426 research outputs found
Fundamental Properties of Cool Stars with Interferometry
We present measurements of fundamental astrophysical properties of nearby,
low-mass, K- and M-dwarfs from our DISCOS survey (DIameterS of COol Stars). The
principal goal of our study is the determination of linear radii and effective
temperatures for these stars. We calculate their radii from angular diameter
measurements using the CHARA Array and Hipparcos distances. Combined with
bolometric flux measurements based on literature photometry, we use our angular
diameter results to calculate their effective surface temperatures. We present
preliminary results established on an assortment of empirical relations to the
stellar effective temperature and radius that are based upon these
measurements. We elaborate on the discrepancy seen between theoretical and
observed stellar radii, previously claimed to be related to stellar activity
and/or metallicity. Our preliminary conclusion, however, is that convection
plays a larger role in the determination of radii of these late-type stars.
Understanding the source of the radius disagreement is likely to impact other
areas of study for low-mass stars, such as the detection and characterization
of extrasolar planets in the habitable zones.Comment: Contribution to Proceedings of Cool Stars 16 Workshop; 8 pages in ASP
format; 9 figure
The fundamental parameters of the roAp star 10 Aql
Due to the strong magnetic field and related abnormal surface layers existing
in rapidly oscillating Ap stars, systematic errors are likely to be present
when determining their effective temperatures, which potentially compromises
asteroseismic studies of these pulsators. Using long-baseline interferometry,
our goal is to determine accurate angular diameters of a number of roAp targets
to provide a temperature calibration for these stars. We obtained
interferometric observations of 10 Aql with the visible spectrograph VEGA at
the CHARA array. We determined a limb-darkened angular diameter of
0.275+/-0.009 mas and deduced a linear radius of 2.32+/-0.09 R_sun. We
estimated the star's bolometric flux and used it, in combination with its
parallax and angular diameter, to determine the star's luminosity and effective
temperature. For two data sets of bolometric flux we derived an effective
temperature of 7800+/-170 K and a luminosity of 18+/-1 L_sun or of 8000+/-210 K
and 19+/-2 L_sun. We used these fundamental parameters together with the large
frequency separation to constrain the mass and the age of 10 Aql, using the
CESAM stellar evolution code. Assuming a solar chemical composition and
ignoring all kinds of diffusion and settling of elements, we obtained a mass of
1.92 M_sun and an age of 780 Gy or a mass of 1.95 M_sun and an age of 740 Gy,
depending on the considered bolometric flux. For the first time, we managed to
determine an accurate angular diameter for a star smaller than 0.3 mas and to
derive its fundamental parameters. In particular, by only combining our
interferometric data and the bolometric flux, we derived an effective
temperature that can be compared to those derived from atmosphere models. Such
fundamental parameters can help for testing the mechanism responsible for the
excitation of the oscillations observed in the magnetic pulsating stars
Searching for keV Sterile Neutrino Dark Matter with X-ray Microcalorimeter Sounding Rockets
High-resolution X-ray spectrometers onboard suborbital sounding rockets can
search for dark matter candidates that produce X-ray lines, such as decaying
keV-scale sterile neutrinos. Even with exposure times and effective areas far
smaller than XMM-Newton and Chandra observations, high-resolution, wide
field-of-view observations with sounding rockets have competitive sensitivity
to decaying sterile neutrinos. We analyze a subset of the 2011 observation by
the X-ray Quantum Calorimeter instrument centered on Galactic coordinates l =
165, b = -5 with an effective exposure of 106 seconds, obtaining a limit on the
sterile neutrino mixing angle of sin^2(2 theta) < 7.2e-10 at 95% CL for a 7 keV
neutrino. Better sensitivity at the level of sin^2(2 theta) ~ 2.1e-11 at 95\%
CL for a 7 keV neutrino is achievable with future 300-second observations of
the galactic center by the Micro-X instrument, providing a definitive test of
the sterile neutrino interpretation of the reported 3.56 keV excess from galaxy
clusters.Comment: 13 pages, 13 figures, submitted to Ap
The Ages of A-Stars I: Interferometric Observations and Age Estimates for Stars in the Ursa Major Moving Group
We have observed and spatially resolved a set of seven A-type stars in the
nearby Ursa Major moving group with the Classic, CLIMB, and PAVO beam combiners
on the CHARA Array. At least four of these stars have large rotational
velocities ( 170 ) and are expected to
be oblate. These interferometric measurements, the stars' observed photometric
energy distributions, and values are used to computationally
construct model oblate stars from which stellar properties (inclination,
rotational velocity, and the radius and effective temperature as a function of
latitude, etc.) are determined. The results are compared with MESA stellar
evolution models (Paxton et al. 2011, 2013) to determine masses and ages. The
value of this new technique is that it enables the estimation of the
fundamental properties of rapidly rotating stars without the need to fully
image the star. It can thus be applied to stars with sizes comparable to the
interferometric resolution limit as opposed to those that are several times
larger than the limit. Under the assumption of coevality, the spread in ages
can be used as a test of both the prescription presented here and the MESA
evolutionary code for rapidly rotating stars. With our validated technique, we
combine these age estimates and determine the age of the moving group to be 414
23 Myr, which is consistent with, but much more precise than previous
estimates.Comment: Accepted by Ap
TC21/RRas2 regulates glycoprotein VI–FcRγ‐mediated platelet activation and thrombus stability
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/145353/1/jth14197.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/145353/2/jth14197_am.pd
Interferometric radii of bright Kepler stars with the CHARA Array: {\theta} Cygni and 16 Cygni A and B
We present the results of long-baseline optical interferometry observations
using the Precision Astronomical Visual Observations (PAVO) beam combiner at
the Center for High Angular Resolution Astronomy (CHARA) Array to measure the
angular sizes of three bright Kepler stars: {\theta} Cygni, and both components
of the binary system 16 Cygni. Supporting infrared observations were made with
the Michigan Infrared Combiner (MIRC) and Classic beam combiner, also at the
CHARA Array. We find limb-darkened angular diameters of 0.753+/-0.009 mas for
{\theta} Cyg, 0.539+/-0.007 mas for 16 Cyg A and 0.490+/-0.006 mas for 16 Cyg
B. The Kepler Mission has observed these stars with outstanding photometric
precision, revealing the presence of solar-like oscillations. Due to the
brightness of these stars the oscillations have exceptional signal-to-noise,
allowing for detailed study through asteroseismology, and are well constrained
by other observations. We have combined our interferometric diameters with
Hipparcos parallaxes, spectrophotometric bolometric fluxes and the
asteroseismic large frequency separation to measure linear radii ({\theta} Cyg:
1.48+/-0.02 Rsun, 16 Cyg A: 1.22+/-0.02 Rsun, 16 Cyg B: 1.12+/-0.02 Rsun),
effective temperatures ({\theta} Cyg: 6749+/-44 K, 16 Cyg A: 5839+/-42 K, 16
Cyg B: 5809+/-39 K), and masses ({\theta} Cyg: 1.37+/-0.04 Msun, 16 Cyg A:
1.07+/-0.05 Msun, 16 Cyg B: 1.05+/-0.04 Msun) for each star with very little
model dependence. The measurements presented here will provide strong
constraints for future stellar modelling efforts.Comment: 9 pages, 4 figures, and 5 tables, accepted for publication in Monthly
Notices of the Royal Astronomical Societ
Toward Direct Detection of Hot Jupiters with Precision Closure Phase: Calibration Studies and First Results from the CHARA Array
Direct detection of thermal emission from nearby hot Jupiters has greatly
advanced our knowledge of extrasolar planets in recent years. Since hot Jupiter
systems can be regarded as analogs of high contrast binaries, ground-based
infrared long baseline interferometers have the potential to resolve them and
detect their thermal emission with precision closure phase - a method that is
immune to the systematic errors induced by the Earth's atmosphere. In this
work, we present closure phase studies toward direct detection of nearby hot
Jupiters using the CHARA interferometer array outfitted with the MIRC
instrument. We carry out closure phase simulations and conduct a large number
of observations for the best candidate {\upsion} And. Our experiments suggest
the method is feasible with highly stable and precise closure phases. However,
we also find much larger systematic errors than expected in the observations,
most likely caused by dispersion across different wavelengths. We find that
using higher spectral resolution modes (e.g., R=150) can significantly reduce
the systematics. By combining all calibrators in an observing run together, we
are able to roughly recalibrate the lower spectral resolution data, allowing us
to obtain upper limits of the star-planet contrast ratios of {\upsion} And b
across the H band. The data also allow us to get a refined stellar radius of
1.625\pm0.011 R\odot. Our best upper limit corresponds to a contrast ratio of
2.1\times10^3:1 with 90% confidence level at 1.52{\mu}m, suggesting that we are
starting to have the capability of constraining atmospheric models of hot
Jupiters with interferometry. With recent and upcoming improvements of
CHARA/MIRC, the prospect of detecting emission from hot Jupiters with closure
phases is promising.Comment: 30 pages, including 9 figures and 4 tables. Published in PASP in
August 201
Fundamental Properties of Stars using Asteroseismology from Kepler & CoRoT and Interferometry from the CHARA Array
We present results of a long-baseline interferometry campaign using the PAVO
beam combiner at the CHARA Array to measure the angular sizes of five
main-sequence stars, one subgiant and four red giant stars for which solar-like
oscillations have been detected by either Kepler or CoRoT. By combining
interferometric angular diameters, Hipparcos parallaxes, asteroseismic
densities, bolometric fluxes and high-resolution spectroscopy we derive a full
set of near model-independent fundamental properties for the sample. We first
use these properties to test asteroseismic scaling relations for the frequency
of maximum power (nu_max) and the large frequency separation (Delta_nu). We
find excellent agreement within the observational uncertainties, and
empirically show that simple estimates of asteroseismic radii for main-sequence
stars are accurate to <~4%. We furthermore find good agreement of our measured
effective temperatures with spectroscopic and photometric estimates with mean
deviations for stars between T_eff = 4600-6200 K of -22+/-32 K (with a scatter
of 97K) and -58+/-31 K (with a scatter of 93 K), respectively. Finally we
present a first comparison with evolutionary models, and find differences
between observed and theoretical properties for the metal-rich main-sequence
star HD173701. We conclude that the constraints presented in this study will
have strong potential for testing stellar model physics, in particular when
combined with detailed modelling of individual oscillation frequencies.Comment: 18 pages, 12 figures, 7 tables; accepted for publication in Ap
Precise measurements help gauge Pacific Northwest\u27s Earthquake potential
Except for the recent rumblings of a few moderate earthquakes and the eruption of Mt. St. Helen\u27s, all has been relatively quiet on the Pacific Northwestern front. The Cascades region in the Pacific Northwest, a sporadically active earthquake and volcanic zone, still has great seismic potential [Atwater, 1987], as comparisons with other subduction zones around the world have shown [Heaton and Kanamori, 1984]. Recent tsunami propagation models [Satake, 1996] and tree ring studies suggest that the last great Cascadia earthquake occurred in the winter of 1700 A.D. and had a magnitude of −8.9. The North Cascades or Wenatchee earthquake followed in 1872. With an estimated magnitude greater than 7, it was the largest earthquake in the written history of Washington and Oregon
Carbon nanotubes-semiconductor networks for organic electronics: The pickup stick transistor
We demonstrate an alternative path for achieving high transconductance organic transistors in spite of relatively large source to drain distances. The improvement of the electronic characteristic of such a scheme is equivalent to a 60-fold increase in mobility of the underlying organic semiconductor. The method is based on percolating networks, which we create from a dispersion of individual single-wall carbon nanotubes and narrow ropes within an organic semiconducting host. The majority of current paths between source and drain follow the metallic nanotubes but require a short, switchable semiconducting link to complete the circuit. With these nanotube-semiconducting composites we achieve effectively a 60X reduction in source to drain distance, which is equivalent to a 60-fold increase of the '' effective '' mobility of the starting semiconducting material with a minor decrease of the on/off current ratio. These field-induced percolating networks allow for the fabrication of high-transconductance transistors having relatively large source to drain distances that can be manufactured inexpensively by commercially available printing techniques. (c) 2005 American Institute of Physicsclose516
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