5,945 research outputs found
J-Band Infrared Spectroscopy of a Sample of Brown Dwarfs Using Nirspec on Keck II
Near-infrared spectroscopic observations of a sample of very cool, low-mass
objects are presented with higher spectral resolution than in any previous
studies. Six of the objects are L-dwarfs, ranging in spectral class from L2 to
L8/9, and the seventh is a methane or T-dwarf. These new observations were
obtained during commissioning of NIRSPEC, the first high-resolution
near-infrared cryogenic spectrograph for the Keck II 10-meter telescope on
Mauna Kea, Hawaii. Spectra with a resolving power of R=2500 from 1.135 to 1.360
microns (approximately J-band) are presented for each source. At this
resolution, a rich spectral structure is revealed, much of which is due to
blending of unresolved molecular transitions. Strong lines due to neutral
potassium (K I), and bands due to iron hydride (FeH) and steam (H2O) change
significantly throughout the L sequence. Iron hydride disappears between L5 and
L8, the steam bands deepen and the K I lines gradually become weaker but wider
due to pressure broadening. An unidentified feature occurs at 1.22 microns
which has a temperature dependence like FeH but has no counterpart in the
available FeH opacity data. Because these objects are 3-6 magnitudes brighter
in the near-infrared compared to the I-band, spectral classification is
efficient. One of the objects studied (2MASSW J1523+3014) is the coolest
L-dwarf discovered so far by the 2-Micron All-Sky Survey (2MASS), but its
spectrum is still significantly different from the methane-dominated objects
such as Gl229B or SDSS 1624+0029.Comment: New paper, Latex format, 2 figures, accepted to ApJ Letter
Fabrication and Assessment of 3D Printed Anatomical Models of the Lower Limb for Anatomical Teaching and Femoral Vessel Access Training in Medicine
For centuries, cadaveric dissection has been the touchstone of anatomy education. It offers a medical student intimate access to his or her first patient. In contrast to idealized artisan anatomical models, it presents the natural variation of anatomy in fine detail. However, a new teaching construct has appeared recently in which artificial cadavers are manufactured through three-dimensional (3D) printing of patient specific radiological data sets. In this article, a simple powder based printer is made more versatile to manufacture hard bones, silicone muscles and perfusable blood vessels. The approach involves blending modern approaches (3D printing) with more ancient ones (casting and lost-wax techniques). These anatomically accurate models can augment the approach to anatomy teaching from dissection to synthesis of 3D-printed parts held together with embedded rare earth magnets. Vascular simulation is possible through application of pumps and artificial blood. The resulting arteries and veins can be cannulated and imaged with Doppler ultrasound. In some respects, 3D-printed anatomy is superior to older teaching methods because the parts are cheap, scalable, they can cover the entire age span, they can be both dissected and reassembled and the data files can be printed anywhere in the world and mass produced. Anatomical diversity can be collated as a digital repository and reprinted rather than waiting for the rare variant to appear in the dissection room. It is predicted that 3D printing will revolutionize anatomy when poly-material printing is perfected in the early 21st century. (C) 2015 American Association of Anatomists
Fabrication and Assessment of 3D Printed Anatomical Models of the Lower Limb for Anatomical Teaching and Femoral Vessel Access Training in Medicine
For centuries, cadaveric dissection has been the touchstone of anatomy education. It offers a medical student intimate access to his or her first patient. In contrast to idealized artisan anatomical models, it presents the natural variation of anatomy in fine detail. However, a new teaching construct has appeared recently in which artificial cadavers are manufactured through three-dimensional (3D) printing of patient specific radiological data sets. In this article, a simple powder based printer is made more versatile to manufacture hard bones, silicone muscles and perfusable blood vessels. The approach involves blending modern approaches (3D printing) with more ancient ones (casting and lost-wax techniques). These anatomically accurate models can augment the approach to anatomy teaching from dissection to synthesis of 3D-printed parts held together with embedded rare earth magnets. Vascular simulation is possible through application of pumps and artificial blood. The resulting arteries and veins can be cannulated and imaged with Doppler ultrasound. In some respects, 3D-printed anatomy is superior to older teaching methods because the parts are cheap, scalable, they can cover the entire age span, they can be both dissected and reassembled and the data files can be printed anywhere in the world and mass produced. Anatomical diversity can be collated as a digital repository and reprinted rather than waiting for the rare variant to appear in the dissection room. It is predicted that 3D printing will revolutionize anatomy when poly-material printing is perfected in the early 21st century. (C) 2015 American Association of Anatomists
Dynamical (e, 2e) studies of tetrahydrofurfuryl alcohol
Cross section data for electron scattering from DNA are important for modelling radiation damage
in biological systems. Triply differential cross sections for the electron impact ionization of the
highest occupied outer valence orbital of tetrahydrofurfuryl alcohol, which can be considered as
an analogue to the deoxyribose backbone molecule in DNA, have been measured using the (e,2e)
technique. The measurements have been performed with coplanar asymmetric kinematics at an incident
electron energy of 250 eV, an ejected electron energy of 20 eV, and at scattered electron
angles of −5°, −10°, and −15°. Experimental results are compared with corresponding theoretical
calculations performed using the molecular 3-body distorted wave model. Some important differences
are observed between the experiment and calculations
The Spectra of T Dwarfs I: Near-Infrared Data and Spectral Classification
We present near-infrared spectra for a sample of T dwarfs, including eleven
new discoveries made using the Two Micron All Sky Survey. These objects are
distinguished from warmer (L-type) brown dwarfs by the presence of methane
absorption bands in the 1--2.5 \micron spectral region. A first attempt at a
near-infrared classification scheme for T dwarfs is made, based on the
strengths of CH and HO bands and the shapes of the 1.25, 1.6, and 2.1
\micron flux peaks. Subtypes T1 V through T8 V are defined, and spectral
indices useful for classification are presented. The subclasses appear to
follow a decreasing T scale, based on the evolution of CH and
HO bands and the properties of L and T dwarfs with known distances.
However, we speculate that this scale is not linear with spectral type for cool
dwarfs, due to the settling of dust layers below the photosphere and subsequent
rapid evolution of spectral morphology around T 1300--1500 K.
Similarities in near-infrared colors and continuity of spectral features
suggest that the gap between the latest L dwarfs and earliest T dwarfs has been
nearly bridged. This argument is strengthened by the possible role of CH as
a minor absorber shaping the K-band spectra of the latest L dwarfs. Finally, we
discuss one peculiar T dwarf, 2MASS 0937+2931, which has very blue
near-infrared colors (J-K = 0.24) due to suppression of the 2.1
\micron peak. The feature is likely caused by enhanced collision-induced
H absorption in a high pressure or low metallicity photosphere.Comment: 74 pages including 26 figures, accepted by ApJ v563 December 2001;
full paper including all of Table 3 may be downloaded from
http://www.gps.caltech.edu/~pa/adam/classification ;also see submission
010844
In situ epitaxial MgB2 thin films for superconducting electronics
A thin film technology compatible with multilayer device fabrication is
critical for exploring the potential of the 39-K superconductor magnesium
diboride for superconducting electronics. Using a Hybrid Physical-Chemical
Vapor Deposition (HPCVD) process, it is shown that the high Mg vapor pressure
necessary to keep the MgB phase thermodynamically stable can be achieved
for the {\it in situ} growth of MgB thin films. The films grow epitaxially
on (0001) sapphire and (0001) 4H-SiC substrates and show a bulk-like of
39 K, a (4.2K) of A/cm in zero field, and a
of 29.2 T in parallel magnetic field. The surface is smooth with a
root-mean-square roughness of 2.5 nm for MgB films on SiC. This deposition
method opens tremendous opportunities for superconducting electronics using
MgB
Nickel-Catalyzed Carbon–Carbon Bond-Forming Reactions of Unactivated Tertiary Alkyl Halides: Suzuki Arylations
The first Suzuki cross-couplings of unactivated tertiary alkyl electrophiles are described. The method employs a readily accessible catalyst (NiBr[subscript 2]·diglyme/4,4′-di-tert-butyl-2,2′-bipyridine, both commercially available) and represents the initial example of the use of a group 10 catalyst to cross-couple unactivated tertiary electrophiles to form C–C bonds. This approach to the synthesis of all-carbon quaternary carbon centers does not suffer from isomerization of the alkyl group, in contrast with the umpolung strategy for this bond construction (cross-coupling of a tertiary alkylmetal with an aryl electrophile). Preliminary mechanistic studies are consistent with the generation of a radical intermediate along the reaction pathway.National Institute of General Medical Sciences (U.S.) (R01-GM62871)Merck Research Laboratories (Summer Fellowship
Uplifting and Inflation with D3 Branes
Back-reaction effects can modify the dynamics of mobile D3 branes moving
within type IIB vacua, in a way which has recently become calculable. We
identify some of the ways these effects can alter inflationary scenarios, with
the following three results: (1) By examining how the forces on the brane due
to moduli-stabilizing interactions modify the angular motion of D3 branes
moving in Klebanov-Strassler type throats, we show how previous slow-roll
analyses can remain unchanged for some brane trajectories, while being modified
for other trajectories. These forces cause the D3 brane to sink to the bottom
of the throat except in a narrow region close to the D7 brane, and do not
ameliorate the \eta-problem of slow roll inflation in these throats; (2) We
argue that a recently-proposed back-reaction on the dilaton field can be used
to provide an alternative way of uplifting these compactifications to Minkowski
or De Sitter vacua, without the need for a supersymmetry-breaking anti-D3
brane; and (3) by including also the D-term forces which arise when
supersymmetry-breaking fluxes are included on D7 branes we identify the 4D
supergravity interactions which capture the dynamics of D3 motion in D3/D7
inflationary scenarios. The form of these potentials sheds some light on recent
discussions of how symmetries constrain D term interactions in the low-energy
theory.Comment: JHEP.cls, 35 pages, 3 .eps figure
The Evolution of Bat Vestibular Systems in the Face of Potential Antagonistic Selection Pressures for Flight and Echolocation
PMCID: PMC3634842This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited
Design of Experiments for Screening
The aim of this paper is to review methods of designing screening
experiments, ranging from designs originally developed for physical experiments
to those especially tailored to experiments on numerical models. The strengths
and weaknesses of the various designs for screening variables in numerical
models are discussed. First, classes of factorial designs for experiments to
estimate main effects and interactions through a linear statistical model are
described, specifically regular and nonregular fractional factorial designs,
supersaturated designs and systematic fractional replicate designs. Generic
issues of aliasing, bias and cancellation of factorial effects are discussed.
Second, group screening experiments are considered including factorial group
screening and sequential bifurcation. Third, random sampling plans are
discussed including Latin hypercube sampling and sampling plans to estimate
elementary effects. Fourth, a variety of modelling methods commonly employed
with screening designs are briefly described. Finally, a novel study
demonstrates six screening methods on two frequently-used exemplars, and their
performances are compared
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