141 research outputs found
Fiducial Stellar Population Sequences for the u'g'r'i'z' System
We describe an extensive observational project that has obtained high-quality
and homogeneous photometry for a number of different Galactic star clusters
(including M 92, M 13, M 3, M 71, and NGC 6791) spanning a wide range in
metallicity (-2.3<[Fe/H]<+0.4), as observed in the u'g'r'i'z' passbands with
the MegaCam wide-field imager on the Canada-France-Hawaii Telescope. By
employing these purest of stellar populations, fiducial sequences have been
defined from color-magnitude diagrams that extend from the tip of the red-giant
branch down to approximately 4 magnitudes below the turnoff: these sequences
have been accurately calibrated to the standard u'g'r'i'z' system via a set of
secondary photometric standards located within these same clusters.
Consequently, they can serve as a valuable set of empirical fiducials for the
interpretation of stellar populations data in the u'g'r'i'z' system.Comment: 16 pages, 7 tables, 13 figures; accepted for publication in A
High-resolution measurements of surface topography with airborne laser altimetry and the global positioning system
Recently, an airborne lidar system that measures laser pulse time-of-flight and the distortion of the pulse waveform upon reflection from earth surface terrain features was developed and is now operational. This instrument is combined with Global Positioning System (GPS) receivers and a two-axis gyroscope for accurate recovery of aircraft position and pointing attitude. The laser altimeter system is mounted on a high-altitude aircraft platform and operated in a repetitively-pulsed mode for measurements of surface elevation profiles at nadir. The laser transmitter makes use of recently developed short-pulse diode-pumped solid-state laser technology in Q-switched Nd:YAG operating at its fundamental wavelength of 1064 nm. A reflector telescope and silicon avalanche photodiode are the basis of the optical receiver. A high-speed time-interval unit and a separate high-bandwidth waveform digitizer under microcomputer control are used to process the backscattered pulses for measurements of terrain. Other aspects of the lidar system are briefly discussed
Geometry-dependent critical currents in superconducting nanocircuits
In this paper we calculate the critical currents in thin superconducting
strips with sharp right-angle turns, 180-degree turnarounds, and more
complicated geometries, where all the line widths are much smaller than the
Pearl length . We define the critical current as the
current that reduces the Gibbs free-energy barrier to zero. We show that
current crowding, which occurs whenever the current rounds a sharp turn, tends
to reduce the critical current, but we also show that when the radius of
curvature is less than the coherence length this effect is partially
compensated by a radius-of-curvature effect. We propose several patterns with
rounded corners to avoid critical-current reduction due to current crowding.
These results are relevant to superconducting nanowire single-photon detectors,
where they suggest a means of improving the bias conditions and reducing dark
counts. These results also have relevance to normal-metal nanocircuits, as
these patterns can reduce the electrical resistance, electromigration, and hot
spots caused by nonuniform heating.Comment: 29 pages, 24 figure
Bright Variable Stars in NGC 6819 - An Open Cluster in the Kepler Field
We describe a variability study of the moderately old open cluster NGC 6819.
We have detected 4 new detached eclipsing binaries near the cluster turnoff
(one of which may be in a triple system). Several of these systems should be
able to provide mass and radius information, and can therefore constrain the
age of the cluster. We have also newly detected one possible detached binary
member about 3.5 magnitudes below the turnoff. One EW-type binary (probably not
a cluster member) shows unusually strong night-to-night light curve variations
in sets of observations separated by 8 years. According to the best current
information, the three brightest variables we detected (2 of them new) are
cluster members, making them blue stragglers. One is a delta Scu pulsating
variable, one is a close but detached binary, and the third contains a detached
short period binary that shows total eclipses. In each case, however, there is
evidence hinting that the system may have been produced through the interaction
of more than two stars.Comment: 33 pages, 15 figures, accepted to A
Prazosin during threat discrimination boosts memory of the safe stimulus
The α-1 adrenoreceptor antagonist prazosin has shown promise in the treatment of post-traumatic stress disorder (PTSD) symptoms, but its mechanisms are not well understood. Here we administered prazosin or placebo prior to threat conditioning (day 1) and tested subsequent extinction (day 2) and reextinction (day 3) in healthy human participants. Prazosin did not affect threat conditioning but augmented stimulus discrimination during extinction and reextinction, via lower responding to the safe stimulus. These results suggest that prazosin during threat acquisition may have influenced encoding or consolidation of safety processing in particular, subsequently leading to enhanced discrimination between the safe and threatening stimuli
The RR Lyrae variables and Horizontal Branch of NGC6656 (M22)
The first calibrated broadband UBVI time-series photometry is presented for
the RR Lyrae variable stars in NGC6656 (M22), with observations spanning a
range of twenty-two years. We have also redetermined the variability types and
periods for the RR Lyrae stars identified previously by photographic
observations, revising the number of fundamental-mode RR Lyrae variables (RR0)
to 10 and the number of first-overtone variables (RR1) to 16. The mean periods
of the RR0 and RR1 variables are _{RR0}=0.66 \pm 0.02 d and _{RR1}=0.33
\pm 0.01 d, respectively, supporting an Oosterhoff II classification for the
cluster. The number ratio of RR1- to all RR-type variables is N_1/N_{RR}=0.61,
also consistent with an Oosterhoff II designation. Both the RR Lyrae stars'
minimum light colors and the blue edge of the RR Lyrae instability strip
suggest E(B-V)=0.36 \pm 0.02 mag toward M22. Regarding the HB morphology of
M22, we find (B-R)/(B+V+R)=+0.97 \pm 0.1 and at least one "gap" located in an
unusual part of the blue HB, in the middle of the so-called hot HB stars.Comment: accepted to A
Surface Superconductivity in Niobium for Superconducting RF Cavities
A systematic study is presented on the superconductivity (sc) parameters of
the ultrapure niobium used for the fabrication of the nine-cell 1.3 GHz
cavities for the linear collider project TESLA. Cylindrical Nb samples have
been subjected to the same surface treatments that are applied to the TESLA
cavities: buffered chemical polishing (BCP), electrolytic polishing (EP),
low-temperature bakeout (LTB). The magnetization curves and the complex
magnetic susceptibility have been measured over a wide range of temperatures
and dc magnetic fields, and also for di erent frequencies of the applied ac
magnetic field. The bulk superconductivity parameters such as the critical
temperature Tc = 9.26 K and the upper critical field Bc2(0) = 410 mT are found
to be in good agreement with previous data. Evidence for surface
superconductivity at fields above Bc2 is found in all samples. The critical
surface field exceeds the Ginzburg-Landau field Bc3 = 1.695Bc2 by about 10% in
BCP-treated samples and increases even further if EP or LTB are applied. From
the field dependence of the susceptibility and a power-law analysis of the
complex ac conductivity and resistivity the existence of two different phases
of surface superconductivity can be established which resemble the Meissner and
Abrikosov phases in the bulk: (1) coherent surface superconductivity, allowing
sc shielding currents flowing around the entire cylindrical sample, for
external fields B in the range between Bc2 and Bcohc3, and (2) incoherent
surface superconductivity with disconnected sc domains between Bcohc3 and Bc3.
The coherent critical surface field separating the two phases is found to be
Bcoh c3 = 0.81Bc3 for all samples. The exponents in the power law analysis are
different for BCP and EP samples, pointing to different surface topologies.Comment: 15 pages, 21 figures, DESY-Report 2004-02
Galactic Globular and Open Clusters in the Sloan Digital Sky Survey. I. Crowded Field Photometry and Cluster Fiducial Sequences in ugriz
We present photometry for globular and open cluster stars observed with the
Sloan Digital Sky Survey (SDSS). In order to exploit over 100 million stellar
objects with r < 22.5 mag observed by SDSS, we need to understand the
characteristics of stars in the SDSS ugriz filters. While star clusters provide
important calibration samples for stellar colors, the regions close to globular
clusters, where the fraction of field stars is smallest, are too crowded for
the standard SDSS photometric pipeline to process. To complement the SDSS
imaging survey, we reduce the SDSS imaging data for crowded cluster fields
using the DAOPHOT/ALLFRAME suite of programs and present photometry for 17
globular clusters and 3 open clusters in a SDSS value-added catalog. Our
photometry and cluster fiducial sequences are on the native SDSS 2.5-meter
ugriz photometric system, and the fiducial sequences can be directly applied to
the SDSS photometry without relying upon any transformations. Model photometry
for red giant branch and main-sequence stars obtained by Girardi et al. cannot
be matched simultaneously to fiducial sequences; their colors differ by
~0.02-0.05 mag. Good agreement (< ~0.02 mag in colors) is found with Clem et
al. empirical fiducial sequences in u'g'r'i'z' when using the transformation
equations in Tucker et al.Comment: 30 pages, 25 figures. Accepted for publication in ApJS. Version with
high resolution figures available at
http://www.astronomy.ohio-state.edu/~deokkeun/AnJohnson.pd
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