4,953 research outputs found
Research relative to a model of the Orion nebula
Research basically has been directed along two avenues. First of all, there is a long-standing interest in modeling H II regions in order to understand the physical processes involved and to extract important astrophysical information. This includes knowledge of chemical elemental abundances and properties of the exciting stars, such as their far ultraviolet spectrum. The Orion Nebula is a prime candidate to study because it is nearby, bright, the extinction is not large, and its appearance is reasonably circular. Such a shape is consistent with a geometrical structure that is spherically symmetric (1-D) or one that is axisymmetric (2-D) seen nearly face-on. Previously all detailed modeling of the ionization and thermal structure of H II regions has been confined to 1-D because of computational complexity. There is now the capability to treat the 2-D case with much of the level of physical sophistication as the 1-D case. The interpretation of the spectra of gaseous nebulae in terms of underlying physical processes requires measurements of line intensities at different points in the object and over as wide a set of excitation and ionization conditions as practical.Observations of nebulae have been made for many years in the optical and radio but only recently in the infrared and ultraviolet. These relatively new windows allow observations of lines of ionization states not available in the radio or optical--providing a much more complete set of known quantities to undertake a meaningful modeling effort
Molecular hydrogen emission from W51
The detection of emission from the v = 1 approaches 0 S(1) quadrupole transition of H2 toward the cluster of intense infrared and H2O maser sources in W51 (north) is reported. The apparent luminosity of this line in W51 (north) is only about 4% of the luminosity of the same line toward the Kleinmann-Low infrared cluster in Orion; however, additional line-of-sight extinction and spatial extent of the source may account for the lower apparent power in W51. Similarity in the infrared and H2O properties of these clusters is addressed. The implications of the H2 emission for mass loss in the W51 region is discussed and some proposed models of radiation-driven mass outflow from pre-main sequence stars are briefly considered
Rapid Determination of Multiple Reaction Pathways in Molecular Systems: The Soft-Ratcheting Algorithm
We discuss the ``soft-ratcheting'' algorithm which generates targeted
stochastic trajectories in molecular systems with scores corresponding to their
probabilities. The procedure, which requires no initial pathway guess, is
capable of rapidly determining multiple pathways between known states.
Monotonic progress toward the target state is not required. The soft-ratcheting
algorithm is applied to an all-atom model of alanine dipeptide, whose unbiased
trajectories are assumed to follow overdamped Langevin dynamics. All possible
pathways on the two-dimensional dihedral surface are determined. The associated
probability scores, though not optimally distributed at present, may provide a
mechanism for estimating reaction rates
Scratching the Surface and Digging Deeper: Exploring Ecological Theories in Urban Soils
Humans have altered the Earth more extensively during the past 50 years than at any other time in history (Millennium Assessment 2003). A significant part of this global change is the conversion of land covers from native ecosystems to those dominated by human activities (Kareiva et al. 2007; Ellis and Ramankutty 2008). Although agricultural needs have historicall
Direct Imaging of Multiple Planets Orbiting the Star HR 8799
Direct imaging of exoplanetary systems is a powerful technique that can
reveal Jupiter-like planets in wide orbits, can enable detailed
characterization of planetary atmospheres, and is a key step towards imaging
Earth-like planets. Imaging detections are challenging due to the combined
effect of small angular separation and large luminosity contrast between a
planet and its host star. High-contrast observations with the Keck and Gemini
telescopes have revealed three planets orbiting the star HR 8799, with
projected separations of 24, 38, and 68 astronomical units. Multi-epoch data
show counter-clockwise orbital motion for all three imaged planets. The low
luminosity of the companions and the estimated age of the system imply
planetary masses between 5 and 13 times that of Jupiter. This system resembles
a scaled-up version of the outer portion of our Solar System.Comment: 30 pages, 5 figures, Research Article published online in Science
Express Nov 13th, 200
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