276 research outputs found
Separated Fringe Packet Observations with the CHARA Array II: Andromeda, HD 178911, and {\xi} Cephei
When observed with optical long-baseline interferometers (OLBI), components
of a binary star which are sufficiently separated produce their own
interferometric fringe packets; these are referred to as Separated Fringe
Packet (SFP) binaries. These SFP binaries can overlap in angular separation
with the regime of systems resolvable by speckle interferometry at single,
large-aperture telescopes and can provide additional measurements for
preliminary orbits lacking good phase coverage, help constrain elements of
already established orbits, and locate new binaries in the undersampled regime
between the bounds of spectroscopic surveys and speckle interferometry. In this
process, a visibility calibration star is not needed, and the separated fringe
packets can provide an accurate vector separation. In this paper, we apply the
SFP approach to {\omega} Andromeda, HD 178911, and {\xi} Cephei with the CLIMB
three-beam combiner at the CHARA Array. For these systems we determine
component masses and parallax of 0.9630.049 and
0.8600.051 and 39.541.85 milliarcseconds (mas) for
{\omega} Andromeda, for HD 178911 of 0.8020.055 and
0.6220.053 with 28.261.70 mas, and masses of
1.0450.031 and 0.4080.066 and
38.102.81 mas for {\xi} Cephei.Comment: 28 pages, 4 tables, 6 figures, accepted to AJ May 201
Anisotropic intrinsic lattice thermal conductivity of phosphorene from first principles
Phosphorene, the single layer counterpart of black phosphorus, is a novel
two-dimensional semiconductor with high carrier mobility and a large
fundamental direct band gap, which has attracted tremendous interest recently.
Its potential applications in nano-electronics and thermoelectrics call for a
fundamental study of the phonon transport. Here, we calculate the intrinsic
lattice thermal conductivity of phosphorene by solving the phonon Boltzmann
transport equation (BTE) based on first-principles calculations. The thermal
conductivity of phosphorene at is
(zigzag) and
(armchair), showing an obvious anisotropy along different directions. The
calculated thermal conductivity fits perfectly to the inverse relation with
temperature when the temperature is higher than Debye temperature (). In comparison to graphene, the minor contribution around
of the ZA mode is responsible for the low thermal conductivity of
phosphorene. In addition, the representative mean free path (MFP), a critical
size for phonon transport, is also obtained.Comment: 5 pages and 6 figures, Supplemental Material available as
http://www.rsc.org/suppdata/cp/c4/c4cp04858j/c4cp04858j1.pd
Cepheid distances from the SpectroPhoto-Interferometry of Pulsating Stars (SPIPS) - Application to the prototypes delta Cep and eta Aql
The parallax of pulsation, and its implementations such as the
Baade-Wesselink method and the infrared surface bright- ness technique, is an
elegant method to determine distances of pulsating stars in a quasi-geometrical
way. However, these classical implementations in general only use a subset of
the available observational data. Freedman & Madore (2010) suggested a more
physical approach in the implementation of the parallax of pulsation in order
to treat all available data. We present a global and model-based
parallax-of-pulsation method that enables including any type of observational
data in a consistent model fit, the SpectroPhoto-Interferometric modeling of
Pulsating Stars (SPIPS). We implemented a simple model consisting of a
pulsating sphere with a varying effective temperature and a combina- tion of
atmospheric model grids to globally fit radial velocities, spectroscopic data,
and interferometric angular diameters. We also parametrized (and adjusted) the
reddening and the contribution of the circumstellar envelopes in the
near-infrared photometric and interferometric measurements. We show the
successful application of the method to two stars: delta Cep and eta Aql. The
agreement of all data fitted by a single model confirms the validity of the
method. Derived parameters are compatible with publish values, but with a
higher level of confidence. The SPIPS algorithm combines all the available
observables (radial velocimetry, interferometry, and photometry) to estimate
the physical parameters of the star (ratio distance/ p-factor, Teff, presence
of infrared excess, color excess, etc). The statistical precision is improved
(compared to other methods) thanks to the large number of data taken into
account, the accuracy is improved by using consistent physical modeling and the
reliability of the derived parameters is strengthened thanks to the redundancy
in the data.Comment: 10 pages, 4 figures, A&A in pres
The Membership and Distance of the Open Cluster Collinder 419
The young open cluster Collinder 419 surrounds the massive O star, HD 193322,
that is itself a remarkable multiple star system containing at least four
components. Here we present a discussion of the cluster distance based upon new
spectral classifications of the brighter members, UBV photometry, and an
analysis of astrometric and photometric data from the UCAC3 and 2MASS catalogs.
We determine an average cluster reddening of E(B-V)=0.37 +- 0.05 mag and a
cluster distance of 741 +- 36 pc. The cluster probably contains some very young
stars that may include a reddened M3 III star, IRAS~20161+4035
Horizontal Inequalities and Ethnonationalist Civil War: A Global Comparison
Contemporary research on civil war has largely dismissed the role of political and economic grievances, focusing instead on opportunities for conflict. However, these strong claims rest on questionable theoretical and empirical grounds. Whereas scholars have examined primarily the relationship between individual inequality and conflict, we argue that horizontal inequalities between politically relevant ethnic groups and states at large can promote ethnonationalist conflict. Extending the empirical scope to the entire world, this article introduces a new spatial method that combines our newly geocoded data on ethnic groups’ settlement areas with spatial wealth estimates. Based on these methodological advances, we find that, in highly unequal societies, both rich and poor groups fight more often than those groups whose wealth lies closer to the country average. Our results remain robust to a number of alternative sample definitions and specifications.</jats:p
The Search for Stellar Companions to Exoplanet Host Stars Using the CHARA Array
Most exoplanets have been discovered via radial velocity studies, which are
inherently insensitive to orbital inclination. Interferometric observations
will show evidence of a stellar companion if it sufficiently bright, regardless
of the inclination. Using the CHARA Array, we observed 22 exoplanet host stars
to search for stellar companions in low-inclination orbits that may be
masquerading as planetary systems. While no definitive stellar companions were
discovered, it was possible to rule out certain secondary spectral types for
each exoplanet system observed by studying the errors in the diameter fit to
calibrated visibilities and by searching for separated fringe packets.Comment: 26 pages, 5 tables, 8 figure
Extended envelopes around Galactic Cepheids III. Y Oph and alpha Per from near-infrared interferometry with CHARA/FLUOR
Unbiased angular diameter measurements are required for accurate distances to
Cepheids using the interferometric Baade Wesselink method (IBWM). The precision
of this technique is currently limited by interferometric measurements at the
1.5% level. At this level, the center-to-limb darkening (CLD) and the presence
of circumstellar envelopes (CSE) seem to be the two main sources of bias. The
observations we performed aim at improving our knowledge of the interferometric
visibility profile of Cepheids. In particular, we assess the systematic
presence of CSE around Cepheids in order determine accurate distances with the
IBWM free from CSE biased angular diameters. We observed a Cepheid (Y Oph) for
which the pulsation is well resolved and a non-pulsating yellow supergiant
(alpha Per) using long-baseline near-infrared interferometry. We interpreted
these data using a simple CSE model we previously developed. We found that our
observations of alpha Per do not provide evidence for a CSE. The measured CLD
is explained by an hydrostatic photospheric model. Our observations of Y Oph,
when compared to smaller baseline measurements, suggest that it is surrounded
by a CSE with similar characteristics to CSE found previously around other
Cepheids. We have determined the distance to Y Oph to be d=491+/-18 pc.
Additional evidence points toward the conclusion that most Cepheids are
surrounded by faint CSE, detected by near infrared interferometry: after
observing four Cepheids, all show evidence for a CSE. Our CSE non-detection
around a non-pulsating supergiant in the instability strip, alpha Per, provides
confidence in the detection technique and suggests a pulsation driven mass-loss
mechanism for the Cepheids.Comment: accepted for publication in Ap
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