1,721,097 research outputs found
Visible Spectroscopy of Possible Cometary Candidates
No full textIt is now accepted that comets and asteroids are not two well separated classes of objects. In particular, several peculiar objects have been recognized to resemble comets in several aspects: chemical, dynamical, and/or physical. In this context we spectroscopically observed in the visible region three strong cometary candidates: 944 Hidalgo, 2201 Oljato, and 3200 Phaethon. The spectrum of Hidalgo obtained has a reflectivity slope similar to those of known cometary nuclei. Phaethon shows a spectrum with a negative slope. The spectrum of Oljato shows a strong absorption feature around 0.9 mm which is suggestive of the presence of aqueous altered materials. No clear signs of cometary activity have been found in the spectra of these objects
The formation of binary asteroids as outcomes of catastrophic collisions
No full textA semi-empirical model of catastrophic break-up processes among asteroids is used to investigate the possibility that binary asteroids could be formed by collisional events. In particular, numerical integrations of the dynamical evolution of fragments under the effect of mutual gravitational interactions after ejection are performed for a set of simulations, in order to detect the possible formation of binary systems. The results suggest that the occurrence of binaries formed by couples of fragments of comparable mass is fairly rare. Therefore it is estimated that such systems could exist but they should not be very frequent in the asteroid belt. On the other hand, binaries with very small mass ratios, like the Ida-Dactyl binary system, might be more frequent. However, in these cases fragments are dealt with in a size range which is poorly sampled by the model. For this reason, some more detailed analysis seems necessary for assessing more reliably the efficiency of break-up phenomena as sources of Ida-Dactyl-like systems
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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Spectroscopic observations of Edgeworth-Kuiper Belt (EKB) objects
No full textTo increase our knowledge of the physical properties of the Edgeworth-Kuiper Belt (EKB) objects, we started an observational program at the European Southern Observatory with the 3.5 m NTT. The aim of our observations is to obtain UBVRI colors for a large sample to examine the spectral diversity of the population and identify candidate objects for higher resolution studies of the surface composition, to determine precise magnitudes from which diameter estimates can be made, and to measure their lightcurves from which rotation rates and shape constraints can be derived. To minimize systematic errors in the colors caused by rotational lightcurve variations, we interpolated observations through other filters with multiple observations through the same filter so that interpolation can be performed (e.g. V-B-V-R-V-I-V). The first results concern six objects (1994 TB, 1994 JR1, 1995 QY9, 1996 TL66,1996 TP66, and 1996 TO66). Two of them (1996 TL66 and 1996 TO66) have a spectrum similar to those of dark D-type asteroids, while the other have redder spectra. We analysed all the BVRI color data available in the literature (about 20 EKB objects). The data tend to be noisy, due to their faint apparent magnitudes, but there are discrepancies in the results obtained by different observers of the same object that exceed the stated errors by significant amounts, which could be due to contamination by faint background sources. When multiple observations of the same objects were available, we adopted the maximum semi-dispersion for the error bar. Looking for groupings, we did not find any cluster of objects in either the three-dimensional (B-V, V-R, R-I) space or in the projected two-color planes, though considerable spectral diversity appears to exist
Search for Aqueously Altered Materials on Asteroids
No full textThe analysis of visible and near-infrared reflectance spectra of C-class objects has revealed the presence of features that appear to indicate a history of aqueous alteration on their surfaces. This result can provide constraints on our understanding of the early Solar System. We have recorded spectra covering 0.48-0.92 μm for 29 C-class objects with orbital semimajor axes between 2.3 and 3.6 AU. About 70% of the observed objects show features produced as the result of aqueous alteration processes
Compositional properties of Near-Earth Asteroids: spectroscopic comparison with Ordinary Chondrite Meteorites.
No full textSome years ago we started a spectroscopic survey, in the visible region, of Earth-approaching asteroids to investigate their compositional nature in order to improve the comprehension of their origin. To date we have obtained low-resolution spectra, in the range 0.5-1.0μm, of 1 Aten (3753 1986 TO), 4 Apollo (1864 Daedalus, 5786 Talos, 1989 JA, 2063 Bacchus), and 3 Amor (3352 McAuliffe, 4954 Eric, 5836 1993 MF). Most of them show spectra similar to those of the S taxonomic class; Bacchus only has a spectrum which resembles those of more primitive objects (C-type). It has not been possible to definitively distinguish to which S-subclass the observed objects belong because the spectra we obtained do not cover the necessary spectral range to make this investigation as described by Gaffey et al. (1993Icar..106..573G). Nevertheless four of the observed objects have visible spectra similar to those of ordinary chondrites meteorites suggesting a strong relation between the two classes of objects. Moreover 5836 1993 MF shows an absorption feature near 6000Å probably due to the presence of aqueous altered materials
Spectroscopic Comparison Between Near-Earth Asteroids and Ordinary Chondrites Meteorites
No full textThe population of NEA represents one of the most peculiar and heterogeneous classes of objects in the Solar System. In particular, as their dynamical lifetimes are shorter than the age of the Solar System, one of the most interesting aspects in the study of NEA is to understand their origin. To investigate the compositional properties of this population we started a spectroscopic survey in the visible region (0.5<lambda <1.0 mu m). To date we have obtained low resolution spectra of 1 Aten (3753 1986 TO), 4 Apollo (1864 Daedalus, 1786 Talos, 1989 JA, 2063 Bacchus), and 3 Amor (3352 McAuliffe, 4954 Eric, 5836 1993 MF). Most of them show spectra similar to those of the S taxonomic class, while Bacchus is the only one having a spectrum typical of C-type objects. Four of the observed asteroids show a spectrum that match quite consistently the laboratory spectra of Ordinary Chondrites. One of them (5836 1993 MF) shows also an absorption band centered around 0.6 mu m suggestive of the presence of aqueous altered materials. The OC are considered the remnants of the primitive solar nebula: they have been scarcely thermally processed during the evolutionary stages of the Solar System. Binzel et al. (1996, Science, 273, 946) already found a relationship between OC and some NEAs. If the idea that part of the NEAs could be the parent bodies of OC is confirmed, it would help to understand the origin of part of these objects: they would have been injected into near-Earth orbits from the main-belt reservoir
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