3,575 research outputs found
Study of the true performance limits of the Astrometric Multiplexing Area Scanner (AMAS)
The Astrometric Multiplexing Area Scanner (AMAS) is an instrument designed to perform photoelectric long focus astrometry of small fields. Modulation of a telescope focal plane with a rotating Ronchi ruling produces a frequency modulated signal from which relative positions and magnitudes can be extracted. Evaluation instrumental precision, accuracy and resolution characteristics with respect to a variety of instrumental and cosmical parameters indicates 1.5 micron precision and accuracy for single stars under specific conditions. This value decreases for increased number of field stars, particularly for fainter stars
The R Aquarii System at Optical and Radio Wavelengths
Observations of the symbiotic binary R Aquarii environment were obtained with the Very Large Array (VLA) at 2, 6, and 20 em at the same epoch. The observed spectral index and strong linear polarization reveal that emission from the compact double radio source discovered in previous observations is nonthermal; thus, this source is not associated with R Aquarii but is an extragalactic background object. The spectral index of the compact nebula surrounding R Aquarii indicates that the emission is thermal and the nebula is ionized by an unseen, hot companion to the Mira-like variable R Aquarii. As expected, this region shows no indication of linear polarization, and we have determined a steady state mass loss of ~ 2. 7 x 10- 7 M 0 yr- 1 from the system. However, the spectral index and polarization observations of the extended jet ~6 away from R Aquarii indicate that this amorphous source is definitely thermal and optically thin in nature. These new observations place severe constraints on possible models which have been proposed from previous investigations. We suggest that our new observations of the jet can be best explained by enhanced mass exchange occurring periodically in the symbiotic system. Comparison of 6 em data taken with the same VLA configuration but separated by 495 days does not indicate any appreciable morphological change or statistically significant integrated flux difference and thus suggests that on these time scales the jet is now quite stable. High-resolution white light images of R Aquarii and environs obtained with the 4 m telescope at Kitt Peak just prior to the VLA observations show a high degree of correlation with the 6 em radio data and place a limit on the apparent visual magnitude of the compact double radio source; optical speckle interferometry failed to resolve any components in the R Aquarii system
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
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
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
Direct Measurement of the Radius and Density of the Transiting Exoplanet HD 189733B with the CHARA Array
We have measured the angular diameter of the transiting extrasolar planet
host star HD 189733 using the CHARA O/IR interferometric array. Combining our
new angular diameter of 0.377+/-0.024 mas with the Hipparcos parallax leads to
a linear radius for the host star of 0.779+/-0.052 Rsol and a radius for the
planet of 1.19+/-0.08 RJup. Adopting the mass of the planet as derived by its
discoverers, we derive a mean density of the planet of 0.91+/-0.18 g cm-3. This
is the first determination of the diameter of an extrasolar planet through
purely direct means.Comment: 14 pages, 5 figures, to be published in Astrophysical Journal Letter
Multiplicity of Galactic Cepheids from long-baseline interferometry~III. Sub-percent limits on the relative brightness of a close companion of ~Cephei
We report new CHARA/MIRC interferometric observations of the Cepheid
archetype Cep, which aimed at detecting the newly discovered
spectroscopic companion. We reached a maximum dynamic range = 6.4,
5.8, and 5.2 mag, respectively within the relative distance to the Cepheid mas, mas and mas. Our observations did not
show strong evidence of a companion. We have a marginal detection at
with a flux ratio of 0.21\%, but nothing convincing as we found other possible
probable locations. We ruled out the presence of companion with a spectral type
earlier than F0V, A1V and B9V, respectively for the previously cited ranges
. From our estimated sensitivity limits and the Cepheid light curve, we
derived lower-limit magnitudes in the band for this possible companion to
be and 7.77 mag, respectively for mas,
mas and mas. We also found that to be consistent
with the predicted orbital period, the companion has to be located at a
projected separation mas with a spectral type later than a F0V star.Comment: Accepted for publication in MNRA
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
Radial Structure in the TW Hya Circumstellar Disk
We present new near-infrared interferometric data from the CHARA array and the Keck Interferometer on the circumstellar disk of the young star, TW Hya, a proposed "transition disk." We use these data, as well as previously published, spatially resolved data at 10 μm and 7 mm, to constrain disk models based on a standard flared disk structure. We find that we can match the interferometry data sets and the overall spectral energy distribution with a three-component model, which combines elements at spatial scales proposed by previous studies: optically thin, emission nearest the star, an inner optically thick ring of emission at roughly 0.5 AU followed by an opacity gap and, finally, an outer optically thick disk starting at ~4 AU. The model demonstrates that the constraints imposed by the spatially resolved data can be met with a physically plausible disk but this requires a disk containing not only an inner gap in the optically thick disk as previously suggested, but also a gap between the inner and outer optically thick disks. Our model is consistent with the suggestion by Calvet et al. of a planet with an orbital radius of a few AU. We discuss the implications of an opacity gap within the optically thick disk
- …