3,172 research outputs found
Structural Properties and Relative Stability of (Meta)Stable Ordered, Partially-ordered and Disordered Al-Li Alloy Phases
We resolve issues that have plagued reliable prediction of relative phase
stability for solid-solutions and compounds. Due to its commercially important
phase diagram, we showcase Al-Li system because historically density-functional
theory (DFT) results show large scatter and limited success in predicting the
structural properties and stability of solid-solutions relative to ordered
compounds. Using recent advances in an optimal basis-set representation of the
topology of electronic charge density (and, hence, atomic size), we present DFT
results that agree reasonably well with all known experimental data for the
structural properties and formation energies of ordered, off-stoichiometric
partially-ordered and disordered alloys, opening the way for reliable study in
complex alloys.Comment: 7 pages, 2 figures, 2 Table
CHARA/MIRC observations of two M supergiants in Perseus OB1: temperature, Bayesian modeling, and compressed sensing imaging
Two red supergiants of the Per OB1 association, RS Per and T Per, have been
observed in H band using the MIRC instrument at the CHARA array. The data show
clear evidence of departure from circular symmetry. We present here new
techniques specially developed to analyze such cases, based on state-of-the-art
statistical frameworks. The stellar surfaces are first modeled as limb-darkened
discs based on SATLAS models that fit both MIRC interferometric data and
publicly available spectrophotometric data. Bayesian model selection is then
used to determine the most probable number of spots. The effective surface
temperatures are also determined and give further support to the recently
derived hotter temperature scales of red su- pergiants. The stellar surfaces
are reconstructed by our model-independent imaging code SQUEEZE, making use of
its novel regularizer based on Compressed Sensing theory. We find excellent
agreement between the model-selection results and the reconstructions. Our
results provide evidence for the presence of near-infrared spots representing
about 3-5% of the stellar flux
A Spectroscopic Orbit for Regulus
We present a radial velocity study of the rapidly rotating B-star Regulus
that indicates the star is a single-lined spectroscopic binary. The orbital
period (40.11 d) and probable semimajor axis (0.35 AU) are large enough that
the system is not interacting at present. However, the mass function suggests
that the secondary has a low mass (M_2 > 0.30 M_sun), and we argue that the
companion may be a white dwarf. Such a star would be the remnant of a former
mass donor that was the source of the large spin angular momentum of Regulus
itself.Comment: 18 pages, 2 figures, ApJL in pres
Imaging the Algol Triple System in H Band with the CHARA Interferometer
Algol (Beta Per) is an extensively studied hierarchical triple system whose
inner pair is a prototype semi-detached binary with mass transfer occurring
from the sub-giant secondary to the main-sequence primary. We present here the
results of our Algol observations made between 2006 and 2010 at the CHARA
interferometer with the Michigan Infrared Combiner in the H band. The use of
four telescopes with long baselines allows us to achieve better than 0.5 mas
resolution and to unambiguously resolve the three stars. The inner and outer
orbital elements, as well as the angular sizes and mass ratios for the three
components are determined independently from previous studies. We report a
significantly improved orbit for the inner stellar pair with the consequence of
a 15% change in the primary mass compared to previous studies. We also
determine the mutual inclination of the orbits to be much closer to
perpendicularity than previously established. State-of-the-art image
reconstruction algorithms are used to image the full triple system. In
particular an image sequence of 55 distinct phases of the inner pair orbit is
reconstructed, clearly showing the Roche-lobe-filling secondary revolving
around the primary, with several epochs corresponding to the primary and
secondary eclipses
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
Multiplicity of Galactic Cepheids from long-baseline interferometry I. CHARA/MIRC detection of the companion of V1334 Cygni
We aim at determining the masses of Cepheids in binary systems, as well as
their geometric distances and the flux contribution of the companions. The
combination of interferometry with spectroscopy will offer a unique and
independent estimate of the Cepheid masses. Using long-baseline interferometry
at visible and infrared wavelengths, it is possible to spatially resolve binary
systems containing a Cepheid down to milliarcsecond separations. Based on the
resulting visual orbit and radial velocities, we can then derive the
fundamental parameters of these systems, particularly the masses of the
components and the geometric distance. We therefore performed interferometric
observations of the first-overtone mode Cepheid V1334 Cyg with the CHARA/MIRC
combiner. We report the first detection of a Cepheid companion using
long-baseline interferometry. We detect the signature of a companion orbiting
V1334 Cyg at two epochs. We measure a flux ratio between the companion and the
Cepheid f = 3.10+/-0.08%, giving an apparent magnitude mH = 8.47+/-0.15mag. The
combination of interferometric and spectroscopic data have enabled the unique
determination of the orbital elements: P = 1938.6+/-1.2 days, Tp = 2 443
616.1+/-7.3, a = 8.54+/-0.51mas, i = 124.7+/-1.8{\deg}, e = 0.190+/-0.013,
{\omega} = 228.7+/-1.6{\deg}, and {\Omega} = 206.3+/-9.4{\deg}. We derive a
minimal distance d ~ 691 pc, a minimum mass for both stars of 3.6 Msol, with a
spectral type earlier than B5.5V for the companion star. Our measured flux
ratio suggests that radial velocity detection of the companion using
spectroscopy is within reach, and would provide an orbital parallax and
model-free masses.Comment: Published in A&
First visual orbit for the prototypical colliding-wind binary WR 140
Wolf-Rayet stars represent one of the final stages of massive stellar
evolution. Relatively little is known about this short-lived phase and we
currently lack reliable mass, distance, and binarity determinations for a
representative sample. Here we report the first visual orbit for WR
140(=HD193793), a WC7+O5 binary system known for its periodic dust production
episodes triggered by intense colliding winds near periastron passage. The IOTA
and CHARA interferometers resolved the pair of stars in each year from
2003--2009, covering most of the highly-eccentric, 7.9 year orbit. Combining
our results with the recent improved double-line spectroscopic orbit of Fahed
et al. (2011), we find the WR 140 system is located at a distance of 1.67 +/-
0.03 kpc, composed of a WR star with M_WR = 14.9 +/- 0.5 Msun and an O star
with M_O = 35.9 +/- 1.3 Msun. Our precision orbit yields key parameters with
uncertainties times 6 smaller than previous work and paves the way for detailed
modeling of the system. Our newly measured flux ratios at the near-infrared H
and Ks bands allow an SED decomposition and analysis of the component
evolutionary states.Comment: Complete OIFITS dataset included via Data Conservancy Projec
Resolving Vega and the inclination controversy with CHARA/MIRC
Optical and infrared interferometers definitively established that the
photometric standard Vega (alpha Lyrae) is a rapidly rotating star viewed
nearly pole-on. Recent independent spectroscopic analyses could not reconcile
the inferred inclination angle with the observed line profiles, preferring a
larger inclination. In order to resolve this controversy, we observed Vega
using the six-beam Michigan Infrared Combiner on the Center for High Angular
Resolution Astronomy Array. With our greater angular resolution and dense
(u,v)-coverage, we find Vega is rotating less rapidly and with a smaller
gravity darkening coefficient than previous interferometric results. Our models
are compatible with low photospheric macroturbulence and also consistent with
the possible rotational period of ~0.71 days recently reported based on
magnetic field observations. Our updated evolutionary analysis explicitly
incorporates rapid rotation, finding Vega to have a mass of 2.15+0.10_-0.15
Msun and an age 700-75+150 Myrs, substantially older than previous estimates
with errors dominated by lingering metallicity uncertainties
(Z=0.006+0.003-0.002).Comment: Accepted for publication in ApJ Letter
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