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  • 1
    ISSN: 1573-0794
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The impacts of Comet Shoemaker-Levy 9 left spots on Jupiter with diameters on the order of tens of thousands of kilometers, which have the appearance of debris fields strewn upon the Jovian cloud tops. In this note we employ a measurement of the optical depth of the debris at the impact site of fragment G to estimate mass in the debris field and lower limits to the G fragment mass of 4×1012 – 4×1013 g and diameter of 0.1 – 0.3 km. The masses and sizes of the fragments of Comet Shoemaker-Levy 9 are still uncertain, with estimated sizes ranging from 0.1 to 4 km. The size of the cometary body before breakup is believed to have been between 1 and 10 km. (Asphaug & Benz 1994; Solen 1994; Weaver et al. 1994, Scott & Melosh 1993). These estimates were based on pre-impact images of the cometary fragments. A complimentary technique is to use post-impact images of the spots left on Jupiter to infer the sizes and masses of the fragments. Structure in the underlying clouds is clearly visible through spots imaged by the Hubble Space Telescope, implying that the debris fields are relatively thin. Shortly after the G impact, A'Hearn and collaborators (paper in preparation) used the University of Maryland CCD System at the Perth Observatory to image Jupiter in a variety of bandpasses. While a complete reduction is still underway, a preliminary examination of the raw data shows that the spot at the impact site of fragment G, when near the central meridian roughly three hours after impact, had an average optical depth of roughly 0.05 in several bandpasses between 0.62 and 0.73µm. The measured diameter of the spot was approximately D = 15,000 km. In this note we do not present the data for optical depth, but rather we show that measurements of this type can be used to determine the mass of the solid particles in the clouds and thus to set limits on the mass of the impactor. We assume that the spot consisted of a thin layer of dust in the upper atmosphere. Assuming a one-particle layer covering a fraction of 0.05 of the spot area (a valid assumption for an optically thin cloud), the mass of matter in the spot is M = (0.05π/4) ρdD2, where ρ and d are the particle density and diameter. Particle sizes are not directly measured. However, the particle diameters cannot be much less than 1 µm because the CCD observations when compared with HST ultraviolet images show that extinction is not strongly wavelength dependent at optical and near-uv wavelengths. Typical grain sizes in comets and in the zodiacal dust range from 1 to 10 µm. For particle densities of 0.5 g cm−3 and assumed particle diameters in the range 1 – 10 µm, we find masses, M = 4×1012 – 4×1013 g. Assuming an impactor density of 0.5 g cm−3 (Asphaug & Benz 1994), the corresponding fragment diameters are 0.1 – 0.3 km. Larger sizes for the grains would increase the estimated mass. The observed debris may not be actual comet dust. Since temperatures in the fireball are estimated to be several thousand degrees, all the material in the fragment should have been vaporized (Sekanina et al 1995; Takata et al 1994; Zahnle & MacLow 1994). Therefore the debris material could consist of recondensed matter, perhaps organics, from the fireball. An impactor collides with roughly its own mass of atmospheric material before disruption, so the estimates for the impactor mass hold to order of magnitude even if the debris contains matter with contributions from originally atmospheric gases. The estimate of 0.1 – 0.3 km diameter for the G fragment is a lower limit because the object would also contain material, for example ices, that would not appear in the debris field. Furthermore, since the HST images show structure in the spots that is unresolved in the observations used here, the spot may not be optically thin at all points, but only on average, and this leads to our estimate being a lower limit for the mass of particles. As noted above, the particles are unlikely to be much less than 1 µm in size; particles much larger than 10µm would also imply a larger mass of particles. The derived fragment size is comparable to those estimated from pre-impact observations.
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  • 2
    ISSN: 1572-9672
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract EDISON, a large-aperture, radiatively-cooled telescope, is proposed as the major international mission to follow the current generation of cryogenically-cooled infrared space telescopes. It is being studied at present as a 2.5–3.5 m mixed radiatively- and mechanically-cooled facility optimized to investigate the wavelength range 3–100+ μm. This paper outlines the status of the project, discusses some aspects of a smaller-aperture ‘precursor’ mission, and describes a portion of the baseline science mission.
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  • 3
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The composition of the interstellar silicate dust is investigated. Condensation or alteration of silicate grains at temperatures of a few hundred degrees, in the presence of H2O, would result in hydrous or phyllosilicates, the silicate type most abundant in the type I carbonaceous chondrites. Infrared spectra of small particles (∼0.1 μ) of the high temperature condensates, olivine and pyroxene, at 300 K and 4 K do not give a good match to the interstellar absorption band near 9.8 μ. Laboratory spectra of several phyllosilicates give better agreement as does the spectrum of a carbonaceous chondrite. We propose that the silicates in the interstellar grains are predominantly phyllosilicates and suggest additional spectral tests for this hypothesis.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Infrared spectra (7–40 μ) of the carbonaceous chondrites, Cold Bokkeveld, Murray, and Orgueil are compared with phyllosilicate minerals and interstellar grain spectra. Similarities in the position and shape of the silicate feature near 9.8 μ suggest that similar silicate minerals are present in primitive meteorites and the interstellar dust.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1573-0794
    Keywords: Comet Hale-Bopp ; ISOPHOT ; spectral energy distribution ; dust temperatures
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Comet Hale-Bopp has been observed five times with ISOPHOT, the photometer on board the Infrared Space Observatory (ISO), four times before its perihelion passage at heliocentric distances of 4.92, 4.58, 2.93 and 2.81 AU, and at 3.91 AU postperihelion. Each time, multi-filter photometry covering the range between 3.6–175 μm with eight to ten filters was performed to sample the spectral energy distribution of the comet. These measurements were used to determine dust temperatures for the cometary coma. The evolution of the strength of the silicate feature can be followed in the data as well as the flux deficit at longer wavelengths.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 238 (1972), S. 263-263 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Markarian 231 is such a Seyfert galaxy with a very luminous nucleus. Arakelian et al.6 find an emission redshift of z = 0.041. Using a Hubble constant of 50 km s?1 Mpc-?1 and the photographic magnitude 14.1 given by Zwicky and Herzog7, we find an intrinsic photographic magnitude Mp= ?22.9. (The ...
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  • 7
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 231 (1971), S. 254-255 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] These observations indicate that the spectrum of BL Lac is that of synchrotron radiation from non-thermal electrons, but otherwise little is known abbut the object, for there are no spectral lines from which the usual astronomical analyses might be done, one consequence of which is that even the ...
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  • 8
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 280 (1979), S. 215-217 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] A clear distinction exists between the variable, steep slope QSOs which often show strong and variable optical polarisation, and the less variable QSOs with lower spectral index which do not6'7. The former resemble the BL Lac objects in the steepness of their optical spectra and in their optical ...
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  • 9
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 269 (1977), S. 132-134 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The infrared bands for which there are no positive identifications, or for which the identifications rest on the assignment of a single band, are listed in Table 1. The band at 3.3 Mm has been studied in greatest detail. It is definitely broader than a single emission line and is unresolved at a ...
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  • 10
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 217 (1968), S. 44-45 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The refractive index of quartz varies between 1.52 and 1.57 in the visible region of the spectrum. To determine approximately the size of interstellar silica grains, we have compared the Mie calculation of Lowen2 for m = 1.55 with the extinction curve of Boggess and Borgman3 between 0.3 and ...
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