142 research outputs found
Pupil plane optimization for single-mode multiaxial optical interferometry with a large number of telescopes
Incoming optical interferometers will allow spectro-imaging at high angular
resolution. Non-homothetic Fizeau concept combines good sensitivity and high
spectral resolution capabilities. However, one critical issue is the design of
the beam recombination scheme, at the heart of the instrument. We tackle the
possibility of reducing the number of pixels that are coding the fringes by
compressing the pupil plane. Shrinking the number of pixels -- which
drastically increases with the number of recombined telescopes -- is indeed a
key issue that enables to reach higher limiting magnitude, but also allows to
lower the required spectral resolution and fasten the fringes reading process.
By means of numerical simulations, we study the performances of existing
estimators of the visibility with respect to the compression process. We show
that, not only the model based estimator lead to better signal to noise ratio
(SNR) performances than the Fourier ones, but above all it is the only one
which prevent from introducing baseline mixing biases in the visibilities as
the pupil plane compression rate increases. Furthermore, we show that moderate
compression allows to keep the visibilities SNR unaffected. In the light of
these conclusions, we propose an optimized pupil arrangements for 6 and 8 beam
recombiners
Masses and age of the Chemically Peculiar double-lined binary ~Lupi
We aim at measuring the stellar parameters of the two Chemically Peculiar
components of the B9.5Vp HgMn + A2 Vm double-lined spectroscopic binary
HD141556, whose period is 15.25 days. We combined historical radial velocity
measurements with new spatially resolved astrometric observations from
PIONIER/VLTI to reconstruct the three-dimensional orbit of the binary, and thus
obtained the individual masses. We fit the available photometric points
together with the flux ratios provided by interferometry to constrain the
individual sizes, which we compared to predictions from evolutionary models.The
individual masses of the components are \Ma = 2.84 \pm 0.12\ \Msun and \Mb =
1.94 \pm 0.09\ \Msun. The dynamical distance is compatible with the Hipparcos
parallax. We find linear stellar radii of \Ra=2.85 \pm 0.15\ \Rsun and
\Rb=1.75 \pm 0.18\ \Rsun. This result validates a posteriori the flux ratio
used in previous detailed abundance studies. We determine a sub-solar initial
metallicity and an age of years. Our
results imply that the primary rotates more slowly than its synchronous
velocity, while the secondary is probably synchronous. We show that strong
tidal coupling during the pre-main sequence evolution followed by a full
decoupling at zero-age main sequence provides a plausible explanation for these
very low rotation rates.Comment: 8 pages, accepted in Aand
Refined masses and distance of the young binary Haro 1-14 C
We aim to refine the dynamical masses of the individual component of the
low-mass pre-main sequence binary Haro 1-14 C. We combine the data of the
preliminary orbit presented previously with new interferometric observations
obtained with the four 8m telescopes of the Very Large Telescope
Interferometer. The derived masses are M_a=0.905\pm0.043\,\Msun and
M_b=0.308\pm0.011\,\Msun for the primary and secondary components,
respectively. This is about five times better than the uncertainties of the
preliminary orbit. Moreover, the possibility of larger masses is now securely
discarded. The new dynamical distance, pc, is smaller than the
distance to the Ophiuchus core with a significance of . Fitting
the spectral energy distribution yields apparent diameters of
\phi_a=0.13\pm0.01\mas and \phi_b=0.10\pm0.01\mas (corresponding to
\Ra=1.50\,\Rsun and \Rb=1.13\,\Rsun) and a visual extinction of
. Although the revised orbit has a nearly edge-on geometry, the
system is unlikely to be a long-period eclipsing binary. The secondary in
Haro~1-14C is one of the few low-mass, pre-main sequence stars with an
accurately determined dynamical mass and distance
A PIONIER View on Mass-Transferring Red Giants
Symbiotic stars display absorption lines of a cool red giant together with
emission lines of a nebula ionized by a hotter star, indicative of an active
binary star system in which mass transfer is occurring. PIONIER at the VLT has
been used to combine the light of four telescopes at a time to study in
unprecedented detail how mass is transferred in symbiotic stars. The results of
a mini-survey of symbiotic stars with PIONIER are summarised and some tentative
general results about the role of Roche lobe overflow are presented.Comment: Report for the ESO Messenger June issu
Unveiling new stellar companions from the PIONIER exozodi survey
The main goal of the EXOZODI survey is to detect and characterize
circumstellar dust and to propose the first statistical study of exozodiacal
disks in the near-infrared using telescopes in both hemispheres. For this
purpose, Ertel et al. have conducted in 2012 a survey of nearby main sequence
stars with VLTI/PIONIER to search for the presence of circumstellar dust. This
survey, carried out during 12 nights, comprises about 100 stars. For each star,
we obtained typically three OBs and we searched for circumstellar emission
based on the measurement of squared visibilities at short baselines. A drop in
the measured visibilities with respect to the expected photospheric visibility
indicates the presence of resolved emission around the target star. It is
however generally not possible to conclude on the morphology of the detected
emission based solely on the squared visibilities. Here, we focus on closure
phases to search for faint companions around the whole sample. Indeed, to
derive robust statistics on the occurrence rate of bright exozodiacal disks, we
need to discriminate between companions and disks. For this reason, the main
goal of this paper is to discriminate between circumstellar disks (which show
no closure phase provided that they are point-symmetric) and faint companions
(point-like sources, creating non-zero closure phases). We also aim to reveal
new companions that do not necessarily produce a significant signature in the
squared visibilities, as the signature of the companion may show up more
prominently in the closure phases. In this process, we reveal four new stellar
companions with contrasts ranging from 2% to 95% (i.e., up to equal flux
binaries). We also tentatively detect faint companions around one other target
that will require follow-up observations to be confirmed or infirmed. We
discuss the implications of these discoveries on the results of the exozodi
survey.Comment: To appear in SPIE proceedings vol. 914
Searching for faint companions with VLTI/PIONIER. II. 92 main sequence stars from the Exozodi survey
The Exozodi survey aims to determine the occurrence rate of bright
exozodiacal discs around nearby main sequence stars using infrared
interferometry. Although the Exozodi survey targets have been carefully
selected to avoid the presence of binary stars, the results of this survey can
still be biased by the presence of unidentified stellar companions. Using the
PIONIER data set collected within the Exozodi survey, we aim to search for the
signature of point-like companions around the Exozodi target stars. We use both
the closure phases and squared visibilities collected by PIONIER to search for
companions within the ~100 mas interferometric field of view. The presence of a
companion is assessed by computing the goodness of fit to the data for a series
of binary models with various separations and contrasts. Five stellar
companions are resolved for the first time around five A-type stars: HD 4150,
HD 16555, HD 29388, HD 202730, and HD 224392 (although the companion to HD
16555 was independently resolved by speckle interferometry while we were
carrying out the survey). In the most likely case of main sequence companions,
their spectral types range from A5V to K4V. Three of these stars were already
suspected to be binaries from Hipparcos astrometric measurements, although no
information was available on the companions themselves so far. In addition to
debiasing the statistics of the Exozodi survey, these results can also be used
to revise the fraction of visual binaries among A-type stars, suggesting that
an extra ~13% A-type stars are visual binaries in addition to the ones detected
in previous direct imaging surveys. We estimate that about half the population
of nearby A-type stars could be resolved as visual binaries using a combination
of state-of-the-art interferometry and single-aperture imaging, and we suggest
that a significant fraction of these binaries remains undetected to date.Comment: Accepted for publication in A&
Observational calibration of the projection factor of Cepheids. II. Application to nine Cepheids with HST/FGS parallax measurements
The distance to pulsating stars is classically estimated using the
parallax-of-pulsation (PoP) method, which combines spectroscopic radial
velocity measurements and angular diameter estimates to derive the distance of
the star. An important application of this method is the determination of
Cepheid distances, in view of the calibration of their distance scale. However,
the conversion of radial to pulsational velocities in the PoP method relies on
a poorly calibrated parameter, the projection factor (p-factor). We aim to
measure empirically the value of the p-factors of a homogeneous sample of nine
Galactic Cepheids for which trigonometric parallaxes were measured with the
Hubble Space Telescope Fine Guidance Sensor. We use the SPIPS algorithm, a
robust implementation of the PoP method that combines photometry,
interferometry, and radial velocity measurements in a global modeling of the
pulsation. We obtained new interferometric angular diameters using the PIONIER
instrument at the Very Large Telescope Interferometer, completed by data from
the literature. Using the known distance as an input, we derive the value of
the p-factor and study its dependence with the pulsation period. We find the
following p-factors: 1.20 0.12 for RT Aur, 1.48 0.18 for T Vul,
1.14 0.10 for FF Aql, 1.31 0.19 for Y Sgr, 1.39 0.09 for X
Sgr, 1.35 0.13 for W Sgr, 1.36 0.08 for Dor, 1.41
0.10 for Gem, and 1.23 0.12 for Car. These values are
consistently close to p = 1.324 0.024. We observe some dispersion around
this average value, but the observed distribution is statistically consistent
with a constant value of the p-factor as a function of the pulsation period.
The error budget of our determination of the p-factor values is presently
dominated by the uncertainty on the parallax, a limitation that will soon be
waived by Gaia.Comment: 18 pages, 13 figure
Observational calibration of the projection factor of Cepheids I. The Type II Cepheid kappa Pavonis
The distances of pulsating stars, in particular Cepheids, are commonly
measured using the parallax of pulsation technique. The different versions of
this technique combine measurements of the linear diameter variation (from
spectroscopy) and the angular diameter variation (from photometry or
interferometry) amplitudes, to retrieve the distance in a quasi-geometrical
way. However, the linear diameter amplitude is directly proportional to the
projection factor (hereafter p-factor), which is used to convert spectroscopic
radial velocities (i.e., disk integrated) into pulsating (i.e., photospheric)
velocities. The value of the p-factor and its possible dependence on the
pulsation period are still widely debated. Our goal is to measure an
observational value of the p-factor of the type-II Cepheid kappa Pavonis, whose
parallax was measured with an accuracy of 5% using HST/FGS. We used this
parallax as a starting point to derive the p-factor of kappa Pav, using the
SPIPS technique, which is a robust version of the parallax-of-pulsation method
that employs radial velocity, interferometric and photometric data. We applied
this technique to a combination of new VLTI/PIONIER optical interferometric
angular diameters, new CORALIE and HARPS radial velocities, as well as
multi-colour photometry and radial velocities from the literature. We obtain a
value of p = 1.26 +/- 0.07 for the p-factor of kappa Pav. This result agrees
with several of the recently derived Period-p-factor relationships from the
literature, as well as previous observational determinations for Cepheids.
Individual estimates of the p-factor are fundamental to calibrating the
parallax of pulsation distances of Cepheids. Together with previous
observational estimates, the projection factor we obtain points to a weak
dependence of the p-factor on period.Comment: 8 pages, 6 figures, accepted in A&
Astronomical interferometry with near-IR e-APD at CHARA: characterization, optimization and on-sky operation
We characterize a near-infrared C-RED ONE camera from First Light Imaging
(FLI). This camera uses a SAPHIRA electron avalanche photo-diode array (e-APD)
from Leonardo (previously Selex). To do so, we developed a model of the signal
distribution. This model allows a measurement of the gain and the Excess Noise
Factor (ENF) independently of preexisting calibration such as the system gain.
The results of this study show a gain which is 0.53 +/- 0.04 times the gain
reported by the manufacturer. The measured ENF is 1.47 +/- 0.03 when we
expected 1.25. For an avalanche gain of 60 and a frame rate larger than 100 Hz,
the total noise can be lower than 1 e-/frame/pixel. The lowest dark current
level is 90e-/s/pixel, in agreement with the expected H-band background passing
through the camera window. These performance values provide a significant
improvement compared to earlier-generation PICNIC camera and allowed us to
improve the performance of the Michigan infrared combiner (MIRC) instrument at
the Center for High Angular Resolution Astronomy (CHARA), as part of our MIRC-X
instrumentation project.Comment: 18 pages, 15 figures, presented at SPIE Astronomical Telescopes +
Instrumentation 2018, Austin, Texas, US
A discontinuity in the -radius relation of M-dwarfs
We report on 13 new high-precision measurements of stellar diameters for
low-mass dwarfs obtained by means of near-infrared long-baseline interferometry
with PIONIER at the Very Large Telescope Interferometer. Together with accurate
parallaxes from Gaia DR2, these measurements provide precise estimates for
their linear radii, effective temperatures, masses, and luminosities. This
allows us to refine the effective temperature scale, in particular towards the
coolest M-dwarfs. We measure for late-type stars with enhanced metallicity
slightly inflated radii, whereas for stars with decreased metallicity we
measure smaller radii. We further show that Gaia DR2 effective temperatures for
M-dwarfs are underestimated by 8.2 % and give an empirical
- relation which is better suited for M-dwarfs with between 2600 and 4000 K. Most importantly, we are able to observationally
identify a discontinuity in the -radius plane, which is likely due
to the transition from partially convective M-dwarfs to the fully convective
regime. We found this transition to happen between 3200 K and 3340 K, or
equivalently for stars with masses . We find that in
this transition region the stellar radii are in the range from 0.18 to
0.42 for similar stellar effective temperatures.Comment: 11 pages, 9 figures, accepted in MNRA
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