Context. Among optical stellar interferometers, the CHARA Array located at Mt Wilson in California offers the potential of very long baselines (up to 330 m) and the prospect of coupling multiple beam ...combiners. This paper presents the principle and the measured performance of VEGA, Visible spEctroGraph and polArimeter installed in September 2007 at the coherent focus of the array. Aims. With 0.3 ms of arc of spatial resolution and up to $30 000$ of spectral resolution, VEGA intends to measure fundamental parameters of stars, to study stellar activities and to image and analyze circumstellar environments. We describe the observing modes that have been implemented for this spectro-polarimeter and show actual performances measured on the sky during the first observing runs. Methods. The astrophysical programs are described in relation to the observing modes of the instrument, the presentation of the spectrograph and of the interface table is shown and finally the data is presented. We discuss the perspectives of further development in the framework of the CHARA Array. Results. We show that VEGA/CHARA is fully operational. The current limiting magnitude is nearly 7 but the results depend on the observing conditions (seeing, spectral resolution, etc.). We have validated the stability of the instrumental visibility at the level of 1 to 2% over half an hour and of the instrumental polarization for various declinations. Some examples of squared visibility and differential visibility are presented. Conclusions. The spectro-polarimeter VEGA has been installed and successfully tested on CHARA. It will permit stellar physics studies at unprecedented spectral and spatial resolutions.
Context. Five different physical processes might be responsible for the formation of decretion disks around Be stars: fast rotation of the star, stellar pulsations, binarity, stellar winds, and ...magnetic fields. Our observations indicate that fast rotation seems to produce a disk in Keplerian rotation, at least in the specific case of the two stars observed. We do not know if this observational result is a generality or not. Aims. We measure the size, orientation, shape, and kinematics of the disks around 2 Be stars, namely 48 Per and ψ Per. Methods. We used the VEGA/CHARA interferometer with a spectral resolution of 5000 to obtain spectrally dispersed visibility modulus and phases within the Hα emission line. Results. We were able to estimate the disk extension in the continuum and in the Hα line, as well as flattening, for both stars. Both stars rotate at nearly a critical rotation, but while the disk of 48 Per seems to be in Keplerian rotation, our preliminary data suggest that the disk of ψ Per is possibly faster than Keplerian, similarly to what has been found for κ CMa with observations carried out in the near-IR. However, more data is needed to confirm the fast rotation of the disk. Conclusions. Assuming a simple uniform disk model for the stellar photosphere in the continuum and a Gaussian brightness distribution in the line emission region, we obtain a ratio of the disk diameter over the photospheric diameter of 8 for 48 Per and 11 for and ψ Per. We also found that the major axis of 48 Per is parallel to the polarization angle and not perpendicular to it as previously observed for many Be stars, including ψ Per. This might be due to the optical thickness of the disk, which is also responsible for the incoherent scattering of a non negligible part of the Hα line emission. To our knowledge, this is the first time that this effect has been measured in a Be star.
Aims. In the context of the future developments of long baseline interferometry at visible wavelengths, we have built a prototype instrument called Fibered spectrally Resolved Interferometer – New ...Design (FRIEND) based on single mode fibers and a new generation detector called Electron Multiplying Charge-Coupled Device (EMCCD). Installed on the Center for High Angular Resolution Astronomy (CHARA) array, it aims to estimate the performance of a fibered instrument in the visible when coupled with telescopes equipped with adaptive optics (AO) in partial correction. Methods. We observed different sequences of targets and reference stars to study the compensation of the birefringence of the fibers, the coupling efficiency in various conditions of correction, and to calibrate our numerical model of signal-to-noise ratio (S/N). We also used a known binary star to demonstrate the reliability and the precision of our squared visibility and closure phase measurements. Results. We firstly present a reliable and stable solution for compensating the birefringence of the fibers with an improvement of a factor of 1.5 of the instrumental visibility. We then demonstrate an improvement by a factor of between 2.5 and 3 of the coupling efficiency when using the LABAO systems in closed loop. The third results of our paper is the demonstration of the correct calibration of the parameters of our S/N estimator provided the correct excess noise factor of EMCCD is correctly taken into account. Finally with the measurements of the angular separation, difference of magnitude and individual diameters of the two components of ζ Ori A, we demonstrate the reliability and precision of our interferometric estimators, and in particular a median residual on the closure phase of 1.2°.
The enigmatic binary, ε Aur, is yielding its parameters as a result of new methods applied to the recent eclipse, including optical spectro-interferometry with the VEGA beam combiner at the CHARA ...Array. VEGA/CHARA visibility measurements from 2009 to 2011 indicate the formation of emission wings of Hα in an expanding zone almost twice the photospheric size of the F star, namely, in a stellar wind. These may be caused by shocks in the atmosphere from large scale convective or multi-periodic pulsation modes emerging from the star. During the total eclipse phase in 2010, when the disk was in the line of sight, we saw broadening of the Hα absorption and a less steep drop of the visibility curve, consistent with the addition of neutral hydrogen in the line of sight but extended above and below the plane of the interferometrically imaged disk itself. This provides a unique constraint on the scale height of the gaseous component of the disk material, and, based on some additional assumptions, points to a mass of the central object being 2.4 to 5.5 M⊙ for a distance of 650 pc or 3.8 to 9.1 M⊙ for a distance of 1050 pc. These results can be tested during coming observing seasons as the star moves from eclipse phase toward quadrature.
Context. Since the discovery of the first exoplanet in 1995 around a solar-type star, the interest in exoplanetary systems has kept increasing. Studying exoplanet host stars is of the utmost ...importance to establish the link between the presence of exoplanets around various types of stars and to understand the respective evolution of stars and exoplanets. Aims. Using the limb-darkened diameter (LDD) obtained from interferometric data, we determine the fundamental parameters of four exoplanet host stars. We are particularly interested in the F4 main-sequence star, θ Cyg, for which Kepler has recently revealed solar-like oscillations that are unexpected for this type of star. Furthermore, recent photometric and spectroscopic measurements with SOPHIE and ELODIE (OHP) show evidence of a quasi-periodic radial velocity of ~150 days. Models of this periodic change in radial velocity predict either a complex planetary system orbiting the star, or a new and unidentified stellar pulsation mode. Methods. We performed interferometric observations of θ Cyg, 14 Andromedae, υ Andromedae and 42 Draconis for two years with VEGA/CHARA (Mount Wilson, California) in several three-telescope configurations. We measured accurate limb darkened diameters and derived their radius, mass and temperature using empirical laws. Results. We obtain new accurate fundamental parameters for stars 14 And, υ And and 42 Dra. We also obtained limb darkened diameters with a minimum precision of ~1.3%, leading to minimum planet masses of Msini = 5.33 ± 0.57, 0.62 ± 0.09 and 3.79 ± 0.29 MJup for 14 And b, υ And b and 42 Dra b, respectively. The interferometric measurements of θ Cyg show a significant diameter variability that remains unexplained up to now. We propose that the presence of these discrepancies in the interferometric data is caused either by an intrinsic variation of the star or an unknown close companion orbiting around it.
The Carlina-type diluted telescope Le Coroller, H; Dejonghe, J; Hespeels, F ...
Astronomy and astrophysics (Berlin),
1/2015, Volume:
573
Journal Article
Peer reviewed
The performance of interferometers has been much increased over the past ten years. Studies have been conducted with the aim to propose a new generation of interferometers. The Carlina concept ...studied at the Haute-Provence Observatory consists of an optical interferometer configured as a diluted version of the Arecibo radio telescope. Since 2003, we have been building a technical demonstrator of this diluted telescope. The main goals of this project were to find opto-mechanical solutions to stabilize the optics attached to cables at several tens of meters above the ground, and to characterize this diluted telescope under real conditions. By detecting fringes on Deneb, we confirm that the entire system conceptually has worked correctly. It also proves that when the primary mirrors are aligned using the metrology system, we can directly record fringes in the focal gondola, even in blind operation. Its optical architecture has many advantages for future projects: Planet Formation Imager (PFI), post-ELTs, interferometer in space.
Abstract
We present the Mid-infrared stellar Diameters and Fluxes compilation Catalogue (MDFC) dedicated to long-baseline interferometry at mid-infrared wavelengths (3–13 $\mu$m). It gathers data for ...half a million stars, i.e. nearly all the stars of the Hipparcos-Tycho catalogue whose spectral type is reported in the SIMBAD data base. We cross-match 26 data bases to provide basic information, binarity elements, angular diameter, magnitude and flux in the near and mid-infrared, as well as flags that allow us to identify the potential calibrators. The catalogue covers the entire sky with 465 857 stars, mainly dwarfs and giants from B to M spectral types closer than 18 kpc. The smallest reported values reach 0.16 $\mu$Jy in L and 0.1 $\mu$Jy in N for the flux, and 2 microarcsec for the angular diameter. We build four lists of calibrator candidates for the L and Nbands suitable with the Very Large Telescope Interferometer (VLTI) sub- and main arrays using the MATISSE instrument. We identify 1621 candidates for L and 44 candidates for N with the Auxiliary Telescopes (ATs), 375 candidates for both bands with the ATs, and 259 candidates for both bands with the Unit Telescopes (UTs). Predominantly cool giants, these sources are small and bright enough to belong to the primary lists of calibrator candidates. In the near future, we plan to measure their angular diameter with 1 per cent accuracy.
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