Engineering topics

Ahearn, M. F.

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

At least 55 records · Page 3

NASA NTRS1981

Introductory remarks: Photometry section

The principal goals of photometry and polarimetry are summarized. These goals include studies of the emission features of gaseous species of comets, the wavelength depenence of albedo, the phase function of scatter distribution, and time variation of rotation. Recent advances in photometric techniques were reviewed and include: use of proper filters, extension to the shortest and longest wavelengths, higher spectral resolving power, and polarizaton techniques.

Ahearn, M. F.↗

NASA NTRS1981

Ground-based photometry of comets in the spectral interval 3000 to 3500 angstrom

Abundances and production rates of CN, C3, and C2 for fifteen comets were determined and the variation of these parameters with helicentric distance for two comets were monitored. While these measurements provide much information about the similarities and differences among comets, only a small fraction of the total material in any of the comets observed was sampled.

Willis, R. L.↗

NASA NTRS1981

Correlated Ground-based and IUE Observations

The value of coordination among different observers particularly, those working in different wavelength regions is highlighted. This coordination is more important among observers of comets than among observers of almost any other class of astronomical object. The basic reason for this is that comets are highly time-variable, often erratically so, and observations of a particular comet usually cannot be repeated. As a result, some uncoordinated observations cannot be interpreted at all while others are susceptible to misinterpretation. Several specific examples of coordinated observations are discussed.

Ahearn, M. F.↗

NASA NTRS1981

Comet Bradfield 1979 X - The gassiest comet

The Comet Bradfield 1979 X provided the most recent opportunity to obtain quantitative photometric data on a comet. Although the comet was not unusually luminous, its very close approach to earth made it possible to observe it from the time of discovery at a heliocentric distance of just over 0.5 AU until it was beyond 1.5 AU from the Sun. The observations conducted are discussed along with aspects of data reduction. It was found that the OH production of comets can be experimentally studied on the basis of a routine procedure. The Comet Bradfield 1979 X was one of the gassiest comets ever observed, comparable to Comet P/Encke. The production rates of all molecular species were found to decline sharply with heliocentric distance.

Ahearn, M. F.↗

NASA NTRS1981

Where is the ice in comets

JHKL photometry of comets P/Tuttle, Meier 1980q, P/Stephan-Oterma, and Bowell 1980b has yielded J-H and H-K colors, uncontaminated by thermal emission, which are nearly identical for all four comets. The colors are inconsistent with the reflection spectrum from a cloud of icy particles, if the particles are composed primarily of any of the ices (H2O, CO2, CH4, or NH3) commonly assumed to be present in the nuclei of comet. Circular variable filter spectra of comet Stephan-Oterma confirm the absence of any absorption features due to these ices. The reflection spectra of these comets appear most like those of Cand S-type asteroids and the ring of Jupiter. Mie calculations for core-mantle particles suggest that ices could be present as thin mantles on inherently reddish, refractory cores, such as magnetite.

Ahearn, M. F.↗

NASA NTRS1981

IUE observations of faint comets

Ultraviolet spectra of seven comets taken with the same instrument are given. The comets P/Encke (1980), P/Tuttle (1980 h), P/Stephan-Oterma (1980 g), and Meier (1980 q) were observed in November and December 1980 with the IUE satellite, and comets P/Borrelly (1980 i) and Panther (1980 u) were observed with the IUE on March 6, 1981. The spectra of these comets are compared with one another, as well as with comet Bradfield (1978 X), which was extensively studied earlier in 1980 with the IUE. To simplify the interpretation of the data and to minimize the dependence upon a specific model, the spectra are compared at approximately the same value of heliocentric distance whenever possible. Effects arising from heliocentric velocity, geocentric distance, and optical depth are also discussed. All of the cometary spectra are found to be remarkably similar, suggesting that these comets may have a common composition and origin.

Weaver, H. A.↗

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.↗

NASA NTRS1981

Chemical abundances in comets

Photoelectric filter photometry and spectrophotometry undertaken to establish homogeneous, quantitative data on chemical abundances in comets is discussed. Absolute column densities of a particular species were derived from a comparison of the absolute flux of an emission feature due to the species with a fluorescence calculation, and converted into species production rates by the use of a Haser model and the species lifetime. Observations of C2 and CN reveal a constant production rate ratio at heliocentric distances up to 1.5-2.0 AU, which implies a compositional homogeneity with depth and the formation of most comets from a homogeneous volume in the solar nebula. Observations of the trace species C3 indicate no apparent variation in relative abundances, while observations of low levels of NH2 and CH imply that NH3 and CH4 are not abundant species. H2O is considered to be the most abundant species based on detections of O, H and OH, while variations in CO or CO2 content may account for the very large differences in cometary gas-to-dust ratios and morphologies.

Ahearn, M. F.↗

NASA NTRS1980

Carbon in Comet Bradfield 1979l

Ultraviolet spectra of comet Bradfield obtained with IUE show a surprising absence of CO(+) despite the presence of CO2(+), and they also show, for the first time, the unambiguous presence of the Mulliken bands of C2. With the use of simultaneous ground-based data, a value of 0.00001 is derived for the transition moment of the a 3 Pi-X 1 Sigma g + transition of C2. The anomalous CO(+)/CO2(+) ratio can be accounted for by the different scale lengths of the species rather than by an abundance difference. Using the model of Feldman (1978), the upper limit for CO(+) is found to be consistent with the observed abundance of CO. On that same model, however, CO cannot produce nearly as much atomic carbon as is observed.

Ahearn, M. F.↗

NASA NTRS1980

Abundance correlations among comets

Results of narrowband filter photometry are presented for comets 1977g, 1978b, 1978f, 1978j, 1979c, and 1979i. There appears to be a remarkable homogeneity among comets with regard to the relative production rates for those species which are readily measured, specifically C2, CN, C3, and OH. Most comets appear to show identical relative abundances, except for a rather well-defined variation with heliocentric distance. Molecular production rates appear to be unrelated to either the emission-to-continuum (gas-to-dust) ratio or the dynamical age of the comet. The correlation with visual magnitudes is excellent, though the slope is not what could be intuitively expected.

Ahearn, M. F.↗

NASA NTRS1980

Vaporization in comets - The icy grain halo of Comet West

The variation with heliocentric distance of the production rates of various species in Comet West (1975n = 1976 VI) is explained with a cometary model consisting of a CO2 dominated nucleus plus a halo of icy grains of H2O or clathrate hydrate. It is concluded that the parents of CN and C3 are released primarily from the nucleus but that the parent of C2 is released primarily from the halo of icy grains.

Ahearn, M. F.↗

NASA NTRS1980

IUE observations of the UV spectrum of Comet Bradfield

Comet Bradfield (1979), discovered shortly after perihelion on Christmas day 1979, is characterized by a retrograde orbit of period approximately equal to 250 yr. This orbit was very favorable for observation by the IUE satellite, and the first observations, the preliminary results of which are reported here, were made on 10 and 11 January 1980. Previously, comprehensive vacuum UV cometary spectra were available only for Comet West (1976VI) from rocket observations and Comet Seargent (1978m) with IUE, hence three comets have now been observed in the wavelength region where most of the major constituents are detectable spectroscopically. The UV spectra of these comets are remarkably similar, despite large differences between them in the gas-to-dust ratio and the different heliocentric distances at which the observations were made, and although the statistical basis is still very small, the implication for cosmogony is that nearly all comets have the same primary composition and origin.

Feldman, P. D.↗

NASA NTRS1980

Spectrophotometry of Comet West

Postperihelion observations of Comet West (1975n = 1976 VI) have been made with a Fourier transform spectrometer at heliocentric distances from 0.57 to 1.68 AU. Measurements were made of the emission bands of C2, CN, C3, CH, and NH2, as well as the emission lines of Na D and forbidden (O I), and the flux in the continuum in nine different bandpasses. Several ratios of the band strengths of CN have been used to determine the two free parameters in the fluorescence equilibrium model of CN of Danks and Arpigny (1973). From the values of the parameters it is inferred that the vibrational transition probability for the ground electronic state is between 0.025 and 0.075 per sec and that the ratio of oscillator strengths between the (0-0) bands of the violet and red systems is between 25 and 30. When corrected for field-of-view effects, NH2 shows no systematic variation in abundance relative to C2 while CH shows a small increase. The cometary continuum is found to be slightly redder than the solar continuum, consistent with results for other bright, dusty comets. The equivalent width of the Delta u = 0 sequence of C2 shows a marked decrease at r(H) = 1.2 AU.

Ahearn, M. F.↗

NASA NTRS1979

Vaporization of comet nuclei - Light curves and life times

The effects of vaporization from the nucleus of a comet are examined and it is shown that a latitude dependence of vaporization can explain the asymmetries in cometary light curves. An attempt is made to explain the observed variation in molecular production rates with heliocentric distance when employing CO2 and clathrate hydrate ice as cometary nuclei substances. The energy balance equation and the vapor pressure equations of water and CO2 are used in calculating the vaporization from a surface. Calculations were carried out from both dry-ice and water-ice nuclei, using a variety of different effective visual albedos, but primarily for a thermal infrared of 0 (emission). Attention is given to cometary lifetimes and light curves and it was determined that the asymmetry in light curves occurs (occasionally) as a 'seasonal' effect due to a variation in the angle between the comet's rotation axis and the sun-comet line.

Cowan, J. J.↗

NASA NTRS1979

Gas and dust in some recent periodic comets

Results are presented for narrow-band photometry of four recent periodic comets. The observed fluxes are used to estimate carbon radical production rates, gas-to-dust ratios, and the colors of the dust grains. It is found that in most respects the periodic comets exhibit the same range of properties typical of parabolic comets. Comet P/Chernykh is one of the dustiest comets on record, and its dust is physically different from that of the other comets considered. A correlation between the gas-to-dust ratio and the color of the grains is tentatively proposed. The need for homogeneous data on a large number of comets is emphasized.

Ahearn, M. F.↗

NASA NTRS1978

Synthetic spectra of C2 in comets

With the key result of Krishna Swamy and O'Dell (1977), a complete fluorescence equilibrium calculation has been used to predict the detailed Swan-band spectrum in comets. A rotational Boltzmann temperature of approximately 3000 K and small changes in published molecular constants lead to a theoretical spectrum in excellent agreement with that observed. The Fox-Herzberg bands are predicted to be quite weak compared with previously observed ultraviolet features in cometary spectra. The Ballik-Ramsay bands should be observable in the near-infrared.

Ahearn, M. F.↗

NASA NTRS1977

Evaporation of ices from Comet West

Postperihelion narrow-band filter photometry has been used to derive gas and dust production rates for Comet West. For heliocentric distances less than 1.25 AU, C2 and CN vary as the -2.8 power of distance, while C3 and OH (based on published measurements) vary as -2.1 power. C3 production closely parallels dust production. It is concluded that OH and C3 parents are produced at the nucleus, while CN and C2 parents are produced from the icy-grain halo. It is also found that Comet West is one of the dustiest and most productive of the recent comets, being exceeded only by Comet Bennett.

Ahearn, M. F.↗

NASA NTRS1975

Spectrophotometry of Comet Kohoutek /1973f = 1973XII/

Spectrum scans have been obtained of Comet Kohoutek with a Fourier spectrometer at heliocentric distances from 0.55 to 1.05 AU. The useful spectral range is roughly 3800-8400 A and the resolution is about 10-15 A at 5000 A. Large variations occurred in the gas-to-dust ratio (pre- versus postperihelion) and in the relative strength of the NH2 bands (with heliocentric distance). There is a smaller change in the ratio of CN to C2 emission. The Swan bands of C2 show the customary anomaly in strengths implying highly non-Boltzmann distributions among vibrational levels. The red and violet CN system fluxes are in slightly better agreement with laboratory data on oscillator strengths than results on previous comets. Other species measured include Na, (O I), CH, and C3.

Ahearn, M. F.↗