Browsing by Author "Christensen, Eric J."

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    Description of a very dense meteorite collection area in western Atacama: Insight into the long-term composition of the meteorite flux to Earth
    (2016) Hutzler, Aurore; Gattacceca, Jerome; Rochette, Pierre; Braucher, Regis; Carro, Bertrand; Christensen, Eric J.; Cournede, Cecile; Gounelle, Matthieu; Ouazaa, Nejia Laridhi; Martinez, Rodrigo; Valenzuela, Millarca; Warner, Michael; Bourles, Didier
    We describe the geological, morphological, and climatic settings of two new meteorite collections from Atacama (Chile). The El Medano collection was recovered by systematic on-foot search in El Medano and Caleta el Cobre dense collection areas and is composed of 213 meteorites before pairing, 142 after pairing. The private collection has been recovered by car by three private hunters and consists of 213 meteorites. Similar to other hot desert finds, and contrary to the falls and Antarctica finds, both collections show an overabundance of H chondrites. A recovery density can be calculated only for the El Medano collection and gives 251 and 168 meteorites larger than 10gkm(-2), before and after pairing, respectively. It is by far the densest collection area described in hot deserts. The Atacama Desert is known to have been hyperarid for a long period of time and, based on cosmic-ray exposure ages on the order of 1-10Ma, to have been stable over a period of time of several million years. Such a high meteorite concentration might be explained invoking either a yet unclear concentration mechanism (possibly related to downslope creeping) or a previously underestimated meteorite flux in previous studies or an average terrestrial age over 2Myr. This last hypothesis is supported by the high weathering grade of meteorites and by the common terrestrial fragmentation (with fragments scattered over a few meters) of recovered meteorites.
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    Discovery of 30,000 Periodic Variables in the Southern Sky
    (2016) Drake, Andrew J.; Djorgovski, Stanislav G.; Graham, Matthew; Catelan, Márcio; Torrealba, Gabriel; Mahabal, Ashish A.; Christensen, Eric J.; Larson, Stephen M.; McNaught, Robert; Garradd, Gordon
    We have completed a search for periodic variable stars within the 30,000 square degrees of the sky covered by the Catalina Surveys. Here we present the results from our analysis of six years of optical photometry taken by the Siding Spring Survey (SSS). This data covers 200 million sources at declinations between -20 and -75 degrees in the magnitude range 11 < V < 19. In addition to the 10,000 RR Lyrae that we previously discovered in this data, we find approximately 30,000 new periodic variable stars. These sources include, eclipsing binaries, RR Lyrae, LPVs, RSCVn stars, delta Scutis and Anomalous Cepheids.As part of our ongoing search for structure within the Galactic halo we determine the distances to each of the type-ab RR Lyrae. We discover that many of these stars appear to belong to the old stellar halo of the LMC. Our analysis suggests that the stellar halo of the LMC extends far beyond the limits previously observed....
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    Periodic Variable Stars Across the Southern Sky
    (2015) Drake, Andrew J.; Graham, Matthew; Djorgovski, Stanislav G.; Catelan, Márcio; Torrealba, Gabriel; Mahabal, Ashish A.; Donalek, Ciro; Christensen, Eric J.; Larson, Stephen M.; McNaught, Robert; Garradd, Gordon
    We continue our search for periodic variables within the ~30,000 square degrees of the sky covered by the Catalina Surveys. Here we analyze six years of optical photometry taken by the Siding Spring Survey (SSS), which is sensitive to sources in the range 11 < V < 19. In all, this new analysis covers ten thousand square degrees on the sky at declinations between -20 and -75 degrees. Due to the very large number of periodic variable candidates found, we perform automated classification of the stars using multivariate kernel density estimation based on features selected from previously classified northern data. We test the accuracy of the results by visually validating a large sample of the objects. We also compare the classifications with those from other automated methods. The final SSS catalog contains tens of thousands of new periodic variable stars including eclipsing binaries, RR Lyrae, LPVs, delta Scuti's and Cepheids. By combining the newly discovered LPVs and RR Lyrae with our previous discoveries, we trace the path of the Sagittarius tidal streams system across the entire sky....