Karine Rousselet-Perraut

Asteroseismology and interferometry

Asteroseismology provides us with a unique opportunity to improve our understanding of stellar structure and evolution. Recent developments, including the first systematic studies of solar-like pulsators, have boosted the impact of this field of research within astrophysics and have led to a significant increase in the size of the research community. In the present paper we start by reviewing the basic observational and theoretical properties of classical and solar-like pulsators and present results from some of the most recent and outstanding studies of these stars. We centre our review on those classes of pulsators for which interferometric studies are expected to provide a significant input. We discuss current limitations to asteroseismic studies, including difficulties in mode identification and in the accurate determination of global parameters of pulsating stars, and, after a brief review of those aspects of interferometry that are most relevant in this context, anticipate how interferometric observations may contribute to overcome these limitations. Moreover, we present results of recent pilot studies of pulsating stars involving both asteroseismic and interferometric constraints and look into the future, summarizing ongoing efforts concerning the development of future instruments and satellite missions which are expected to have an impact in this field of research.

INTEGRATED OPTICS FOR ASTRONOMICAL INTERFEROMETRY. I. CONCEPT AND ASTRONOMICAL APPLICATIONS

We propose a new instrumental concept for long-baseline optical single-mode interferometry using integrated optics which were developed for telecommunication. Visible and infrared multi-aperture interferometry requires many optical functions (spatial filtering, beam combination, photometric calibration, polarization control) to detect astronomical signals at very high angular resolution. Since the 80s, integrated optics on planar substrate have become available for telecommunication applications with multiple optical functions like power dividing, coupling, multiplexing, etc. We present the concept of an optical/infrared interferometric instrument based on this new technology. The main advantage is to provide an interferometric combination unit on a single optical chip. Integrated optics are compact, provide stability, low sensitivity to external constrains like temperature, pressure or mechanical stresses, no optical alignment except for coupling, simplicity and intrinsic polarization control. The integrated optics devices are inexpensive compared to devices that have the same functionalities in bulk optics. We think integrated optics will fundamentally change single-mode interferometry. Integrated optics devices are in particular well-suited for interferometric combination of numerous beams to achieve aperture synthesis imaging or for space-based interferometers where stability and a minimum of optical alignments are wished.

An integrated-optics 3-way beam combiner for IOTA

We report here the first visibility and closure-phase measurements done with the IONIC instrument at the IOTA interferometer. The IONIC instrument is presented and preliminary analysis of the results discussed. Future improvements of IONIC are envisioned.

Using the near infrared VLTI instrument AMBER

AMBER is the General User near infrared focal instrument of the Very Large Telescope Interferometer. Its a single mode, dispersed fringes, three telescopes instrument. A limiting magnitude of the order of H=13 will allow to tackle a fair sample of extra galactic targets. A very high accuracy, in particular in color differential phase and closure phase modes gives good hope for very high dynamic range observation, possibly including hot extra solar planets. The relatively high maximum spectral resolution, up to 10000, will allow some stellar activity observations. Between this extreme goals, AMBER should have a wide range of applications including Young Stellar Objects, Evolved Stars, circumstellar material and many others. This paper tries to introduce AMBER to its future users with information on what it measures, how it is calibrated and hopes to give the readers ideas for applications.

Integrated optics for astronomical interferometry - VI. Coupling the light of the VLTI in K band

Aims. Our objective is to prove that integrated optics (IO) is not o nly a good concept for astronomical interferometry but also a working technique with high performance. Methods. We used the commissioning data obtained with the dedicated K-band integrated optics two-telescope beam combiner which now replaces the fiber coupler MONA in the VLTI/VINCI instrument. We characterize the behaviour of this IO device and compare its properties to other single mode beam combiner like the previously used MONA fiber coupler. Results. The IO combiner provides a high optical throughput, a contrast of 89% with a night-to-night stability of a few percent. Even if a dispersive phase is present, we show that it does not bias the measured Fourier visibility estimate. An upper limit of 5× 10 −3 for the cross-talk between linear polarization states has been measured. We take advantage of the intrinsic contrast stability to test a ne w astronomical prodecure for calibrating diameters of simple stars by simultaneousl y fitting the instrumental contrast and the apparent stellar diameters. This method reaches an accuracy with diameter errors of the order of previous ones but without the need of an already known calibrator. Conclusions. These results are an important step of integrated optics, si nce they prove its maturity in an astronomical band where the technology has been specially developed for astronomical conveniences. It paves the road to incoming imaging interferometer projects.

Integrated optics for astronomical interferometry - II. First laboratory white-light interferograms

We report rst white-light interferograms ob- tained with an integrated optics beam combiner on a glass plate. These results demonstrate the feasability of single- mode interferometric beam combination with integrated optics technology presented and discussed in Paper I (Malbet et al. 1999). The demonstration is achieved in laboratory with o-the-shelves components coming from micro-sensor applications, not optimized for astronomi- cal use. These two-telescope beam combiners made by ion exchange technique on glass substrate provide laboratory white-light interferograms simultaneously with photomet- ric calibration. A dedicated interferometric workbench us- ing optical bers is set up to characterize these devices. Despite the rather low match of the component param- eters to astronomical constraints, we obtain stable con- trasts higher than 93% with a 1.54-m laser source and up to 78% with a white-light source in the astronomical H band. Global throughput of 27% for a potassium ion exchange beam combiner and of 43% for a silver one are reached. This work validates our approach for combining several stellar beams of a long baseline interferometer with integrated optics components.

Integrated optics for astronomical interferometry. III. Optical validation of a planar optics two-telescope beam combiner

The optical characterization of a fiber-connected planar optics beam combiner dedicated to astronomical interferometry for two telescopes is presented. The beam combiner, fully integrated on a single 5 mm x 40 mm glass chip, is tested as the central part of an astronomical instrument. The single-mode waveguides are made by silver-ion-exchange technology upon glass substrates and provide spatial filtering, which improves the visibility measurement accuracy by selecting only the fundamental mode of the beams at the telescope focal plane. A global optical throughput of 43% is measured, and the sources of losses are identified and examined in detail. Solutions for improving this throughput are proposed. High and stable contrasts are obtained with a 1.55-mum laser diode (?96%) and with a white-light source (~92%) in the astronomical H filter (1.43 mum; 1.77 mum). The need for accurate control of differential instrumental polarization is demonstrated. In this context the intrinsic polarization-maintaining property of the planar optics component is characterized. This validation of the important potential uses of integrated planar optics should be valuable for future design of optical telescope arrays.

First observations with an H-band integrated optics beam combiner at the VLTI

This paper reports results obtained with the Very Large Telescope Interferometer Commissioning Instrument (VLTI-VINCI) where the fiber interferometric coupler MONA was temporarily replaced by an integrated optics beam com- biner (IONIC). The two-beam combiner operating in the H atmospheric band was tested at Paranal during two weeks in July 2002. Fringes were recorded on several stars with the siderostats and the Unit Telescopes (UT). We demonstrate that in- tegrated optics allows beam combination with a high interferometric efficiency, stability and spatial filtering. The instrumental visibility of VLTI+IONIC is better than 85%, which is just 5% under the visibility of the IONIC device alone. During this run, the flux injection in the IONIC combiner was not optimized since we used the K-band MONA optics. However, we obtain fringes with the 35 cm siderostats on a star of magnitude mH = 2.47 (κ 2 Scl). It allows us to extrapolate a magnitude around mH ∼ 6 with 8 m telescopes without adaptive optics and in the same poor injection conditions.

The Hα line forming region of AB Aurigae spatially resolved at sub-AU with the VEGA/CHARA spectro-interferometer

Context. A crucial issue in star formation is to understand the physical mechanism by which mass is accreted onto and ejected by a young star. To derive key constraints on the launching point of the jets and on the geometry of the winds, the visible spectropolarimeter VEGA installed on the CHARA optical array can be an efficient means of probing the structure and the kinematics of the hot circumstellar gas at sub-AU. Aims. For the first time, we observed the Herbig Ae star AB Aur in the Hα emission line, using the VEGA low spectral resolution (R = 1700) on two baselines of the array. Methods. We computed and calibrated the spectral visibilities of AB Aur between 610 nm and 700 nm in spectral bands of 20.4 nm. To simultaneously reproduce the line profile and the inferred visibility around Hα, we used a 1D radiative transfer code (RAMIDUS/PROFILER) that calculates level populations for hydrogen atoms in a spherical geometry and that produces synthetic spectro-interferometric observables. Results. We clearly resolved AB Aur in the Hα line and in a part of the continuum, even at the smallest baseline of 34 m. The small P-Cygni absorption feature is indicative of an outflow but could not be explained by a spherical stellar wind model. Instead, it favors a magneto-centrifugal X-disk or disk-wind geometry. The fit of the spectral visibilities from 610 to 700 nm could not be accounted for by a wind alone, so another component inducing a visibility modulation around Hα needed to be considered. We thus considered a brightness asymmetry possibly caused by large-scale nebulosity or by the known spiral structures. Conclusions. Thanks to the unique capabilities of VEGA, we managed to simultaneously record for the first time a spectrum at a resolution of 1700 and spectral visibilities in the visible range on a target as faint as mV = 7.1. It was possible to rule out a spherical geometry for the wind of AB Aur and provide realistic solutions to account for the Hα emission compatible with magneto-centrifugal acceleration. It was difficult, however, to determine the exact morphology of the wind because of the surrounding asymmetric nebulosity. The study illustrates the advantages of optical interferometry and motivates observations of other bright young stars in the same way to shed light on the accretion/ejection processes.

Characterization of integrated optics components for the second generation of VLTI instruments

Two of the three instruments proposed to ESO for the second generation instrumentation of the VLTI would use integrated optics for beam combination. Several design are studied, including co-axial and multi-axial recombination. An extensive quantity of combiners are therefore under test in our laboratories. We will present the various components, and the method used to validate and compare the different combiners. Finally, we will discuss the performances and their implication for both VSI and Gravity VLTI instruments.