Subtracting the photon noise bias from single-mode optical interferometer visibilities

I present in this paper a method to subtract the bias due to source photon noise from visibilities measured with a single-mode optical interferometer. Properties of the processed noise are demonstrated and examples of subtraction on real data are presented.Comment: To appear in Astronomy & Astrophysic

Wide field interferometric imaging with single-mode fibers

Classical single-mode fiber interferometers, using one fiber per aperture, have very limited imaging capabilities and small field of view. Observations of extended sources (resolved by one aperture) cannot be fully corrected for wavefront aberrations: accurate measurements of object visibilities are then made very difficult from ground-based fiber interferometers. These limitations are very severe for the new generation of interferometers, which make use of large telescopes equipped with adaptive optics, but can be overcome by using several fibers per aperture. This technique improves the wide field imaging capabilities of both ground-based and space interferometers.Comment: 14 pages, 14 figures. Accepted for publication in A&

The calibration of interferometric visibilities obtained with single-mode optical interferometers. Computation of error bars and correlations

I present in this paper a method to calibrate data obtained from optical and infrared interferometers. I show that correlated noises and errors need to be taken into account for a very good estimate of individual error bars but also when model fitting the data to derive meaningful model parameters whose accuracies are not overestimated. It is also shown that under conditions of high correlated noise, faint structures of the source can be detected. This point is important to define strategies of calibration for difficult programs such as exoplanet detection. The limits of validity of the assumptions on the noise statistics are discussed

Data reduction methods for single-mode optical interferometry - Application to the VLTI two-telescopes beam combiner VINCI

The interferometric data processing methods that we describe in this paper use a number of innovative techniques. In particular, the implementation of the wavelet transform allows us to obtain a good immunity of the fringe processing to false detections and large amplitude perturbations by the atmospheric piston effect, through a careful, automated selection of the interferograms. To demonstrate the data reduction procedure, we describe the processing and calibration of a sample of stellar data from the VINCI beam combiner. Starting from the raw data, we derive the angular diameter of the dwarf star Alpha Cen A. Although these methods have been developed specifically for VINCI, they are easily applicable to other single-mode beam combiners, and to spectrally dispersed fringes.Comment: Accepted for publication in Astronomy & Astrophysics, 17 pages, 19 figure

First radius measurements of very low mass stars with the VLTI

e present 4 very low mass stars radii measured with the VLTI using the 2.2 microns VINCI test instrument. The observations were carried out during the commissioning of the 104-meter-baseline with two 8-meter-telescopes. We measure angular diameters of 0.7-1.5 mas with accuracies of 0.04-0.11 mas, and for spectral type ranging from M0V to M5.5V. We determine an empirical mass-radius relation for M dwarfs based on all available radius measurements. The observed relation agrees well with theoretical models at the present accuracy level, with possible discrepancy around 0.5-0.8 Msolar that needs to be confirmed. In the near future, dozens of M dwarfs radii will be measured with 0.1-1% accuracy, with the VLTI, thanks to the improvements expected from the near infrared instrument AMBER. This will bring strong observational constraints on both atmosphere and interior physics.Comment: Accepted for publication in Astronomy and Astrophysics Letters, 4 pages, 3 figure

Cepheid distances from infrared long-baseline interferometry - I. VINCI/VLTI observations of seven Galactic Cepheids

We report the angular diameter measurements of seven classical Cepheids (X Sgr, eta Aql, W Sgr, zeta Gem, beta Dor, Y Oph and L Car) that we have obtained with the VINCI instrument, installed at ESO's VLT Interferometer (VLTI). We also present reprocessed archive data obtained with the FLUOR/IOTA instrument on zeta Gem, in order to improve the phase coverage of our observations. We obtain average limb darkened angular diameter values of LD(X Sgr) = 1.471 +/- 0.033 mas, LD(eta Aql) = 1.839 +/- 0.028 mas, LD(W Sgr) = 1.312 +/- 0.029 mas, LD(beta Dor) = 1.891 +/- 0.024 mas, LD(zeta Gem) =1.747 +/- 0.061 mas, LD(Y Oph) = 1.437 +/- 0.040 mas and LD(L Car) = 2.988 +/- 0.012 mas. For four of these stars (eta Aql, W Sgr, beta Dor, and L Car) we detect the pulsational variation of their angular diameter. This enables us to compute directly their distances, using a modified version of the Baade-Wesselink method: d(eta Aql) = 276 [+55 -38] pc, d(W Sgr) = 379 [+216 -130] pc, d(beta Dor) = 345 [+175 -80] pc, d(L Car) = 603 [+24 -19] pc. The stated error bars are statistical in nature. Applying a hybrid method, that makes use of the Gieren et al. (1998) Period-Radius relation to estimate the linear diameters, we obtain the following distances (statistical and systematic error bars are mentioned): d(X Sgr) = 324 +/- 7 +/- 17 pc, d(eta Aql) = 264 +/- 4 +/- 14 pc, d(W Sgr) = 386 +/- 9 +/- 21 pc, d(beta Dor) = 326 +/- 4 +/- 19 pc, d(zeta Gem) = 360 +/- 13 +/- 22 pc, d(Y Oph) = 648 +/- 17 +/- 47 pc and d(L Car) = 542 +/- 2 +/- 49 pc.Comment: 16 pages, 12 figures, accepted for publication in Astronomy & Astrophysic

Estimating the phase in ground-based interferometry: performance comparison between single-mode and multimode schemes

In this paper we compare the performance of multi and single-mode interferometry for the estimation of the phase of the complex visibility. We provide a theoretical description of the interferometric signal which enables to derive the phase error in presence of detector, photon and atmospheric noises, for both multi and single-mode cases. We show that, despite the loss of flux occurring when injecting the light in the single-mode component (i.e. single-mode fibers, integrated optics), the spatial filtering properties of such single-mode devices often enable higher performance than multimode concepts. In the high flux regime speckle noise dominated, single-mode interferometry is always more efficient, and its performance is significantly better when the correction provided by adaptive optics becomes poor, by a factor of 2 and more when the Strehl ratio is lower than 10%. In low light level cases (detector noise regime), multimode interferometry reaches better performance, yet the gain never exceeds 20%, which corresponds to the percentage of photon loss due to the injection in the guides. Besides, we demonstrate that single-mode interferometry is also more robust to the turbulence in both cases of fringe tracking and phase referencing, at the exception of narrow field of views (<1 arcsec).Comment: 9 pages (+ 11 online material appendices) -- 8 Figures. Accepted in A&

An active fiber sensor for mirror vibration metrology in astronomical interferometers

We present a fiber sensor based on an active integrated component which could be effectively used to measure the longitudinal vibration modes of telescope mirrors in an interferometric array. We demonstrate the possibility to measure vibrations with frequencies up to $\simeq 100$ Hz with a precision better than 10 nm.Comment: 7 pages, 6 figure

The angular diameter and distance of the Cepheid Zeta Geminorum

Cepheids are the primary distance indicators for extragalactic astronomy and therefore are of very high astrophysical interest. Unfortunately, they are rare stars, situated very far from Earth.Though they are supergiants, their typical angular diameter is only a few milliarcseconds, making them very challenging targets even for long-baseline interferometers. We report observations that were obtained in the K prime band (2-2.3 microns), on the Cepheid Zeta Geminorum with the FLUOR beam combiner, installed at the IOTA interferometer. The mean uniform disk angular diameter was measured to be 1.64 +0.14 -0.16 mas. Pulsational variations are not detected at a significant statistical level, but future observations with longer baselines should allow a much better estimation of their amplitude. The distance to Zeta Gem is evaluated using Baade-Wesselink diameter determinations, giving a distance of 502 +/- 88 pc.Comment: 8 pages, 3 figure

Nulling interferometry: performance comparison between space and ground-based sites for exozodiacal disc detection

Context: Characterising the circumstellar dust around nearby main sequence stars is a necessary step in understanding the planetary formation process and is crucial for future life-finding space missions such as ESA's Darwin or NASA's terrestrial planet finder (TPF). Besides paving the technological way to Darwin/TPF, the space-based infrared interferometers Pegase and FKSI (Fourier-Kelvin Stellar Interferometer) will be valuable scientific precursors. Aims: We investigate the performance of Pegase and FKSI for exozodiacal disc detection and compare the results with ground-based nulling interferometers. Methods: We used the GENIEsim software (Absil et al. 2006, A&A, 448, 787) which was designed and validated to study the performance of ground-based nulling interferometers. The software has been adapted to simulate the performance of space-based nulling interferometers by disabling all atmospheric effects and by thoroughly implementing the perturbations induced by payload vibrations in the ambient space environment. Results: Despite using relatively small telescopes (<=0.5 m), Pegase and FKSI are very efficient for exozodiacal disc detection. They are capable of detecting exozodiacal discs 5 and 1 time respectively, as dense as the solar zodiacal cloud, and they outperform any ground-based instrument. Unlike Pegase, FKSI can achieve this sensitivity for most targets of the Darwin/TPF catalogue thanks to an appropriate combination of baseline length and observing wavelength. The sensitivity of Pegase could, however, be significantly boosted by considering a shorter interferometric baseline length. Conclusions: Besides their main scientific goal (characterising hot giant extrasolar planets), the space-based nulling interferometers Pegase and FKSI will be very efficient in assessing within a few minutes the level of circumstellar dust in the habitable zone around nearby main sequence stars down to the density of the solar zodiacal cloud. These space-based interferometers would be complementary to Antarctica-based instruments in terms of sky coverage and would be ideal instruments for preparing future life-finding space missions