G. Zins

NAOS, the first AO system of the VLT: on-sky performance

NAOS is the first adaptive optics system installed at the VLT 8m telescopes. It was designed, manufactured and tested by a french Consortium under an ESO contract, to provide compensated images to the high angular resolution IR spectro-imaging camera (CONICA) in the 1 to 5 μm spectral range. It is equipped with a 185 actuator deformable mirror, a tip/tilt mirror and two wavefront sensors, one in the visible and one in the near IR spectral range. It has been installed in November at the Nasmyth focus B of the VLT UT4. During the first light run in December 2001, NAOS has delivered a Strehl ratio of 50 under average seeing conditions for bright guide stars. The diffraction limit of the telescope has been achieved at 2.2 μm. The closed loop operation has been very robust under bad seeing conditions. It was also possible to obtain a substantial correction with mV=17.6 and mK=13.1 reference stars. The on-sky acceptance tests of NAOS-CONICA were completed in May 2002 and the instrument will be made available to the European astronomical community in October by ESO. This paper describes the system and present the on-sky performance in terms of Strehl ratio, seeing conditions and guide star magnitude.

PIONIER: a 4-telescope visitor instrument at VLTI

PIONIER stands for Precision Integrated-Optics Near-infrared Imaging ExpeRiment. It combines four 1.8m Auxilliary Telescopes or four 8m Unit Telescopes of the Very Large Telescope Interferometer (ESO, Chile) using an integrated optics combiner. The instrument has been integrated at IPAG starting in December 2009 and commissioned at the Paranal Observatory in October 2010. It provides scientific observations since November 2010. In this paper, we detail the instrumental concept, we describe the standard operational modes and the data reduction strategy. We present the typical performance and discuss how to improve them. This paper is based on laboratory data obtained during the integrations at IPAG, as well as on-sky data gathered during the commissioning at VLTI. We illustrate the imaging capability of PIONIER on the binaries deltaSco and HIP11231. PIONIER provides 6 visibilities and 3 independent closure phases in the H band, either in a broadband mode or with a low spectral dispersion (R=40), using natural light (i.e. unpolarized). The limiting magnitude is Hmag=7 in dispersed mode under median atmospheric conditions (seeing 3ms) with the 1.8m Auxiliary Telescopes. We demonstrate a precision of 0.5deg on the closure phases. The precision on the calibrated visibilities ranges from 3 to 15% depending on the atmospheric conditions. PIONIER has been installed and successfully tested as a visitor instrument for the VLTI. It permits high angular resolution imaging studies at an unprecedented level of sensitivity. The successful combination of the four 8m Unit Telescopes in March 2011 demonstrates that VLTI is ready for 4-telescope operation.

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 milliarcsec (mas) remain mostly unknown due to intrinsic observational limitations. At a typical distance of 2 kpc, this corresponds to projected physical separations of 2–200 AU. 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/ Sparse Aperture Masking (SAM), probing the separation ranges 1–45 and 30–250 mas and brightness contrasts of Δ H< 4 and Δ H< 5, respectively. Taking advantage of NACO’s field of view, we further uniformly searched for visual companions in an 8 �� radius down to ΔH = 8. This paper describes observations and data analysis, reports the discovery of almost 200 new companions in the separation range from 1 mas to 8 �� and presents a 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 ( δ< 0 ◦ ; H< 7.5) were observed both with PIONIER and NACO/SAM. The fraction of these stars with at least one resolved companion within 200 mas is 0.53. Accounting for known but unresolved spectroscopic or eclipsing companions, the multiplicity fraction at separation ρ< 8 �� increases to fm = 0.91 ± 0.03. The fraction of luminosity class V stars that have a bound companion reaches 100% at 30 mas while their average number of physically connected companions within 8 �� is fc = 2.2 ± 0.3. This demonstrates that massive stars form nearly exclusively in multiple systems. The nine non-thermal radio emitters observed by smash+ are all resolved, including the newly discovered pairs HD 168112 and CPD−47 ◦ 2963. This lends strong support to the universality of the wind-wind collision

SearchCal: a Virtual Observatory tool for searching calibrators in optical long-baseline interferometry - II. The faint-object case

Context. In long-baseline interferometry, images or astrophysical parameters are obtained from the raw fringe contrast after a careful calibration process. We have already developed the software (SearchCal) to select suitable bright calibration stars (V ≤ 10; K ≤ 5.0) for obtaining the ultimate precision of current interferometric instruments like the VLTI. With the expected gain in sensitivity of AMBER and PRIMA on the VLTI, the need for fainter calibrators should now be adressed. Aims. We present a new version of SearchCal dedicated to the creation of an evolving catalog of stars suitable as calibrators with K magnitude >5 around the scientific target. Methods. Star catalogs available at the CDS are searched via web requests and provide the useful astrometric and photometric informations for selecting calibrators. The missing photometries are computed with an accuracy of about 0.1 mag. The stellar angular diameter is estimated with a precision of about 10% through newly determined surface-brightness versus color-index relations based on the I, J, H and K magnitudes. For each star the squared visibility is computed taking into account the central wavelength and the maximum baseline of the predicted observations. Results. The version of SearchCal for faint objects that allows to find calibrators for interferometric observations up to K ∼ 15 is available as a web service at the address: http://www.jmmc.fr/searchcal

Status of the VLT Nasmyth adaptive optics system (NAOS)

NAOS is the adaptive optics system to be installed at one of the Nasmyth focus of the VLT. It was designed and manufactured by a French Consortium to provide compensated images to the high angular resolution IR spectro-imaging camera (CONICA) in the 1 to 5 micrometer spectral range. For bright sources, NAOS will achieve a Strehl ratio of 70% under average seeing conditions. It is equipped with a 185 actuator deformable mirror, a tip/tilt mirror and two wavefront sensors, one in the visible and one in the near IR. All the components of NAOS have been delivered and the integration phase is in progress since the beginning of 2000. After extensive tests and performance verifications in France, the system will be shipped to Chile by the end of 2000. The first light at the VLT is foreseen in the beginning of 2001.

VLT/NACO adaptive optics imaging of Titan ?

The advent of the NAOS/CONICA adaptive optics system at the ESO Very Large Telescope recently gave us the opportunity to map the surface of Titan and to search for atmospheric variations at high spatial resolution and contrast. We report here the first results from a series of observations of Titan performed with this instrument in a number of near-infrared narrow-band filters, covering various altitude regions and three dierent longitudes (out of the 16 days of Titans orbit). We have achieved unequaled contrast on images showing complex topography on Titans trailing hemisphere and have found robust evidence for the north-south asymmetry inversion. The presence of other interesting atmospheric features at Titans South Pole is described.

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 χ^2 goodness of fit for a series of binary star models at different positions and with various flux ratios. The resulting χ^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 3σ on the companion flux ratio are respectively of 2.3 × 10^(-3) (in 4 h), 3.5 × 10^(-3) (in 3 h) and 5.4 × 10^(-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 2.05 × 10^(-2) ± 0.16 × 10^(-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 on unresolved targets, but significant improvements are still required to reach the ultimate goal of directly detecting hot giant extrasolar planets.

The 2011 outburst of the recurrent nova T Pyxidis. Evidence for a face-on bipolar ejection

We report on near-IR interferometric observations of the outburst of the recurrent nova T Pyx. We obtained near-IR observations of T Pyx at dates ranging from t=2.37d to t=48.2d after the outburst, with the CLASSIC recombiner, located at the CHARA array, and with the PIONIER and AMBER recombiners, located at the VLTI array. These data are supplemented with near-IR photometry and spectra obtained at Mount Abu, India. Slow expansion velocities were measured (<300km/s) before t=20d (assuming D=3.5kpc). From t=28d on, the AMBER and PIONIER continuum visibilities (K and H band, respectively) are best simulated with a two component model consisting of an unresolved source plus an extended source whose expansion velocity onto the sky plane is lower than 700km/s. The expansion of the Brgamma line forming region, as inferred at t=28d and t=35d is slightly larger, implying velocities in the range 500-800km/s, still strikingly lower than the velocities of 1300-1600km/s inferred from the Doppler width of the line. Moreover, a remarkable pattern was observed in the Brgamma differential phases. A semi-quantitative model using a bipolar flow with a contrast of 2 between the pole and equator velocities, an inclination of i=15° and a position angle P.A.=110° provides a good match to the AMBER observables (spectra, differential visibilities and phases). At t=48d, a PIONIER dataset confirms the two component nature of the H band emission, consisting of an unresolved stellar source and an extended region whose appearance is circular and symmetric within error bars.These observations are most simply interpreted within the frame of a bipolar model, oriented nearly face-on. This finding has profound implications for the interpretation of past, current and future observations of the expanding nebula.

SPHERE IRDIS and IFS astrometric strategy and calibration

We present the current results of the astrometric characterization of the VLT planet finder SPHERE over 2 years of on-sky operations. We first describe the criteria for the selection of the astrometric fields used for calibrating the science data: binaries, multiple systems, and stellar clusters. The analysis includes measurements of the pixel scale and the position angle with respect to the North for both near-infrared subsystems, the camera IRDIS and the integral field spectrometer IFS, as well as the distortion for the IRDIS camera. The IRDIS distortion is shown to be dominated by an anamorphism of 0.60±0.02% between the horizontal and vertical directions of the detector, i.e. 6 mas at 1 arcsec. The anamorphism is produced by the cylindrical mirrors in the common path structure hence common to all three SPHERE science subsystems (IRDIS, IFS, and ZIMPOL), except for the relative orientation of their field of view. The current estimates of the pixel scale and North angle for IRDIS are 12.255±0.009 milliarcseconds/pixel for H2 coronagraphic images and -1.70±0.08°. Analyses of the IFS data indicate a pixel scale of 7.46±0.02 milliarcseconds/pixel and a North angle of -102.18±0.13°. We finally discuss plans for providing astrometric calibration to the SPHERE users outside the instrument consortium.

The VLTI/PIONIER near-infrared interferometric survey of southern T Tauri stars - I. First results

Context. The properties of the inner disks of bright Herbig AeBe stars have been studied with near-infrared (NIR) interferometry and high resolution spectroscopy. The continuum (dust) and a few molecular gas species have been studied close to the central star; however, sensitivity problems limit direct information about the inner disks of the fainter T Tauri stars. Aims. Our aim is to measure some of the properties (inner radius, brightness profile, shape) of the inner regions of circumstellar disk surrounding southern T Tauri stars. Methods. We performed a survey with the VLTI/PIONIER recombiner instrument at H-band of 21 T Tauri stars. The baselines used ranged from 11 m to 129 m, corresponding to a maximum resolution of ~3 mas (~0.45 au at 150 pc). Results. Thirteen disks are resolved well and the visibility curves are fully sampled as a function of baseline in the range 45–130 m for these 13 objects. A simple qualitative examination of visibility profiles allows us to identify a rapid drop-off in the visibilities at short baselines( 3 au, at 150 pc) contribution of light from the disk. We demonstrate that this component is compatible with scattered light, providing strong support to an earlier prediction. The amplitude of the drop-off and the amount of dust thermal emission changes from source to source suggesting that each disk is different. A by-product of the survey is the identification of a new milli-arcsec separation binary: WW Cha. Spectroscopic and interferometric data of AK Sco have also been fitted with a binary + disk model. Conclusions. The visibility data are reproduced well when thermal emission and scattering from dust are fully considered. The inner radii measured are consistent with the expected dust sublimation radii. The modelling of AK Sco suggests a likely coplanarity between the disk and the binary’s orbital plane.