21,362 research outputs found
Apparatus for reducing aerodynamic noise in a wind tunnel
An apparatus is described for reducing the background noise produced by the porous walls of the test section of a wind tunnel. A finely meshed screen member is placed over the perforations in the test section walls. The mesh wire screen attached to the interior wall provides a smoother surface for the air stream to flow against reducing the vorticies produced by the edges of the perforations in the test section walls
Retired A Stars and Their Companions IV. Seven Jovian Exoplanets from Keck Observatory
We report precise Doppler measurements of seven subgiants from Keck
Observatory. All seven stars show variability in their radial velocities
consistent with planet-mass companions in Keplerian orbits. The host stars have
masses ranging from 1.1 < Mstar/Msun < 1.9, radii 3.4 < Rstar/Rsun < 6.1, and
metallicities -0.21 < [Fe/H] < +0.26. The planets are all more massive than
Jupiter (Msini > 1 Mjup) and have semimajor axes > 1 AU. We present
millimagnitude photometry from the T3 0.4m APT at Fairborn observatory for five
of the targets. Our monitoring shows these stars to be photometrically stable,
further strengthening the interpretation of the observed radial velocity
variability. The orbital characteristics of the planets thus far discovered
around former A-type stars are very different from the properties of planets
around dwarf stars of spectral type F, G and K, and suggests that the formation
and migration of planets is a sensitive function of stellar mass. Three of the
planetary systems show evidence of long-term, linear trends indicative of
additional distant companions. These trends, together with the high planet
masses and increased occurrence rate, indicate that A-type stars are very
promising targets for direct imaging surveys.Comment: PASP Accepted, final submission awaiting comments from the communit
The California Planet Survey II. A Saturn-Mass Planet Orbiting the M Dwarf Gl649
We report precise Doppler measurements of the nearby (d = 10.34 pc) M dwarf
Gl649 that reveal the presence of a planet with a minimum mass Msini = 0.328
Mjup in an eccentric (e = 0.30), 598.3 day orbit. Our photometric monitoring
reveals Gl649 to be a new variable star with brightness changes on both
rotational and decadal timescales. However, neither of these timescales are
consistent with the 600-day Doppler signal and so provide strong support for
planetary reflex motion as the best interpretation of the observed radial
velocity variations. Gl649b is only the seventh Doppler-detected giant planet
around an M dwarf. The properties of the planet and host-star therefore
contribute significant information to our knowledge of planet formation around
low-mass stars. We revise and refine the occurrence rate of giant planets
around M dwarfs based on the California Planet Survey sample of low-mass stars
(M* < 0.6 Msun). We find that f = 3.4^{+2.2}_{-0.9}% of stars with M* < 0.6
Msun harbor planets with Msini > 0.3$ Mjup and a < 2.5 AU. When we restrict our
analysis to metal-rich stars with [Fe/H] > +0.2 we find the occurrence rate is
10.7^{+5.9}_{-4.2}%.Comment: 8 pages, 4 figures, 3 tables, PASP accepte
Three Super-Earths Orbiting HD 7924
We report the discovery of two super-Earth mass planets orbiting the nearby
K0.5 dwarf HD 7924 which was previously known to host one small planet. The new
companions have masses of 7.9 and 6.4 M, and orbital periods of 15.3
and 24.5 days. We perform a joint analysis of high-precision radial velocity
data from Keck/HIRES and the new Automated Planet Finder Telescope (APF) to
robustly detect three total planets in the system. We refine the ephemeris of
the previously known planet using five years of new Keck data and high-cadence
observations over the last 1.3 years with the APF. With this new ephemeris, we
show that a previous transit search for the inner-most planet would have
covered 70% of the predicted ingress or egress times. Photometric data
collected over the last eight years using the Automated Photometric Telescope
shows no evidence for transits of any of the planets, which would be detectable
if the planets transit and their compositions are hydrogen-dominated. We detect
a long-period signal that we interpret as the stellar magnetic activity cycle
since it is strongly correlated with the Ca II H and K activity index. We also
detect two additional short-period signals that we attribute to
rotationally-modulated starspots and a one month alias. The high-cadence APF
data help to distinguish between the true orbital periods and aliases caused by
the window function of the Keck data. The planets orbiting HD 7924 are a local
example of the compact, multi-planet systems that the Kepler Mission found in
great abundance.Comment: Accepted to ApJ on 4/7/201
Coupling of Two Motor Proteins: a New Motor Can Move Faster
We study the effect of a coupling between two motor domains in
highly-processive motor protein complexes. A simple stochastic discrete model,
in which the two parts of the protein molecule interact through some energy
potential, is presented. The exact analytical solutions for the dynamic
properties of the combined motor species, such as the velocity and dispersion,
are derived in terms of the properties of free individual motor domains and the
interaction potential. It is shown that the coupling between the motor domains
can create a more efficient motor protein that can move faster than individual
particles. The results are applied to analyze the motion of helicase RecBCD
molecules
Actively stressed marginal networks
We study the effects of motor-generated stresses in disordered three
dimensional fiber networks using a combination of a mean-field, effective
medium theory, scaling analysis and a computational model. We find that motor
activity controls the elasticity in an anomalous fashion close to the point of
marginal stability by coupling to critical network fluctuations. We also show
that motor stresses can stabilize initially floppy networks, extending the
range of critical behavior to a broad regime of network connectivities below
the marginal point. Away from this regime, or at high stress, motors give rise
to a linear increase in stiffness with stress. Finally, we demonstrate that our
results are captured by a simple, constitutive scaling relation highlighting
the important role of non-affine strain fluctuations as a susceptibility to
motor stress.Comment: 8 pages, 4 figure
Adjustable mount for electro-optic transducers in an evacuated cryogenic system
The invention is an adjustable mount for positioning an electro-optic transducer in an evacuated cryogenic environment. Electro-optic transducers are used in this manner as high sensitivity detectors of gas emission lines of spectroscopic analysis. The mount is made up of an adjusting mechanism and a transducer mount. The adjusting mechanism provided five degrees of freedom, linear adjustments and angular adjustments. The mount allows the use of an internal lens to focus energy on the transducer element thereby improving the efficiency of the detection device. Further, the transducer mount, although attached to the adjusting mechanism, is isolated thermally such that a cryogenic environment can be maintained at the transducer while the adjusting mechanism remains at room temperature. Radiation shields also are incorporated to further reduce heat flow to the transducer location
Effective Potential of a Black Hole in Thermal Equilibrium with Quantum Fields
Expectation values of one-loop renormalized thermal equilibrium stress-energy
tensors of free conformal scalars, spin- fermions and U(1) gauge
fields on a Schwarzschild black hole background are used as sources in the
semi-classical Einstein equation. The back-reaction and new equilibrium metric
are solved for at for each spin field. The nature of the modified
black hole spacetime is revealed through calculations of the effective
potential for null and timelike orbits. Significant novel features affecting
the motions of both massive and massless test particles show up at lowest order
in , where is the renormalized black hole mass,
and is the Planck mass. Specifically, we find the tendency for
\underline{stable} circular photon orbits, an increase in the black hole
capture cross sections, and the existence of a gravitationally repulsive region
associated with the black hole which is generated from the U(1) back-reaction.
We also consider the back-reaction arising from multiple fields, which will be
useful for treating a black hole in thermal equilibrium with field ensembles
belonging to gauge theories.Comment: 25 pages (not including seven figures), VAND-TH-93-6. Typed in Latex,
uses RevTex macro
Bostonia. Volume 5
Founded in 1900, Bostonia magazine is Boston University's main alumni publication, which covers alumni and student life, as well as university activities, events, and programs
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