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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS2021

The Volatile Composition of the Inner Coma of Comet 46P/Wirtanen: Coordinated Observations Using iSHELL at the NASA-IRTF and Keck/NIRSPEC-2

The 2018 perihelion passage of comet 46P/Wirtanen afforded an opportunity to measure the abun-dances and spatial distributions of coma volatiles in a Jupiter-family comet with exceptional spatialresolution for several weeks surrounding its closest approach to Earth (∆min∼0.078 au on UT Decem-ber 16). We conducted near-infrared spectroscopic observations of 46P/Wirtanen using iSHELL at theNASA Infrared Telescope Facility on UT 2018 December 18 in direct coordination with observations using the newly upgraded NIRSPEC-2 instrument at the W. M. Keck Observatory, and securely de-tected fluorescent emission from CH3OH, C2H6, and H2O. This coordinated campaign utilizing the twopremier near-infrared facilities in the northern hemisphere enabled us to sample distinct projections ofthe coma into the plane of the sky simultaneously, and provided an unprecedented view into the innercoma of 46P/Wirtanen near closest approach. We report rotational temperatures, production rates,and abundance ratios (i.e., mixing ratios) for all sampled species and compare ouriSHELLresultsto simultaneous (or near-simultaneous) measurements taken with NIRSPEC-2. We demonstrate theextraordinary synergy of coordinated measurements using iSHELL and NIRSPEC-2, and advocate forfuture cometary studies that jointly leverage the capabilities of these two facilities.

Molecular spectroscopy;↗
NASA NTRS2003

Improved models for the evolution of the coma cluster of galaxies

The analysis by Fitchett & Webster of the observations of the Coma Cluster of galaxies has demonstrated that the center of the Coma Cluster consists of two subclusters. Therefore, it is important to construct realistic dynamical models of a galaxy cluster with two mass centers.

dark↗
NASA NTRS1972

Observations of the extended X-ray sources in the Perseus and Coma clusters from Uhuru.

The X-ray source in Perseus identified as NGC 1275 is found to have a finite angular extent of about 35 arc minutes. The improved location of the center of the emission is consistent with NGC 1275. An improved location for the center of the Coma X-1 source is consistent both with the kinematic center of the cluster and with NGC 4874. These extended sources - Coma, Perseus, and the source in the Virgo cluster - may be a new class of X-ray objects associated with active galaxies in rich clusters.

Forman, W.↗
NASA NTRS1974

Current constraints on hidden mass in the Coma Cluster

Various possible means of accounting for the dynamical mass of the Coma Cluster are reviewed. We intentionally choose to emphasize conventional forms of hidden mass, specifically gas and low-luminosity stars, while adopting conventional mass-to-luminosity ratios for the observed luminous material in galaxies. We have attempted to combine all previous work relating to assumed gas parameters in order to examine precisely what constraints current observations, together with theoretical and observational upper limits, place on any gas in the Coma Cluster. In particular, we make use of a density distribution function that is consistent with the dynamical data and express all constraints in a manner which explicitly displays their dependence on Hubble constant, current observational flux levels, distance from the dynamical center of the cluster, and the mass of gas postulated.

Tarter, J.↗
NASA NTRS1975

Studies of cluster X-ray sources, energy spectra for the Perseus, Virgo, and Coma clusters

Final Uhuru X-ray differential-energy spectra are presented for the Perseus, Virgo, and Coma clusters. Power-law and isothermal bremsstrahlung model spectra with low-energy cutoffs are given, and the energy-dependent Gaunt factor is calculated for the bremsstrahlung. The spectra, which are best fits to the Uhuru data between 2 and 10 keV, are compared with previous observations of these sources in the energy range from 0.1 to 100 keV. The problem of parameter estimation is discussed, error bars with 68% confidence are given for the independently determined slope and cutoff parameters, and the 68% confidence limits are plotted for the fitted spectral functions. The data for Perseus above 20 keV marginally favor the bremsstrahlung fit, those for Virgo between 0.25 and 1.0 keV clearly favor that curve, and those for Coma indicate a low-energy turnover or cutoff. Implications of such a cutoff are briefly discussed.

Kellogg, E.↗
NASA NTRS1976

Coma of modified Gregorian and Cassegrainian mirror systems

The equivalence of the classical Newtonian, Cassegrainian, and Gregorian mirror systems with respect to the first two Seidel aberrations is rederived by means of a simple congruence. The effects of arbitrary small modifications of the two mirror systems are then studied and general formulas are derived for the effects of such modifications on the spherical aberration and coma. Spherical aberration is corrected to the third order if the amount of glass removed from one surface is replaced at the corresponding zone of the other surface. Modifications in which one surface is made spherical while the other is adjusted to eliminate spherical aberration result in large increases of coma for systems having the usual amplifying ratios.

Jones, R. T.↗
NASA NTRS1976

An ultrasoft X-ray source in Coma Berenices

An intense soft X-ray source with an extraordinary spectrum has been observed in Coma Berenices, 4 deg northeast of and not associated with the Coma cluster of galaxies. Two spectra, obtained at different times in a sounding-rocket flight, indicate that the source temperature in thermal models is less than 1 million K; a power-law model requires photon power-law indices steeper than n = -3. The intensity in the 44-165 A band is of the order of 5 by 10 to the -10th power erg/sq cm/sec, but no flux is present at energies 0.3-2.1 keV to a limit of 1 by 10 to the -10th power erg/sq cm/sec. The lack of bright stars or a supernova remnant in the error box implies that this may be a new class of soft X-ray sources.

Margon, B.↗
NASA NTRS1976

The Perseus and Coma clusters of galaxies at energies above 20 keV

Observations of the Perseus and Coma clusters of galaxies were made on June 21, 1974, with a balloon-borne X-ray telescope (energies of approximately 20-150 keV). No positive detection was made. The data favor a thermal bremsstrahlung mechanism for the X-ray production in the Perseus cluster of galaxies over the inverse Compton mechanism. In the case of the Coma cluster of galaxies, the data are inconclusive with respect to determining the origin of the X-rays.

Scheepmaker, A.↗
NASA NTRS1976

Soft X-ray morphology of the Virgo, Coma, and Perseus clusters of galaxies

Soft X-ray emission profiles of the Virgo, Coma, and Perseus clusters of galaxies with 9.2-arcmin resolution are presented for the energy band from 0.5 to 3 keV. For Virgo, a profile is also given for the band from 0.1 to 0.28 keV. The profiles are compared with various models including extended and point sources, particularly a model with an extended source involving thermal bremsstrahlung from a hot gas located in the gravitational potential well of the cluster. It is found that the soft X-ray emission from Virgo is extended, symmetric, and centered on M87; that the emission from Coma is not dominated by any single galaxy; and that the emission from Perseus is centered within 3 arcmin of NGC 1275 and is narrower along a north-south axis than along the east-west axis containing the line of bright galaxies. Limits are set on the strength of possible point sources.

Malina, R.↗
NASA NTRS1979

Soft X-ray structure of the Coma cluster of galaxies

Results are reported for an observation of the Coma cluster with an imaging X-ray telescope system aboard a sounding rocket. A two-dimensional X-ray map of the cluster in the energy range from 0.15 to 2.0 keV is presented. The radial distribution, granularity, and ellipticity of the X-ray surface brightness are investigated, and the spectrum and flux within the field of view are analyzed. These properties are then examined quantitatively by comparing the observations with models through the use of a chi-squared test. It is found that: (1) no single galaxy contributes more than a few percent of the total X-ray flux of the Coma cluster; (2) the X-ray source is diffuse but granular on a scale of several arcmin, particularly in the central region; (3) the centroid of the X-ray source is at a local intensity minimum; and (4) the data are most consistent with an isothermal hydrostatic model.

Gorentstein, P.↗
NASA NTRS1981

On the equilibrium distribution of the elements in the gas in the Coma cluster

The equilibrium distribution of the elements for the case of an isothermal plasma in a gravitational potential well is given governing equations and a rapidly converging iterative procedure for their solution. Results are applied to the intracluster gas in clusters of galaxies, and implications are identified for the controversy as to whether iron nuclei settle out in the cluster core, and with respect to the interpretation of distribution size and shape for continuum X-ray emission from clusters. A model for the Coma cluster is considered in which the intracluster gas is in equilibrium and partially processed. It is found that this model can account for the angular dependence of the continuum X-ray emission from Coma, as determined by Einstein Observatory observations, at least as well as the uniform composition model.

Abramopoulos, F.↗
NASA NTRS1981

Ultraviolet absorption studies of H2O and other species in comet comae with satellite telescope-spectrometers

It is noted that ultraviolet absorption by H2O and other species in the comae of comets could be detected by studying, with satellite telescope-spectrometers, the occultation of hot stars by comets. Observations of this type could produce the first detection of H2O, the fundamental parent molecule in comet comae, and give measures of molecular level populations. The capabilities of the High Resolution Spectrograph on Space Telescope, the first instrument suitable for such observations, are discussed. A Haser model is used to estimate the molecular column densities and to predict equivalent widths for lines of H2O, OH, CO, C, and O as functions of time and angular distance from a comet with a high H2O production rate. The minimum detectable equivalent widths are determined, thereby giving the maximum angular separation from such a comet at which H2O, OH, and CO could be studied. Estimated equivalent widths for CO, OH, and the resonance lines of C and O suggest that these species may also be detected.

Smith, P. L.↗
NASA NTRS1982

A model of a comet coma with interstellar molecules in the nucleus

The coma of a comet is modeled assuming the icy nucleus contains interstellar molecules. This composition overcomes discrepancies between observation and earlier model predictions for CN, C2, C3, and NH2 abundances. It is found that the abundances of CN, C2, and C3-bearing compounds in the nucleus must be constrained to trace amounts in order to account for the observed column densities. NH3 also cannot be abundant by more than about 1%. The model gives good agreement with observed relative ranges of neutral species in the coma as well as with their observed relative intensity dependence on heliocentric distance. The size of the nucleus and the heliocentric ranges considered are relevant to comet Halley.

Biermann, L.↗
NASA NTRS1982

Coma-type radio halos and cluster X-ray morphology

It is pointed out that there seems to be a characteristic X-ray morphology for clusters with Coma-type radio halos. This correlation may be the first evidence that the conditions conducive to formation of a radio halo influence or are influenced by the thermal gas distribution. It is suggested that heating of the intracluster gas by the cosmic rays that power the radio halo may be the source of the correlation. The anomalous gas temperature of Coma may be the result of this process. Future observations of cluster temperatures and velocity dispersions should provide a test of the cosmic-ray heating hypothesis

Vestrand, W. T.↗
NASA NTRS1984

Coma morphology and dust-emission pattern of periodic Comet Halley. I - High-resolution images taken at Mount Wilson in 1910

A radial/rotational shift-difference algorithm has been developed to improve visibility of the May-June 1910 high-resolution images taken at Mount Wilson of Comet Halley. Dust features for a period of up to three days were identified, which consisted of discrete active areas emitting dust continuously from the sunlit hemisphere of the rotating nucleus. A lower limit of the comet's rotation period was approximated at one day, and expansion velocities of the dust features were determined to be in the range of 0.2-0.3 km/s. Based on relative photometry of a bright jet, it was proposed that the column density of dust ejecta in the jet exceeded the density in the coma by a factor greater than or approximately equal to two, and that, if narrow along the line of light, the jet might have had a particle number concentration much higher than the coma background. Also noted are numerous ion features.

Larson, S. M.↗
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 NTRS1985

Nucleus precession of periodic comet Comas Sola

The nuclear properties of the periodic comet Comas Sola are studied based on a precession model applied previously to the periodic comets Encke, Kopff, and Giacobini-Zinner. The results imply that, for a few revolutions about the sun, Comas Sola was precessing more rapidly than any comet studied to date. An explanation is offered for this behavior in terms of a perturbation in the comet's obliquity shortly after the 1952 passage through perihelion. The equatorial radius of the nucleus is close to 1 km and its rotation period is 1.5-2.3 days, according to the model results. The calculated shape of the nucleus is compared with those of other comets studied using this technique.

Sekanina, Z.↗
NASA NTRS1985

C2 imagery of the inner coma of Comet IRAS-Araki-Alcock

May 10, 1983 observations of the comet IRAS-Araki-Alcock are analyzed to determine the spatial molecular abundance of C2 in the inner coma, via the Delta-v = +1 Swan band sequence near 4690 A. The observations indicate a deficiency of C2 emission across a projected diameter of about 2000 km that is centered on the peak of continuum emission. Comet imagery indicates a sunward-pointing coma that suggests an outburst of subsurface volatile ices through a nonvolatile surface crust, as predicted for periodic comets. These results are discussed in the framework of instrumental and observational constraints.

Oliversen, R. J.↗