C2 were isolated, along with continuum points at 3650 A and 4845 A. All observed species showed pronounced asymmetry about perihelion, with preperihelion abundances being greater by more than a factor of 2 than postperihelion abundances over the heliocentric distance range of 1.0-1.5 AU. The characteristics of the asymmetry were significantly different than observed in previous apparitions, with the peak production rates occurring in late July/early August. The unusually low C2/CN abundance ratio seen in previous apparitions is confirmed by the present observations. Furthermore, the QH data demonstrate that the C2 in Giacobini-Zinner (along with C3 and NH) is depleted when compared to water (the dominant species), rather than CN being unusually over abundant.

Schleicher, D. G.↗

NASA NTRS1986

Rotational period of comet Halley

Results are reported from ground-based photometric measurements of the variations in Comet Halley emissions at different wavelengths in order to calculate the comet rotation rate. The data, gathered at Cerro Tololo, included the OH, NH, CN, C3, CO(+) and C2 bands and continuum emissions at 3650 and 4845 A. Emphasis is placed on the C2 data at 5140 A. Both a periodic variation and a secular decrease was detected through March-April 1986. Light curves generated with the data support a rotational period just short of 7.4 days.

Millis, R. L.↗

NASA NTRS1986

The variability of Halley's Comet during the Vega, Planet-A, and Giotto encounters

Narrowband photometry of Halley obtained at Cerro Tololo Inter-American Observatory (CTIO), Mauna Kea Observatory (MKO), and Perth Observatory was combined to determine the relative level of activity during the interval spanning the spacecraft encounters. Measurements of the flux from the comet in emission bands of OH, NH, CN, C3, and C2, as well as at 2 continuum points, were obtained at CTIO on each night between 5 March and 17 March 1986. Observations were made on many of these same dates at MKO and Perth using comparable interference filters. The date clearly show variation of a factor of 2.5 in the production of all observed species with a characteristic time scale of a few days.

Schleicher, D. G.↗

NASA NTRS1985

Emission by OD in comets

A calculation has been made of the complete fluorescent equilibrium of OD in sunlight including ultraviolet (A-X) transitions, vibrational and rotational transitions (A-A and X-X), and Lambda-doubling transitions in the X state. The dependence on heliocentric radial velocity is strong. No lines of OD are visible in any cometary spectra taken with IUE, but the limits vary, considerably from one comet to another. In the most favorable case, comet Austin 1982 VI, the limit is OD/OH less than 0.004. Optimum conditions for detecting OD in comet Halley are also discussed.

Ahearn, M. F.↗

NASA NTRS1985

Onset of sublimation in comet P/Halley (1982i)

The first direct evidence for the onset of sublimation of a comet nucleus is reported. Emission due to CN observed in spectra of the comet P/Halley provides evidence for the development of the gas coma. Broad-band photometric observations of the comet indicate that the dust coma developed near a preperihelion heliocentric distance r of about 6 AU. Rates of gas production and brightening for the comet have been derived at r of 4-6 AU. The mean preperihelion nuclear magnitude derived for the comet was used to calculate an effective radius of the nucleus, which for plausible values of the geometric albedo lies in the range 1-4 km.

Wyckoff, S.↗

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 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

A high resolution solar atlas for fluorescence calculations

The characteristics required of a solar atlas to be used for studying the fluorescence process in comets are examined. Several sources of low resolution data were combined to provide an absolutely calibrated spectrum from 2250 A to 7000A. Three different sources of high resolution data were also used to cover this same spectral range. The low resolution data were then used to put each high resolution spectrum on an absolute scale. The three high resolution spectra were then combined in their overlap regions to produce a single, absolutely calibrated high resolution spectrum over the entire spectral range.

Hearn, 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

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 NTRS1981

Fluorescence equilibrium in the ultraviolet spectra of Comets Seargent (1978 m) and Bradfield (1979 l)

Detailed fluorescence calculations for OH including the Swings effect were performed. These calculations were used to reproduce the high resolution spectra of Comets Seargent and Bradfield taken with IUE. The calculations also provide the OH fluorescence efficiencies (g factors), as a function of heliocentric radial velocity, which are needed to derive OH abundances from measured fluxes. A close examination of the spectra shows no sign of the corresponding emission bands of OD allowing upper limits to be placed on the ratio N(OD)/N(OH). Preliminary attempts to reproduce the CO+ band structure by fluorescence are discussed.

Donn, B.↗