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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

The X-ray spectrm of the Coma Cluster of galaxies

The HEAO 1 A-2 and OSO 8 observations of the Coma Cluster of galaxies in the 2-60 keV energy band are presented. The spectrum is not well fitted by an isothermal bremsstrahlung model with an Fe line. To model the spectrum more accurately, we have performed fits of polytropic models and find that the gas is well fitted by an equation of state with polytropic index ranging between 1.38 and 1.6. This analysis reduces the total cluster mass by a factor of 3-5 below the optically derived virial mass. Using the best-fitting parameters, the gas is found to be a significant fraction (25 percent) of the total cluster mass. The Fe K-alpha line blend is strongly detected, from which we infer 0.18-0.36 solar abundance in the X-ray emitting gas. The Fe K-beta line blend is also weakly detected. The spectrum is not consistent with the existence of a hard X-ray tail at the flux level reported by Bazzano el al. (1984). In addition, the HEAO I scanning mode data place strong constraints on possible contaminating sources in the region surrounding Coma.

Henriksen, M. J.↗
NASA NTRS1986

The distribution of dust in the inner coma of comet IRAS-Araki-Alcock (1983d)

The behavior and characteristics of the inner coma of the Comet IRAS-Araki-Alcock (IAA) were examined for five nights in May 1983 using a ground-based 2.2 m CCD telescope. A highly variable, asymmetric sunward emission was detected, along with a long filamentary structure. The brightness was highest in the sunward direction, with radial brightnesses diminishing more slowly than predicted with a 1/r model. The slow fall-off is attributed to the emission of conglomerate dust grains which fragment while traveling through the inner coma and increase the effective surface area of the dust. Significant day-to-day variations were observed, and the filamentary features were indicative of a centralized region on a mantled nucleus which rotated. The nucleus is estimated to be no more than 17 km in diameter.

Storrs, A. D.↗
NASA NTRS1986

Thermal emission from the dust coma of Comet Bowell and a model for the grains

An analysis of the physical characteristics of Comet Bowell is presented based on 10 and 20 microns emissions observed with the NASA 3 m IR telescope during 1981-82. Numerical models are defined for quantifying the number of dust particles at a given radius and at a given angular distance from the nucleus, the contribution of icy grains to the 20 microns flux and the average albedo, and the dust distribution in the coma. Large, dirty ice grains are judged to be responsible for production of OH at a rate of about 10 to the 29th mol/sec during the spring of 1981. The total surface area of the dirty ice grains is projected to be 10 to the 15th sq cm. The coma in 1982 would therefore have consisted of nonvolatile grains, i.e., old dirty ice grains. Further observations of other, similar comets are needed to identify the composition of the emitted grains and the heliocentric distance at which high water production is initiated.

Hanner, M. S.↗
NASA NTRS1986

Origin of metal ions in the coma of P/Giacobini-Zinner

During the crossing of the ICE spacecraft through the coma of the comet Giacobini-Zinner, ions with a mass/charge ratio 24 + or - 1 were unexpectedly found. Na(+) and/or Mg(+) would fit into this mass range. Processes that could lead to high metal ion abundances in cometary comas are discussed and it is concluded that evaporation or sputtering from silicate grains are not likely to be responsible. A remaining possibility is the occurrence of alkali metals in the icy phase as a result of either cometary differentiation or cocondensation of these elements with the icy component prior to the accretion of cometary bodies.

Geiss, J.↗
NASA NTRS1987

Magnetic field draping in the Comet Halley coma - Comparison of Vega observations with computer simulations

During the Vega-1 encounter with Comet Halley, the magnetometer observed draping and compression of the interplanetary magnetic field. These are reproduced well by a three-dimensional MHD simulation of the cometary interaction. Rotations in the magnetic field similar to those at closest approach are also observed 2.75 hours earlier. It is suggested that both rotations correspond to the same IMF interval and that the spacecraft had overtaken the plasma and encountered 'older' magnetic field as it penetrated the coma. Analysis of the MHD model indicates that it should take about 3 to 5 hours for a solar wind parcel to pass from the unperturbed solar wind to Vega-1 at closest approach. A simulated magnetic field profile composed of nested sections for different IMF orientations closely resembles the observations. This result supports the hypothesis of layered magnetic orientations in the coma.

Schwingenschuh, K.↗
NASA NTRS1987

Solar wind effects on the outer ion coma of Comet Halley

A simple two-dimensional model is developed to examine the composition of the cometary ion coma in the region outside the ionopause which is strongly affected by the solar wind. Two-dimensional ion distributions are obtained assuming a cylindrically symmetric ion coma which accounts for the dynamic effects of the mass-loaded solar wind flow around the cometary ionosphere. The results of this model are discussed in the context of analyzing the GIOTTO ion data.

Flammer, K. R.↗
NASA NTRS1987

Infrared emission from P/Halley's dust coma during March 1986

Photometry at 2-20 microns of the inner dust coma of comet Halley was obtained at the NASA IRTF on March 6.85, 12.8, 13.75, 17.7, and 24.8. Positions offset 10 arcsec were measured as well as the central brightness. The strength of the 10 micron emission feature was observed to vary with location in the coma. The infrared emission is in general agreement with the dust size distribution measured from the Vega and Giotto spacecraft. March 6.8, 17.7, and 24.8 corresponded to strong dust jet activity. The strength of the 10 micron silicate emission is shown to be a sensitive indicator of grain size and thus of jet activity. Dust production rate on March 13.75, 6 hr before Giotto encounter, was about 10 to the 7th gm/s.

Hanner, M. S.↗
NASA NTRS1988

On the ammonia abundance in the coma of Halley's Comet

Consideration is given to recent work by Allen et al. (1987) which suggests that the value of the ratio of the abundance of the atomic masses 19 to 18 observed in situ in the inner coma of Comet Halley is incompatible with a pure H2O nucleus and requires an NH3 abundance of 1-2 percent. The present authors discuss possible ways of explaining the radial profile of r = 19/18, which deemphasizes the presence of NH3. It is shown that the observed value of the 19/18 mass ratio in the inner coma does not necessarily imply the existence of a significant abundance of NH3. An elevated UV flux, for which there is circumstantial evidence, could produce the same effect.

Marconi, M. L.↗
NASA NTRS1988

Monte Carlo modeling of the net effects of coma scattering and thermal reradiation on the energy input to cometary nucleus

A Monte Carlo simulation method is presented that can, to an accuracy of a few percent, calculate the effects of a dusty coma on the total energy input to the cometary nucleus. This method treats nonconservative nonisotropic scattering, as well as the reflection from the nucleus surface. Results are presented as a function of the optical thickness of the dust column in the sun-comet axis. The total energy input to the nucleus appears to be only weakly dependent on the opacity of the coma, the radial distribution of the dust, or the details of the extinction processes.

Salo, H.↗
NASA NTRS1989

Confirmation of dust clusters in the coma of Comet Halley

Measurements from the Ducma dust counter and mass analyzer on the Vega spacecraft are compared with those from the SP-1 sensor on the same spacecraft. It is shown that the sequence of pulses recorded by the SP-1 instruments show deviations from a random distribution. Some of the nonrandom sequences observed by SP-1 are in the same time intervals as the clusters observed by the Ducma instrument. The results support the analysis of Ducma data which showed highly structured variations of dust intensity through the coma and large fluxes of low mass particles extending to great distances beyond the coma (Simpson et al., 1986).

Simpson, J. A.↗
NASA NTRS1990

A multigenerational Monte Carlo model of a comet coma

A steady state multigenerational Monte Carlo model was developed to describe the distribution of a neutral coma species which was produced after several photodissociation steps. This distribution can be compared with either line profiles derived from long slit spectroscopic observations or narrow band imaging, and be used to determine the elemental abundances present in the comet coma. A Monte Carlo model was chosen due to limitations in standard forms of more analytic models, such as those of Haser and Festou. In contrast to the analytical models, the Monte Carlo method can be modified to include new physical parameters.

Ferro, Anthony J.↗
NASA NTRS1991

HUT observations of carbon monoxide in the coma of Comet Levy (1990c)

Observations of comet Levy (1990c) were made with the Hopkins Ultraviolet Telescope during the Astro-1 Space Shuttle mission on 10 Dec. 1990. The spectrum, covering the wavelength range 415 to 1850 A at a spectral emission of 3 A (in first order), shows the presence of carbon monoxide and atomic hydrogen, carbon, and sulfur in the coma. Aside from H I Lyman-beta, no cometary features are detected below 1200 A, although cometary O I and O II would be masked by the same emissions present in the day airglow spectrum. The 9.4 x 116 arcsec aperture corresponds to 12,000 x 148,000 km at the comet. The derived production rate of CO relative to water, 0.13 + or - 0.02, compared with the same ratio derived from IUE observations (made in Sep. 1990) which sample a much smaller region of the coma, 0.04 + or - 0.01, suggests the presence of an extended source of CO, as was found in comet Halley. Upper limits on Ne and Ar abundance are within an order of magnitude or solar abundances.

Feldman, P. D.↗
NASA NTRS1990

Distributed coma sources and the CH4/CO ratio in Comet Halley

Early analyses of the Ion Mass Spectrometer (IMS) data from the Giotto flyby of Comet P/Halley indicated significant abundances of CH(+) sub n (n = 1 to 4). The source of these ions was assumed to be frozen CH4 in the nucleus. An abundance of about 2 percent CH4 was consistent with this interpretation, resulting in a ratio of CH4/CO that is greater than the predicted limits for interstellar clouds or the solar nebula. However, subsequent analyses of data from the Giotto Neutral Mass Spectrometer indicate distributed sources of CO and H2CO in the coma that are most likely associated with organic (CHON) particles, rich in CH-bearing compounds that decay and produce CH(+) sub n species. A model is presented that qualitatively accounts for the measured spatial distribution of CO and H2CO, and indicates that most of the CH(+) sub n deduced from the Giotto IMS data may originate from organic compounds in the coma dust and not from volatiles released directly from the nucleus. As a consequence, the absence of CH4 in the icy phase of the nucleus is consistent with the observations with an upper limit of about 0.5 percent to the ratio CH4/CO. This is an important criterion for theories of comet formation.

Boice, D. C.↗
NASA NTRS1991

Negative ions in the coma of Comet Halley

The detection of negatively charged cometary ions in the inner coma of Comet Halley is reported. These ions are observed in three broad mass peaks at 7-19, 22-65, and 85-110 AMU, with densities reaching greater than about 1/cu cm, about 0.05/cu cm, and about 0.04/cu cm, respectively, at a distance of about 2300 km from the nucleus. The ion species thought to be present include O(-), OH(-), C(-), CH(-), CN(-) and heavier complex CHO molecular ions. As negative ions are easily destroyed by solar radiation at about 1 AU, an efficient production mechanism, so far unidentified, is required to account for the observed densities. The detection of negative ions in the coma near 1 AU implies that negative ions should also be present in similar neutral gas and dust environment farther away from the sun. If the negative-ion densities are large enough, they could play an important part in physical processes such as radiative transfer or charge exchange.

Chaizy, P.↗
NASA NTRS1992

NH3 and NH2 in the coma of Comet Brorsen-Metcalf

Evidence consistent with NH3 ice in the nucleus of Comet Brorsen-Metcalf as the source of the NH2 observed in the comet coma is presented. The distribution of NH2 is symmetric and shows no evidence for jet structure at the 3-sigma significance level above background emission. An azimuthal average of the NH2 image produces an NH2 surface brightness profile for Comet Brorsen-Metcalf which yields a factor of about-10 improvement in the signal-to-noise ratio over previous 1D long-slit NH2 observations, and provides a significant constraint on the NH2 photodissociation time scale in comets. A Monte Carlo simulation of the comet coma, assuming that NH2 is the primary source of NH2, is described and compared with the observations. For an observed production rate, Q(H2O) is approximately equal to 7 x 10 exp 28 molecules/s, collisional effects on the NH3 and NH2 outflow had at most an approximately 10-percent effect on the NH2 surface brightness profile. Because Comet Brorsen-Metcalf showed no significant dust or gas production rate variability, it is argued that steady state conditions best match the comet at the time of the observations.

Tegler, Stephen C.↗
NASA NTRS1992

Inner coma imaging of Comet Levy (1990c) with the Hubble Space Telescope

HST observations of Comet Levy at geocentric and heliocentric distances of about 1 AU show a highly asymmetrical coma in which the sunward-facing hemisphere is brighter than the tailward one by a factor of 2; this is in keeping with dayside volatile sublimation. Radial brightness profiles perpendicular to the sun-comet line are found to be highly symmetric about the nucleus. Detailed image analysis reveals indications of a hemispherical dust arc which propagates through the coma at an average projected velocity of about 0.16 km/sec. It is suggested that periodic occurrences of such dust arcs could account for the temporal variability in IUE continuum photometry.

Weaver, H. A.↗
NASA NTRS1992

The origin of complex organic ions in the coma of Comet Halley

The Giotto-PICCA instrument has obtained heavy-ion mass spectra in the inner coma of Comet Halley; the spectra exhibit a series of distinct mass groups separated by a constant about 15 amu, and extending to at least 120 amu, whose pattern is typical of CHON molecules. It is found that neither coma gas-phase chemistry nor UV photon, solar wind particle, or cosmic irradiation of the nucleus in its current orbit can generate enough heavy molecules to account for the PICCA abundances. It is suggested that these abundances may be supplied by UV and cosmic irradiation of the icy, precometary grains within interstellar molecular clouds and/or the protosolar nebula.

Mitchell, D. L.↗
NASA NTRS1992

CCD imaging of the inner coma jets of comet P/Halley

We analyze the inner coma section of a CCD image of comet P/Halley taken at 1807 UT on 13 March 1986 using a C2 filter (wavelength 5000 to 5200A, half maximum) with the 3.8 m Anglo Australian Telescope at Siding Springs, Australia. Atmospheric turbulence leads to a spreading of the image detail and this produces a blander image of the inner coma region with a slower radial decrease of brightness in comparison to the unaffected image. We remove this smearing by utilizing the point spread function of a star on the same CCD image. Jets were then revealed by removing the average background. Analysis of the jet structure enabled us to estimate the lower limit of the parent molecule velocity. This is found to be 0.3 km s(exp -1).

Boswell, James↗