7,135 research outputs found
2-Diazoacetoacetic acid, an efficient and convenient reagent for the synthesis of alpha-diazo-beta-ketoesters
The formation of various alpha-diazo acetoacetic esters can be obtained in a single transformation with good to excellent yields using readily available 2-diazoacetoacetic acid
Polarization observables in the longitudinal basis for pseudo-scalar meson photoproduction using a density matrix approach
The complete expression for the intensity in pseudo-scalar meson
photoproduction with a polarized beam, target, and recoil baryon is derived
using a density matrix approach that offers great economy of notation. A
Cartesian basis with spins for all particles quantized along a single
direction, the longitudinal beam direction, is used for consistency and clarity
in interpretation. A single spin-quantization axis for all particles enables
the amplitudes to be written in a manifestly covariant fashion with simple
relations to those of the well-known CGLN formalism. Possible sign
discrepancies between theoretical amplitude-level expressions and
experimentally measurable intensity profiles are dealt with carefully. Our
motivation is to provide a coherent framework for coupled-channel partial-wave
analysis of several meson photoproduction reactions, incorporating recently
published and forthcoming polarization data from Jefferson Lab.Comment: 11 pages, 2 figure
An Improbable Solution to the Underluminosity of 2M1207B: A Hot Protoplanet Collision Afterglow
We introduce an alternative hypothesis to explain the very low luminosity of
the cool (L-type) companion to the ~25 M_Jup ~8 Myr-old brown dwarf 2M1207A.
Recently, Mohanty et al. (2007) found that effective temperature estimates for
2M1207B (1600 +- 100 K) are grossly inconsistent with its lying on the same
isochrone as the primary, being a factor of ~10 underluminous at all bands
between I (0.8 um) and L' (3.6 um). Mohanty et al. explain this discrepency by
suggesting that 2M1207B is an 8 M_Jup object surrounded by an edge-on disk
comprised of large dust grains producing 2.5^m of achromatic extinction. We
offer an alternative explanation: the apparent flux reflects the actual source
luminosity. Given the temperature, we infer a small radius (~49,000 km), and
for a range of plausible densities, we estimate a mass < M_Jup. We suggest that
2M1207B is a hot protoplanet collision afterglow and show that the radiative
timescale for such an object is >~1% the age of the system. If our hypothesis
is correct, the surface gravity of 2M1207B should be an order of magnitude
lower than predicted by Mohanty et al. (2007).Comment: ApJ Letters, in press (11 pages
Evidence for Mass-dependent Circumstellar Disk Evolution in the 5 Myr Old Upper Scorpius OB Association
We present 4.5, 8, and 16 µm photometry from the Spitzer Space Telescope for 204 stars in the Upper Scorpius OB
association. The data are used to investigate the frequency and properties of circumstellar disks around stars with masses between ~0.1 and 20 M_☉ at an age of ~5 Myr. We identify 35 stars that have emission at 8 or 16 µm in excess of the, stellar photosphere. The lower mass stars (~0.1–1.2M_☉) appear surrounded by primordial optically thick disks based on, the excess emission characteristics. Starsmoremassive than ~1.8M_☉ have lower fractional excess luminosities suggesting, that the inner ~10 AU of the disk has been largely depleted of primordial material. None of the G and F stars (~1.2–1.8 M_☉) in our sample have an infrared excess at wavelengths ≤16 µm. These results indicate that the mechanisms for, dispersing primordial optically thick disks operate less efficiently, on average, for low-mass stars, and that longer timescales are available for the buildup of planetary systems in the terrestrial zone for stars with masses ≾1 M_☉
Regional and Cellular Codistribution of Interleukin lß and Nerve Growth Factor rnRNA in the Adult Rat Brain
Visualization of Secondary Flow Development in High Aspect Ratio Channels with Curvature
The results of an experimental project to visually examine the secondary flow structure that develops in curved, high aspect-ratio rectangular channels are presented. The results provide insight into the fluid dynamics within high aspect ratio channels. A water flow test rig constructed out of plexiglass, with an adjustable aspect ratio, was used for these experiments. Results were obtained for a channel geometry with a hydraulic diameter of 10.6 mm (0.417 in.), an aspect ratio of 5.0, and a hydraulic radius to curvature radius ratio of 0.0417. Flow conditions were varied to achieve Reynolds numbers up to 5,100. A new particle imaging velocimetry technique was developed which could resolve velocity information from particles entering and leaving the field of view. Time averaged secondary flow velocity vectors, obtained using this velocimetry technique, are presented for 30 degrees, 60 degrees, and 90 degrees into a 180 degrees bend and at a Reynolds number of 5,100. The secondary flow results suggest the coexistence of both the classical curvature induced vortex pair flow structure and the eddies seen in straight turbulent channel flow
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