2,644 research outputs found
Lyman alpha dominance of the Classical T Tauri FUV Radiation Field
Far-ultraviolet (FUV) radiation plays an important role in determining
chemical abundances in protoplanetary disks. HI Lyman alpha is suspected to be
the dominant component of the FUV emission from Classical T Tauri Stars
(CTTSs), but is difficult to measure directly due to circumstellar and
interstellar HI absorption. To better characterize the intrinsic Lyman alpha
radiation, we present FUV spectra of 14 CTTSs taken with the Hubble Space
Telescope COS and STIS instruments. H2 fluorescence, commonly seen in the
spectra of CTTSs, is excited by Lyman alpha photons, providing an indirect
measure of the Lyman alpha flux incident upon the warm disk surface. We use
observed H2 progression fluxes to reconstruct the CTTS Lyman alpha profiles.
The Lyman alpha flux correlates with total measured FUV flux, in agreement with
an accretion-related source of FUV emission. With a geometry-independent
analysis, we confirm that in accreting T Tauri systems Lyman alpha radiation
dominates the FUV flux (~1150 - 1700 Angstroms). In the systems surveyed this
one line comprises 70 - 90 % of the total FUV flux
"Implicature-Laden" Elicitations in Talk Radio Shows
Indirect elicitations in talk radio programmes on BBC Radio are not uncommon, notwithstanding, misunderstanding between the host and his conversational partner is not frequent. Investigating some of the reasons this paper focuses on how the socio-cultural and cognitive factors of the context interweave in discourse. The author suggests that valid interpretation and appropriate response to inferred elicitations can be best explained within the framework of Relevance Theory, and more specifically, with the presumption of accessibility of schemas obtained from the cognitive environment of the discourse partners. Through examples of empirical research the paper aims to reveal how the mutual knowledge of the participants controls discourse via the mental processes occurring in the interaction of two minds
A Rotating Black Hole Solution for Shape Dynamics
Shape dynamics is a classical theory of gravity which agrees with general
relativity in many important aspects, but which possesses different gauge
symmetries and can present some fundamental global differences with respect to
Einstein spacetimes. Here, we present a general procedure for (locally) mapping
stationary, axisymmetric general relativity solutions onto their shape dynamic
counterparts. We focus in particular on the rotating black hole solution for
shape dynamics and show that many of the properties of the spherically
symmetric solution are preserved in the extension to the axisymmetric case: it
is also free of physical singularities, it does not form a space-time at the
horizon, and it possesses an inversion symmetry about the horizon which leads
to us to interpret the solution as a wormhole.Comment: 13 page
Contact Geometry and Quantum Mechanics
We present a generally covariant approach to quantum mechanics in which
generalized positions, momenta and time variables are treated as coordinates on
a fundamental "phase-spacetime." We show that this covariant starting point
makes quantization into a purely geometric flatness condition. This makes
quantum mechanics purely geometric, and possibly even topological. Our approach
is especially useful for time-dependent problems and systems subject to
ambiguities in choices of clock or observer. As a byproduct, we give a
derivation and generalization of the Wigner functions of standard quantum
mechanics.Comment: 7 pages, 1 figure, LaTeX, references added, journal versio
X-Shooter observations of low-mass stars in the Eta Chamaeleontis association
The nearby Eta Chamaeleontis association is a collection of 4-10 Myr old
stars with a disk fraction of 35-45%. In this study, the broad wavelength
coverage of VLT/X-Shooter is used to measure the stellar and mass accretion
properties of 15 low mass stars in the Eta Chamaeleontis association. For each
star, the observed spectrum is fitted with a non-accreting stellar template and
an accretion spectrum obtained from assuming a plane-parallel hydrogen slab.
Five of the eight stars with an IR disk excess show excess UV emission,
indicating ongoing accretion. The accretion rates measured here are similar to
those obtained from previous measurements of excess UV emission, but tend to be
higher than past measurements from H-alpha modeling. The mass accretion rates
are consistent with those of other young star forming regions.Comment: Accepted for publication in Astronomy & Astrophysic
Empirical Isochrones for Low Mass Stars in Nearby Young Associations
Absolute ages of young stars are important for many issues in pre-main
sequence stellar and circumstellar evolution but are long recognized as
difficult to derive and calibrate. In this paper, we use literature spectral
types and photometry to construct empirical isochrones in HR diagrams for
low-mass stars and brown dwarfs in the eta Cha, epsilon Cha, and TW Hya
Associations and the beta Pic and Tuc-Hor Moving Groups. A successful theory of
pre-main sequence evolution should match the shapes of the stellar loci for
these groups of young stars. However, when comparing the combined empirical
isochrones to isochrones predicted from evolutionary models, discrepancies lead
to a spectral type (mass) dependence in stellar age estimates. Improved
prescriptions for convection and boundary conditions in the latest models of
pre-main sequence models lead to a significantly improved correspondence
between empirical and model isochrones, with small offsets at low temperatures
that may be explained by observational uncertainties or by model limitations.
Independent of model predictions, linear fits to combined stellar loci of these
regions provide a simple empirical method to order clusters by luminosity with
a reduced dependence on spectral type. Age estimates calculated from various
sets of modern models that reproduce Li depletion boundary ages of the beta Pic
Moving Group also imply a ~4 Myr age for the low mass members of the Upper Sco
OB Association, which is younger than the 11 Myr age that has been recently
estimated for intermediate mass members.Comment: Accepted by ApJ, 18 page
Sea Turtle Observations at Explosive Removals of Energy Structures
Observers were placed at offshore sites to monitor and protect sea turtles during explosive removals of oil and gas structures in the Gulf of Mexico off Louisiana and Texas. Data collected during more than 6,500 hours of monitoring at 106 structure removals in 1992 provided information on sea turtle distribution. Eighteen individuals were observed including 10 loggerheads, 2 leatherbacks, 1 hawksbill, and 5 unidentified sea turtles. The observation rate (individuals per monitoring hour) of sea turtles was about 30 times higher during aerial surveys than during day or night suiface surveys
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