623 research outputs found
Data reduction for the AMBER instrument
We present here the general formalism and data processing steps used in the data reduction pipeline of the AMBER instrument. AMBER is a three-telescope interferometric beam combiner in J, H and K bands installed at ESO\'s Very Large Telescope Interferometer. The fringes obtained on the 3 pairs of telescopes are spatially coded and spectrally dispersed. These are monitored on a 512x512 infrared camera at frame rates up to 100 frames per second, and this paper presents the algorithm used to retrieve the complex coherent visibility of the science target and the subsequent squared visibility, dierential phase and phase closure on the 3 bases and in the 3 spectral bands available in AMBER
Southern Massive Stars at High Angular Resolution: Observational Campaign and Companion Detection
Multiplicity is one of the most fundamental observable properties of massive
O-type stars and offers a promising way to discriminate between massive star
formation theories. Nevertheless, companions at separations between 1 and 100
mas remain mostly unknown due to intrinsic observational limitations. [...] The
Southern MAssive Stars at High angular resolution survey (SMASH+) was designed
to fill this gap by providing the first systematic interferometric survey of
Galactic massive stars. We observed 117 O-type stars with VLTI/PIONIER and 162
O-type stars with NACO/SAM, respectively probing the separation ranges 1-45 and
30-250mas and brightness contrasts of Delta H < 4 and Delta H < 5. Taking
advantage of NACO's field-of-view, we further uniformly searched for visual
companions in an 8''-radius down to Delta H = 8. This paper describes the
observations and data analysis, reports the discovery of almost 200 new
companions in the separation range from 1mas to 8'' and presents the catalog of
detections, including the first resolved measurements of over a dozen known
long-period spectroscopic binaries.
Excluding known runaway stars for which no companions are detected, 96
objects in our main sample (DEC < 0 deg; H<7.5) were observed both with PIONIER
and NACO/SAM. The fraction of these stars with at least one resolved companion
within 200mas is 0.53. Accounting for known but unresolved spectroscopic or
eclipsing companions, the multiplicity fraction at separation < 8'' increases
to f_m = 0.91 +/- 0.03. The fraction of luminosity class V stars that have a
bound companion reaches 100% at 30mas while their average number of physically
connected companions within 8'' is f_c = 2.2 +/- 0.3. This demonstrates that
massive stars form nearly exclusively in multiple systems. Additionally, the
nine non-thermal (NT) radio emitters observed by SMASH+ are all resolved [...]Comment: 57 pages, 20 figures, 7 tables; accepted for publication in ApJ
Masses and age of the Chemically Peculiar double-lined binary ~Lupi
8 pages, accepted in AandAWe 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 ~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
Calibration of quasi-static aberrations in exoplanet direct-imaging instruments with a Zernike phase-mask sensor. II. Concept validation with ZELDA on VLT/SPHERE
Warm or massive gas giant planets, brown dwarfs, and debris disks around
nearby stars are now routinely observed by dedicated high-contrast imaging
instruments on large, ground-based observatories. These facilities include
extreme adaptive optics (ExAO) and state-of-the-art coronagraphy to achieve
unprecedented sensitivities for exoplanet detection and spectral
characterization. However, differential aberrations between the ExAO sensing
path and the science path represent a critical limitation for the detection of
giant planets with a contrast lower than a few at very small
separations (<0.3\as) from their host star. In our previous work, we proposed a
wavefront sensor based on Zernike phase contrast methods to circumvent this
issue and measure these quasi-static aberrations at a nanometric level. We
present the design, manufacturing and testing of ZELDA, a prototype that was
installed on VLT/SPHERE during its reintegration in Chile. Using the internal
light source of the instrument, we performed measurements in the presence of
Zernike or Fourier modes introduced with the deformable mirror. Our
experimental and simulation results are consistent, confirming the ability of
our sensor to measure small aberrations (<50 nm rms) with nanometric accuracy.
We then corrected the long-lived non-common path aberrations in SPHERE based on
ZELDA measurements. We estimated a contrast gain of 10 in the coronagraphic
image at 0.2\as, reaching the raw contrast limit set by the coronagraph in the
instrument. The simplicity of the design and its phase reconstruction algorithm
makes ZELDA an excellent candidate for the on-line measurements of quasi-static
aberrations during the observations. The implementation of a ZELDA-based
sensing path on the current and future facilities (ELTs, future space missions)
could ease the observation of the cold gaseous or massive rocky planets around
nearby stars.Comment: 13 pages, 12 figures, A&A accepted on June 3rd, 2016. v2 after
language editin
SearchCal: a Virtual Observatory tool for searching calibrators in optical long baseline interferometry. I: The bright object case
In long baseline interferometry, the raw fringe contrast must be calibrated
to obtain the true visibility and then those observables that can be
interpreted in terms of astrophysical parameters. The selection of suitable
calibration stars is crucial for obtaining the ultimate precision of
interferometric instruments like the VLTI. We have developed software SearchCal
that builds an evolutive catalog of stars suitable as calibrators within any
given user-defined angular distance and magnitude around the scientific target.
We present the first version of SearchCal dedicated to the bright-object case
V<=10; K<=5). Star catalogs available at the CDS are consulted via web
requests. They provide all the useful information for selecting of calibrators.
Missing photometries are computed with an accuracy of 0.1 mag and the missing
angular diameters are calculated with a precision better than 10%. For each
star the squared visibility is computed by taking the wavelength and the
maximum baseline of the foreseen observation into account.} SearchCal is
integrated into ASPRO, the interferometric observing preparation software
developed by the JMMC, available at the address: http://mariotti.fr
Benefit of the Vittel criteria to determine the need for whole body scanning in a severe trauma patient.
OBJECTIVE: To evaluate the use of the Vittel criteria in addition to a clinical examination to determine the need for a whole body scan (WBS) in a severe trauma patient.
MATERIALS AND METHODS: Between December 2008 and November 2009, 339 severe trauma patients with at least one Vittel criterion were prospectively evaluated with a WBS. The following data were collected: the Vittel criteria present, circumstances of the accident, traumatic injury on the WBS, and irradiation. The original intent to prescribe a computed tomography (CT) scan (whole body or a targeted region), based solely on clinical signs, was specified.
RESULTS: Injuries were diagnosed in 55.75% of the WBS (n=189). The most common Vittel criteria were "global assessment" (n=266), "thrown, run over" (n=116), and "ejected from vehicle" (n=94). The multivariate analysis used the following as independent criteria for predicting severe traumatic injury on the WBS: Glasgow score less than 13, penetrating trauma, and colloid resuscitation greater than 11. Based solely on clinical factors, 164 patients would not have had any scan or (only) a targeted scan. In that case, 15% of the severe injuries would have been missed.
CONCLUSION: Using the Vittel criteria to determine the need for a WBS in a severe trauma patient makes it possible to find serious injuries not suspected on the clinical examination, but at the cost of an increased number of normal scans
Searching for faint companions with VLTI/PIONIER. I. Method and first results
Context. A new four-telescope interferometric instrument called PIONIER has
recently been installed at VLTI. It provides improved imaging capabilities
together with high precision. Aims. We search for low-mass companions around a
few bright stars using different strategies, and determine the dynamic range
currently reachable with PIONIER. Methods. Our method is based on the closure
phase, which is the most robust interferometric quantity when searching for
faint companions. We computed the chi^2 goodness of fit for a series of binary
star models at different positions and with various flux ratios. The resulting
chi^2 cube was used to identify the best-fit binary model and evaluate its
significance, or to determine upper limits on the companion flux in case of non
detections. Results. No companion is found around Fomalhaut, tau Cet and
Regulus. The median upper limits at 3 sigma on the companion flux ratio are
respectively of 2.3e-3 (in 4 h), 3.5e-3 (in 3 h) and 5.4e-3 (in 1.5 h) on the
search region extending from 5 to 100 mas. Our observations confirm that the
previously detected near-infrared excess emissions around Fomalhaut and tau Cet
are not related to a low-mass companion, and instead come from an extended
source such as an exozodiacal disk. In the case of del Aqr, in 30 min of
observation, we obtain the first direct detection of a previously known
companion, at an angular distance of about 40 mas and with a flux ratio of
2.05e-2 \pm 0.16e-2. Due to the limited u,v plane coverage, its position can,
however, not be unambiguously determined. Conclusions. After only a few months
of operation, PIONIER has already achieved one of the best dynamic ranges
world-wide for multi-aperture interferometers. A dynamic range up to about
1:500 is demonstrated, but significant improvements are still required to reach
the ultimate goal of directly detecting hot giant extrasolar planets.Comment: 11 pages, 6 figures, accepted for publication in A&
Non-monotonic field-dependence of the ZFC magnetization peak in some systems of magnetic nanoparticles
We have performed magnetic measurements on a diluted system of gamma-Fe2O3
nanoparticles (~7nm), and on a ferritin sample. In both cases, the ZFC-peak
presents a non-monotonic field dependence, as has already been reported in some
experiments,and discussed as a possible evidence of resonant tunneling. Within
simple assumptions, we derive expressions for the magnetization obtained in the
usual ZFC, FC, TRM procedures. We point out that the ZFC-peak position is
extremely sensitive to the width of the particle size distribution, and give
some numerical estimates of this effect. We propose to combine the FC
magnetization with a modified TRM measurement, a procedure which allows a more
direct access to the barrier distribution in a field. The typical barrier
values which are obtained with this method show a monotonic decrease for
increasing fields, as expected from the simple effect of anisotropy barrier
lowering, in contrast with the ZFC results. From our measurements on
gamma-Fe2O3 particles, we show that the width of the effective barrier
distribution is slightly increasing with the field, an effect which is
sufficient for causing the observed initial increase of the ZFC-peak
temperatures.Comment: LaTeX file 19 pages, 9 postscript figures. To appear in Phys. Rev. B
(tentative schedule: Dec.97
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