1,516 research outputs found

    The Position of Sgr A^* at the Galactic Center

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    The absolute position of the compact radio source at the dynamical center of the Galaxy, Sgr A^*, was known only to an accuracy of 0.20.2'' in spite of its accurate location with respect to near-IR stellar sources to within 30 milliarcsecond (mas). To remedy this poor positional accuracy, we have selected 15 high-resolution, high-frequency VLA observations of Sgr A^* carried out in the last 13 years and determined the weighted average position with the average epoch 1992.4 to be at α\alpha, δ\delta[1950] = 17h42m17^{\rm h} 42^{\rm m} 29\dsec3076±0.0007\pm0.0007, 285918.484±0.014-28^\circ 59^\prime 18.484\pm0.014^{\prime\prime}, or α\alpha, δ\delta [2000] = 17h45m17^{\rm h} 45^{\rm m} 40\dsec0383±0.0007\pm0.0007, 290028.069±0.014-29^\circ 00^\prime 28.069\pm0.014^{\prime\prime} which agrees with earlier published values to within the 0.20.2'' error bars of the earlier measurements. An accurate absolute position of Sgr A^* can be useful for its identification with sources at other wavelengths, particularly, in soft and hard X-rays with implications for the models of a massive black hole at the Galactic center.Comment: 11 pages, one figure and one table. ApJL (in press

    NASA/MSFC Large Stretch Press Study

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    The purpose of this study was to: A. assess and document the advantages/disadvantages of a government agency investment in a large stretch form press on the order of 5000 tons capacity (per jaw); B. develop a procurement specification for the press; and C. provide trade study data that will permit an optimum site location. Tasks were separated into four major elements: cost study, user survey, site selection, and press design/procurement specification

    Ecological Effects of Fear: How Spatiotemporal Heterogeneity in Predation Risk Influences Mule Deer Access to Forage in a Sky‐Island System

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    Forage availability and predation risk interact to affect habitat use of ungulates across many biomes. Within sky‐island habitats of the Mojave Desert, increased availability of diverse forage and cover may provide ungulates with unique opportunities to extend nutrient uptake and/or to mitigate predation risk. We addressed whether habitat use and foraging patterns of female mule deer (Odocoileus hemionus) responded to normalized difference vegetation index (NDVI), NDVI rate of change (green‐up), or the occurrence of cougars (Puma concolor). Female mule deer used available green‐up primarily in spring, although growing vegetation was available during other seasons. Mule deer and cougar shared similar habitat all year, and our models indicated cougars had a consistent, negative effect on mule deer access to growing vegetation, particularly in summer when cougar occurrence became concentrated at higher elevations. A seemingly late parturition date coincided with diminishing NDVI during the lactation period. Sky‐island populations, rarely studied, provide the opportunity to determine how mule deer respond to growing foliage along steep elevation and vegetation gradients when trapped with their predators and seasonally limited by aridity. Our findings indicate that fear of predation may restrict access to the forage resources found in sky islands

    Lunar surface mechanical properties

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    The surface material at the Surveyor 5 site is granular and slightly cohesive. Spacecraft footpads plowed trenches in this material as the spacecraft slid during landing. For a compressible soil model, a static bearing capacity of 2.7 newtons/cm^2 gave best agreement with the observations. Static firing of the vernier engines against the surface moved surface particles; a crater 20 cm in diameter and about 1 cm deep was produced, apparently at engine shutdown. The permeability of the soil to gases, to a depth of about 25 cm, is 1 × 10^(−8) cm^2, corresponding to soil particles mostly 2 to 60 μ in diameter

    Petrography of a "cryptic" mixed magma system – the Mount Waldo granite, coastal Maine

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    The Mount Waldo granite (MWG), dated at 371 ± 2 Ma, is typical of the younger Devonian intrusions of the coastal Maine magmatic province. It is a coarse-grained, seriate to porphyritic, biotite ± hornblende, titanite-bearing granite with abundant intermediate to felsic magmatic enclaves and less abundant mafic schlieren. The MWG contains feldspars of various textures and composition, including abundant plagioclase-mantled K-feldspar (rapakivi texture), boxy and spongy cellular plagioclase, and plagioclase that show An spikes and/or central cores of higher Ca content. In addition, the magmatic enclaves also preserve textural evidence for mixing and hybridization. Specifically, abundant mafic clots, acicular apatite, inherited megacrysts of all the feldspar varieties common in the host granite, and titanite-plagioclase intergrowths are all observed. Although very little mafic material is observed in the MWG at its present level of erosion, abundant textural evidence suggests that mixing occurred at depth in this intrusion. The thermal, chemical, and physical gradients initiated by the injection of mafic material produced not only the array of mixing textures observed in the MWG but also the large-scale convection needed to distribute these features throughout the pluton including its highest levels. RÉSUMÉ Le granite du mont Waldo (GMW), situé à 371±2 Ma, est caractéristique des intrusions récentes du Dévonien de la province magmatique côtière du Maine. Il s'agit d'un granite titanitifère à gros grain, sérié à porphyrique de biotite ± hornblende comportant une abondance d'enclaves magmatiques intermédiaires à felsiques et une abondance moindre de schlierens mafiques. Le GMW renferme des feldspaths de textures et de compositions diverses, notamment une abondance de feldspath potassique recouvert de plagioclase (texture de rapakiwi), de plagioclase alvéolaire massif et spongieux, et de plagioclase présentant des pics d'An ou des noyaux centraux à teneur supérieure en Ca. De plus, les enclaves magmatiques préservent également une preuve texturale de mélange et d'hybridation. On peut, plus précisément observer la totalité de ces traits : une abondance de grumeaux mafiques, de l'apatite aciculaire, des mégacristaux provenant de tous les types de feldspaths courants dans le granite hôte et des enchevêtrements de titanite-plagioclase. Même si l'on observe très peu de matières mafiques dans le GMW à son niveau actuel d'érosion, d'abondants indices texturaux permettent de supposer que le mixage est survenu en profondeur dans cette intrusion. Les gradients thermiques, chimiques et physiques obtenus par l'injection de matières mafiques a non seulement produit l'éventail de textures de mélange observé dans le GMW, mais également la convection à grande échelle nécessaire à la distribution de ces caractéristiques dans l'ensemble du pluton ainsi qu'à ses niveaux les plus élevés. [Traduit par la rédaction

    Automated Propulsion Data Screening demonstration system

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    A fully-instrumented firing of a propulsion system typically generates a very large quantity of data. In the case of the Space Shuttle Main Engine (SSME), data analysis from ground tests and flights is currently a labor-intensive process. Human experts spend a great deal of time examining the large volume of sensor data generated by each engine firing. These experts look for any anomalies in the data which might indicate engine conditions warranting further investigation. The contract effort was to develop a 'first-cut' screening system for application to SSME engine firings that would identify the relatively small volume of data which is unusual or anomalous in some way. With such a system, limited and expensive human resources could focus on this small volume of unusual data for thorough analysis. The overall project objective was to develop a fully operational Automated Propulsion Data Screening (APDS) system with the capability of detecting significant trends and anomalies in transient and steady-state data. However, the effort limited screening of transient data to ground test data for throttle-down cases typical of the 3-g acceleration, and for engine throttling required to reach the maximum dynamic pressure limits imposed on the Space Shuttle. This APDS is based on neural networks designed to detect anomalies in propulsion system data that are not part of the data used for neural network training. The delivered system allows engineers to build their own screening sets for application to completed or planned firings of the SSME. ERC developers also built some generic screening sets that NASA engineers could apply immediately to their data analysis efforts

    Systematic Revision of the Northern Short-tailed Shrew, \u3ci\u3eBlarina brevicauda\u3c/i\u3e (Say)

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    Short-tailed shrews, genus Blarina, are common inhabitants of a variety of terrestrial habitats in most of eastern North America. Of the 4 species currently recognized, the northern short-tailed shrew, Blarina brevicauda (Say, 1823), is the most widely distributed, occurring from southern Canada southward to the central Great Plains and the Appalachian Mountains into Georgia and Alabama and along the East Coast as far south as southeastern North Carolina. It has been more than 65 years since geographic variation within this species has been studied. Accordingly, the objectives of this study were to examine geographic variation in Blarina brevicauda and to revise its intraspecific taxonomy as needed. A total of 12,390 Holocene specimens of Blarina brevicauda from throughout the geographic range of the species and the fossil material of Blarina fossilis, B. ozarkensis, and B. simplicidens were examined during the course of this study. Nine cranial and mandibular measurements were taken from 2,736 Holocene specimens, which were grouped into 114 operational taxonomic units (OTUs) for statistical analysis. We used a single classification ANOVA to test for significant differences among means of OTUs and a principal component analysis (PCA) to extract eigenvectors and generate a 2-dimensional plot of OTUs. Our analysis demonstrates that B. brevicauda consists of 7 well-defined subspecies. Two subspecies, the large-bodied B. b. brevicauda and the medium-sized B. b. talpoides, occupy almost the entire geographic range of the species, with restricted gene flow between these 2 subspecies where their geographic ranges abut in the vicinity of the Mississippi River and its valley. The other 5 subspecies occupy small to modest geographic ranges at the periphery of the range of the species and in isolated geographic areas—an undescribed subspecies on the Cumberland Plateau in Tennessee and adjacent Kentucky, B. b. knoxjonesi along the southeastern coast of North Carolina, another undescribed subspecies on the southern two-thirds of the Delmarva Peninsula, B. b. aloga on Martha’s Vineyard and Nantucket Island, and a third undescribed subspecies in the Kaw River Valley in northeastern Kansas. In each of these geographic areas, gene flow has been stopped or greatly restricted. We have chosen to recognize these 7 subspecies because we believe that each has begun to follow its own evolutionary path. These taxa are arranged in a geographic configuration that fits the pattern termed centrifugal speciation, or the development of small isolated peripheral and sometimes relictual populations as the parent taxon undergoes normal population expansion and contraction cycles. The fact that at least 3 of these peripheral populations are now partially in contact with the parental populations and have not been swamped out genetically indicates to us that they are adapting to their local conditions and are able to maintain their genetic identities. Our morphological data and mitochondrial DNA analyses by other workers indicate that these peripheral subspecies, with the possible exception of the 1 along the Kaw River valley, are derived from B. b. talpoides. These data also indicate that B. b. brevicauda and B. b. talpoides are semi-species. [87 pages
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