1,721,033 research outputs found
MASS EXTINCTION COEFFICIENTS OF VARIOUS SUBMICRON AMORPHOUS-CARBON GRAINS - TABULATED VALUES FROM 40 NM TO 2 MM
No full textAmorphous carbon grains are considered among the main components of cosmic dust. The characterization in the laboratory of their physico-chemical properties may represent a substantial contribution to the interpretation of astronomical observations. In the frame of the experimental activity developed in our laboratory on cosmic grain ''analogues'', we have performed a systematic analysis in order to provide a quantitative determination of the extinction properties of various amorphous carbon samples in a wide spectral range, from the vacuum UV to the mm regions. Particular care has been placed to eliminate possible systematic errors introduced by dust-matrix interaction. The extinction signatures of both hydrogenated and dehydrogenated carbon grains have been analysed, also on the ground of other experimental results obtained by means of complementary analytical techniques. The measured general spectral trend, as well as UV and IR typical features, are interpreted taking into account the intrinsic properties of our samples. The results are compared with previous experimental data and with the output of theoretical models which make use of carbon optical constants. The present quantitative data offer the opportunity to interpret in a more complete way the characteristics of carbonaceous analogues and can be helpful to infer information about cosmic dust. On the other hand, the wide spectral range covered by our measurements provides a set of self-consistent data which can be applied to determine the optical constants of small amorphous carbon grains
The sensitivity of the size distribution to the grain dynamics: Simulation of the dust flux measured by GIOTTO at P/Halley
No full textA cometary dust emission model, based on rigorous keplerian dynamics, is developed and, for the first time, the dependence of fluence on the probability distribution of the dust ejection velocity vector is demonstrated. The results are compared with the fluences measured by the DIDSY experiment on board the GIOTTO spacecraft during the Halley's fly-by in 1986. A fit of the total fluence is obtained and an interpretation of the observed differences, between pre and post fly-by, is proposed. From the best-fitting process we conclude that the dust ejection from P/Halley was strongly anisotropic and mainly Sun-ward oriented with an angular dispersion of 18.4°, for the adopted Gaussian distribution. The most probable velocity at the fly-by is 50 ± 5 m s-1 for 1 mm sized grains and the power index of the velocity size-dependence is -0.5. Both these results agree with those of dust-gas drag models. Moreover, the dust velocity presents a wide dispersion (35 ±5 m s-1), which explains the velocity size-dependence derived by Neck-Line photometry. For grains larger than 20 fim, the power index of the differential size distribution is constant (a = -3.5 ± 0.2). Since a > -4, most of the dust mass is released in the form of large grains. The dust to gas ratio is x = 4 ± 1. The last two conclusions agree with the output of previous DIDSY fitting processes and are compatible with inverse dust tail models; they must be considered the best constrained results coming from the DIDSY experiment. Our results imply that future in-situ cometary experiments will have to measure both mass and velocity vector for each grain, in order to determine the dust size distribution
The radial brightness dependence in the dust coma of comet P/Grigg-Skjellerup
No full textIn a previous paper (Fülle et al. 1993a) we have obtained a three-dimensional coma model of the short period comet P/Grigg-Skjellerup, by fitting ground-based images taken in coincidence with the fly-by of the GIOTTO spacecraft (GEM mission). Concerning the cometary dust environment, we confirmed the excess of large grains measured by the in-situ experiment GEM-DIDSY (McDonnell et al. 1993). In this letter we extend the comparison with space-born data by considering the results provided by the GEM-OPE experiment (Levasseur-Regourd 1993). The same model, once projected along the line of sight of GEM-OPE, is able to produce radial brightness slopes in quite good agreement with those obtained from the in-situ experiment. This result confirms the physical reliability of the proposed dust environment
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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