GRAVITY is a new instrument to coherently combine the light of the European Southern Observatory Very Large Telescope Interferometer to form a telescope with an equivalent 130 m diameter angular ...resolution and a collecting area of 200 m2. The instrument comprises fiber fed integrated optics beam combination, high resolution spectroscopy, built-in beam analysis and control, near-infrared wavefront sensing, phase-tracking, dual-beam operation, and laser metrology. GRAVITY opens up to optical/infrared interferometry the techniques of phase referenced imaging and narrow angle astrometry, in many aspects following the concepts of radio interferometry. This article gives an overview of GRAVITY and reports on the performance and the first astronomical observations during commissioning in 2015/16. We demonstrate phase-tracking on stars as faint as mK ≈ 10 mag, phase-referenced interferometry of objects fainter than mK ≈ 15 mag with a limiting magnitude of mK ≈ 17 mag, minute long coherent integrations, a visibility accuracy of better than 0.25%, and spectro-differential phase and closure phase accuracy better than 0.5°, corresponding to a differential astrometric precision of better than ten microarcseconds (μas). The dual-beam astrometry, measuring the phase difference of two objects with laser metrology, is still under commissioning. First observations show residuals as low as 50 μas when following objects over several months. We illustrate the instrument performance with the observations of archetypical objects for the different instrument modes. Examples include the Galactic center supermassive black hole and its fast orbiting star S2 for phase referenced dual-beam observations and infrared wavefront sensing, the high mass X-ray binary BP Cru and the active galactic nucleus of PDS 456 for a few μas spectro-differential astrometry, the T Tauri star S CrA for a spectro-differential visibility analysis, ξ Tel and 24 Cap for high accuracy visibility observations, and η Car for interferometric imaging with GRAVITY.
Stars orbiting the compact radio source Sgr A* in the Galactic Center serve as precision probes of the gravitational field around the closest massive black hole. In addition to adaptive ...optics-assisted astrometry (with NACO/VLT) and spectroscopy (with SINFONI/VLT, NIRC2/Keck and GNIRS/Gemini) over three decades, we have obtained 30–100 μas astrometry since 2017 with the four-telescope interferometric beam combiner GRAVITY/VLTI, capable of reaching a sensitivity of
m
K
= 20 when combining data from one night. We present the simultaneous detection of several stars within the diffraction limit of a single telescope, illustrating the power of interferometry in the field. The new data for the stars S2, S29, S38, and S55 yield significant accelerations between March and July 2021, as these stars pass the pericenters of their orbits between 2018 and 2023. This allows for a high-precision determination of the gravitational potential around Sgr A*. Our data are in excellent agreement with general relativity orbits around a single central point mass,
M
•
= 4.30 × 10
6
M
⊙
, with a precision of about ±0.25%. We improve the significance of our detection of the Schwarzschild precession in the S2 orbit to 7
σ
. Assuming plausible density profiles, the extended mass component inside the S2 apocenter (≈0.23″ or 2.4 × 10
4
R
S
) must be ≲3000
M
⊙
(1
σ
), or ≲0.1% of
M
•
. Adding the enclosed mass determinations from 13 stars orbiting Sgr A* at larger radii, the innermost radius at which the excess mass beyond Sgr A* is tentatively seen is
r
≈ 2.5″ ≥ 10× the apocenter of S2. This is in full harmony with the stellar mass distribution (including stellar-mass black holes) obtained from the spatially resolved luminosity function.
The GRAVITY instrument on the ESO VLTI pioneers the field of high-precision near-infrared interferometry by providing astrometry at the 10−100
μ
as level. Measurements at this high precision ...crucially depend on the control of systematic effects. We investigate how aberrations introduced by small optical imperfections along the path from the telescope to the detector affect the astrometry. We develop an analytical model that describes the effect of these aberrations on the measurement of complex visibilities. Our formalism accounts for pupil-plane and focal-plane aberrations, as well as for the interplay between static and turbulent aberrations, and it successfully reproduces calibration measurements of a binary star. The Galactic Center observations with GRAVITY in 2017 and 2018, when both Sgr A* and the star S2 were targeted in a single fiber pointing, are affected by these aberrations at a level lower than 0.5 mas. Removal of these effects brings the measurement in harmony with the dual-beam observations of 2019 and 2020, which are not affected by these aberrations. This also resolves the small systematic discrepancies between the derived distance
R
0
to the Galactic Center that were reported previously.
Observed properties of stars and planets in binary/multiple star systems provide clues to planet formation and evolution. We extended our survey for visual stellar companions to the hosts of ...transiting exoplanets by 21 stars, using the Lucky Imaging technique with the two AstraLux instruments: AstraLux Norte at the Calar Alto 2.2-m telescope and AstraLux Sur at the European Southern Observatory 3.5-m New Technology Telescope at La Silla. Typically, a sensitivity to companions of magnitude difference Δz′ 4 is achieved at angular separation ρ = 0.5 arcsec and Δz′ 6 for ρ = 1 arcsec.
We present observations of two previously unknown binary candidate companions, to the transiting planet host stars HAT-P-8 and WASP-12, and derive photometric and astrometric properties of the companion candidates. The common proper motions of the previously discovered companion candidates with the exoplanet host stars TrES-4 and WASP-2 are confirmed from follow-up observations. A Bayesian statistical analysis of 31 transiting exoplanet host stars observed with AstraLux suggests that the companion star fraction of planet hosts is not significantly different from that of solar-type field stars, but that the binary separation is on average larger for planet host stars.
This paper presents the on-sky performance of the unmodulated infrared pyramid wavefront sensor PYRAMIR mounted on the ALFA adaptive optics system at the 3.5 m telescope of the Calar Alto ...Observatory. The performance of the system is compared with the performance of the Shack-Hartmann wavefront sensor of the ALFA system. We carried out a series of measurements to characterize the performance of PYRAMIR under open-loop tip-tilt compensation, and high-order closed-loop conditions, using stars of different magnitudes. We measured the tip-tilt jitter by following the centroid position of a stellar image on a fast series of frames. Additionally from the pyramid wavefront sensor data we could estimate the tip-tilt jitter in closed-loop. Under closed-loop conditions we also measured the long-exposure Strehl ratio. We compared the results of the wavefront sensor measurements with those of the Shack-Hartmann sensor on the same telescope, especially regarding the distribution of the error budged over the Karhunen-Loève modes, and the power spectral density. Our first finding is that we can indeed start up this nonmodulated pyramid system, even under bad seeing conditions. Under good conditions the Strehl ratio reaches≥60%
≥
60
%
in
K
′
K
′
band. We found that the minimum signal-to-noise ratio (S/N) in each subaperture required to close the high-order loop is only 0.4. This is a surprisingly low number. To compare the performance to existing systems, we introduce the S/N per subaperture per loop cycle as a device-independent measure. Using this scheme, we find that the ratio between the low-order residuals and the high-order residuals in the case of PYRAMIR is lower than that of the Shack-Hartmann system, especially in the faint flux regime. This is an important finding because it means that the pyramid-based system removes the halo, i.e., light scattered by the atmosphere, around the target star better than a Shack-Hartmann sensor-based system. A comparison of the power spectral density of the PYRAMIR and the Shack-Hartmann sensor measurements, and a comparison of the noise propagation coefficients of PYRAMIR with theoretical predictions from the literature, confirm this superiority of the pyramid over the Shack-Hartmann sensor.
The GRAVITY Young Stellar Object survey Perraut, K.; Labadie, L.; Lazareff, B. ...
Astronomy and astrophysics (Berlin),
12/2019, Letnik:
632
Journal Article
Recenzirano
Odprti dostop
Context. The formation and the evolution of protoplanetary disks are important stages in the lifetime of stars. Terrestrial planets form or migrate within the innermost regions of these ...protoplanetary disks and so, the processes of disk evolution and planet formation are intrinsically linked. Studies of the dust distribution, composition, and evolution of these regions are crucial to understanding planet formation. Aims. We built a homogeneous observational dataset of Herbig Ae/Be disks with the aim of spatially resolving the sub au-scale region to gain a statistical understanding of their morphological and compositional properties, in addition to looking for correlations with stellar parameters, such as luminosity, mass, and age. Methods. We observed 27 Herbig Ae/Be stars with the GRAVITY instrument installed at the combined focus of the Very Large Telescope Interferometer (VLTI) and operating in the near-infrared K-band, focused on the K-band thermal continuum, which corresponds to stellar flux reprocessed by the dust grains. Our sample covers a large range of effective temperatures, luminosities, masses, and ages for the intermediate-mass star population. The circumstellar disks in our sample also cover a range of various properties in terms of reprocessed flux, flared or flat morphology, and gaps. We developed semi-physical geometrical models to fit our interferometric data. Results. Our best-fit models correspond to smooth and wide rings that support previous findings in the H-band, implying that wedge-shaped rims at the dust sublimation edge are favored. The measured closure phases are generally non-null with a median value of ~10°, indicating spatial asymmetries of the intensity distributions. Multi-size grain populations could explain the closure phase ranges below 20–25° but other scenarios should be invoked to explain the largest ones. Our measurements extend the Radius-Luminosity relation to ~104 L⊙ luminosity values and confirm the significant spread around the mean relation observed by PIONIER in the H-band. Gapped sources exhibit a large N-to-K band size ratio and large values of this ratio are only observed for the members of our sample that would be older than 1 Ma, less massive, and with lower luminosity. In the mass range of 2 M⊙, we do observe a correlation in the increase of the relative age with the transition from group II to group I, and an increase of the N-to-K size ratio. However, the size of the current sample does not yet permit us to invoke a clear, universal evolution mechanism across the Herbig Ae/Be mass range. The measured locations of the K-band emission in our sample suggest that these disks might be structured by forming young planets, rather than by depletion due to EUV, FUV, and X-ray photo-evaporation.
Due to its proximity, youth, and solar-like characteristics with a spectral type of K2V, epsilon Eri is one of the most extensively studied systems in an extrasolar planet context. Based on radial ...velocity, astrometry, and studies of the structure of its circumstellar debris disk, at least two planetary companion candidates to epsilon Eri have been inferred in the literature (\epsilon Eri b, epsilon Eri c). Some of these methods also hint at additional companions residing in the system. Here we present a new adaptive optics assisted high-contrast imaging approach that takes advantage of the favourable planet spectral energy distribution at 4 mum, using narrow-band angular differential imaging to provide an improved contrast at small and intermediate separations from the star. We use this method to search for planets at orbits intermediate between epsilon Eri b (3.4 AU) and epsilon Eri c (40 AU). The method is described in detail, and important issues related to the detectability of planets such as the age of epsilon Eri and constraints from indirect measurements are discussed. The non-detection of companion candidates provides stringent upper limits for the masses of additional planets. Using a combination of the existing dynamic and imaging data, we exclude the presence of any planetary companion more massive than 3 M_{\rm jup} anywhere in the epsilon Eri system. Specifically, with regards to the possible residual linear radial velocity trend, we find that it is unlikely to correspond to a real physical companion if the system is as young as 200 Myr, whereas if it is as old as 800 Myr, there is an allowed semi-major axis range between about 8.5 and 25 AU.
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