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  • ASTROPHYSICS  (40)
  • ISM: dust  (2)
  • Astrophysics  (1)
  • 1
    Digitale Medien
    Digitale Medien
    The Distributed CO in Comet Halley (1999)
    Springer
    Space science reviews 90 (1999), S. 69-74 
    Details
    ISSN: 1572-9672
    Schlagwort(e): ISM: dust ; Comets: general ; Comets: Halley
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract Examination of the spatial distribution of CO intensity of Comet Halley indicates that a large fraction of CO originates from the refractory organic component in the coma, rather than directly from the volatiles in the nucleus. Based on the fluffy aggregate interstellar dust comet model, we have estimated the upper limits of the total amount of CO provided by coma dust. The implications from the comparison of the predicted results with the observed value have been discussed.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1005277410127
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  • 2
    Digitale Medien
    Digitale Medien
    Morphological Structure and Chemical Composition of Cometary Nuclei and Dust (1999)
    Springer
    Space science reviews 90 (1999), S. 149-161 
    Details
    ISSN: 1572-9672
    Schlagwort(e): ISM: dust ; ISM: molecules ; ISM: abundances ; Comets: general ; Comets: individual: Halley, Borrelly, Hale-Bopp ; Stars: individual: β Pictoris ; Stars: circumstellar matter ; Stars: planetary systems
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract The chemical composition of comet nuclei derived from current data on interstellar dust ingredients and comet dust and coma molecules are shown to be substantially consistent with each other in both refractory and volatile components. When limited by relative cosmic abundances the water in comet nuclei is constrained to be close to 30% by mass and the refractory to volatile ratio is close to 1:1. The morphological structure of comet nuclei, as deduced from comet dust infrared continuum and spectral emission properties, is described by a fluffy (porous) aggregate of tenth micron silicate core-organic refractory mantle particle on which outer mantles of predominantly H2O ices contain embedded carbonaceous and polycyclic aromatic hydrocarbon (PAH) type particles of size in the of 1 - 10nm range.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1005298014670
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  • 3
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    Organic Chemistry in Interstellar Ices: Connection to the Comet Halley Results (1997)
    In:  CASI
    Details
    Publikationsdatum: 2004-12-03
    Beschreibung: Mass spectroscopic measurements on the gas and dust in the coma of Comet Halley revealed the presence of considerable amounts of organic species. Greenberg (1973) proposed that prior to the formation of the comet UV processing of the ice mantles on grains in dense clouds could lead to the formation of complex organic molecules. Theoretical predictions of the internal UV field in dense clouds as well as the discovery in interstellar ices of species like OCS and OCN- which have been formed in simulation experiments by photoprocessing of interstellar ice analogues point to the importance of such processing. We undertook a laboratory simulation study of the formation of organic molecules in interstellar ices and their possible relevance to the Comet Halley results.
    Schlagwort(e): Astrophysics
    Materialart: Analysis of Returned Comet Nucleus Samples; 217-224; NASA/CP-1997-10152
    Format: text
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  • 4
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    Explosive desorption of icy grain mantles in dense clouds (1991)
    In:  Other Sources
    Details
    Publikationsdatum: 2011-08-19
    Beschreibung: The cycling of the condensible material in dense clouds between the gas phase and the icy grain mantles is investigated. In the model studied, desorption of the ice occurs due to grain mantle explosions when photochemically stored energy is released after transient heating by a cosmic ray particle. It is shown that, depending on the grain size distribution in dense clouds, explosive desorption can maintain up to about eight percent of the carbon in the form of CO in the gas phase at typical cloud densities.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astronomy and Astrophysics (ISSN 0004-6361); 244; 1, Ap; 190-204
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  • 5
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    Detection of solid methanol toward W33A (1991)
    In:  Other Sources
    Details
    Publikationsdatum: 2011-08-19
    Beschreibung: A recently detected absorption feature at 3.53 microns in the spectrum of W33A has been assigned to methanol (CH3OH). Its optical depth implies that methanol is the second most abundant molecule (7 percent relative to H2O) in the grain mantles in the line of sight toward W33A observed to date. Laboratory experiments have shown that the implied abundance is difficult to explain by UV irradiation of the dust grains alone. Grain surface reactions or condensation directly out of the gas phase must also play roles, but the relative contributions of the various processes are difficult to estimate. The optical depth of the 3.53-micron feature constrains the contribution of methanol to the 6.8-micron feature in W33A to be 10 percent or less, requiring the contribution of at least one other compound to this feature, while the estimated contribution to the 4.9-micron absorption band is even smaller. Only a small contribution of formaldehyde (H2CO) is consistent with the observed 3.53-micron band profile. In contrast to methanol, formaldehyde can be produced readily by photochemical reactions within the ice mantle.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astronomy and Astrophysics (ISSN 0004-6361); 243; 2, Ma
    Format: text
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  • 6
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    Time-dependent chemistry in dense molecular clouds. II - Ultraviolet photoprocessing and infrared spectroscopy of grain mantles (1986)
    In:  Other Sources
    Details
    Publikationsdatum: 2011-08-19
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astronomy and Astrophysics (ISSN 0004-6361); 158; 1-2,
    Format: text
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  • 7
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    Laboratory identification of the emission features near 3.5 microns in the pre-main sequence star HD 97048 (1985)
    In:  Other Sources
    Details
    Publikationsdatum: 2011-08-19
    Beschreibung: This paper presents the results of a laboratory study of formaldehyde (H2CO) suspended in low temperature molecular matrices with compositions similar to what may be found in the 'dirty ice' mantles of grains. It is shown that the emission features near 3.5 microns in the pre-main-sequence star HD 97048 can be matched by a mixture of chemical complexes of H2CO with surrounding molecules in the grain. Furthermore, a discussion is presented of various possible excitation mechanisms for this emission. The conclusion is, that for the features near 3.5 microns in HD 97048, UV pumped IR fluorescence is the most likely mechanism.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astronomy and Astrophysics (ISSN 0004-6361); 145; 1, Ap; 262-268
    Format: text
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  • 8
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    Interstellar circular polarization and the dielectric nature of dust grains (1989)
    In:  CASI
    Details
    Publikationsdatum: 2013-08-31
    Beschreibung: The implications of the observed relationship between the wavelength dependence of interstellar circular and linear polarization were reexamined. Mie theory calculations for grains with various optical constants demonstrate that any population of grains which matches the observed wavelength dependence of linear polarization also yields the correct cross-over wavelength of circular polarization. The coincidence of the peak wavelength of linear polarization and the cross-over of circular polarization is therefore independent of the optical constants of the grains and cannot be used as a critical constraint on grain properties. The observed relationship instead reflects a more fundamental connection between linear and circular polarization which was derived from the Kramers-Kronig relations by Shapiro (1975). Numerical results fully support Shapiro's conclusions and demonstrate that the apparent upper limit on the visual absorptivity of polarizing grains deduced from earlier Mie theory calculations (Martin, 1972) was spurious and resulted from a violation of the Kramers-Kronig relations in the assumed optical constants of the particles. The Kramers-Kronig interpretation of circular polarization can be used to place constraints on linear polarization outside the wavelength range in which it was observed. This approach was used to show that the peak observed in the visual is likely to be the only significant feature in the linear polarization curve, which therefore appears to be well approximated at all wavelengths by the Serkowski formula. A synthesis of available laboratory data was used to analyze the properties of dielectric core-mantel grains as the source of visual extinction and polarization.
    Schlagwort(e): ASTROPHYSICS
    Materialart: NASA, Ames Research Center, Interstellar Dust: Contributed Papers; p 33
    Format: application/pdf
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  • 9
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    A comparative study of the continuum and emission characteristics of comet dust. 1: Are the silicates in Comet Halley and Kohoutek amorphous or crystalline (1989)
    In:  CASI
    Details
    Publikationsdatum: 2013-08-31
    Beschreibung: A continuum emission was subtracted from the 10 micron emission observed towards comets Halley and Kohoutek. The 10 micron excess emissions were compared with BN absorption and laboratory amorphous silicates. The results show that cometary silicates are predominantly amorphous which is consistent with the interstellar dust model of comets. It is concluded that cometary silicates are predominantly similar to interstellar silicates. For a periodic comet like Comet Halley, it is to be expected that some of the silicate may have been heated enough to convert to crystalline form. But apparently, this is only a small fraction of the total. A comparison of Comet Halley silicates with a combination of the crystalline forms observed in interplanetary dust particles (IPDs) seemed reasonable at first sight (Walker 1988, Brownlee 1988). But, if true, it would imply that the total silicate mass in Comet Halley dust is lower than that given by mass spectrometry data of Kissel and Krueger (1987). They estimated m sub org/m sub sil = 0.5 while using crystalline silicate to produce the 10 micron emission would give m sub org/m sub sil = 5 (Greenberg et al. 1988). This is a factor of 10 too high.
    Schlagwort(e): ASTROPHYSICS
    Materialart: NASA, Ames Research Center, Interstellar Dust: Contributed Papers; p 423-427
    Format: application/pdf
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  • 10
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    Scattering by fluffy grains (1989)
    In:  CASI
    Details
    Publikationsdatum: 2013-08-31
    Beschreibung: There are indications (Greenberg et al., 1988), that fluffy (i.e., porous) particles are responsible for the observed 3.4 and 10 micron emissions of comet Halley. The absorption characteristics of small particles both solid and fluffy are needed in order to explain the Halley emissions. How isolated small solid particles react to an external radiation field is well known - the Rayleigh approximation. How these same small particles emit when assembled as fluffy aggregates in another question. To what degree are the emission spectra of isolated and aggregated particles comparable. In order to quantify the assertion that fluffy particles produce the observed Halley infrared emission features, the authors are performing calculations to determine the effect of porosity on the absorption characteristics of aggregates of interstellar grain-type particles. The calculations are based on an integral representation of the scattered electromagnetic field. Results are given with application to comet Halley.
    Schlagwort(e): ASTROPHYSICS
    Materialart: NASA, Ames Research Center, Interstellar Dust: Contributed Papers; p 371
    Format: application/pdf
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