International audienc

Context. We present the early installment of the third Gaia data release, Gaia EDR3, consisting of astrometry and photometry for 1.8 billion sources brighter than magnitude 21, complemented with the list of radial velocities from Gaia DR2. Aims. A summary of the contents of Gaia EDR3 is presented, accompanied by a discussion on the differences with respect to Gaia DR2 and an overview of the main limitations which are present in the survey. Recommendations are made on the responsible use of Gaia EDR3 results. Methods. The raw data collected with the Gaia instruments during the first 34 months of the mission have been processed by the Gaia Data Processing and Analysis Consortium and turned into this early third data release, which represents a major advance with respect to Gaia DR2 in terms of astrometric and photometric precision, accuracy, and homogeneity. Results. Gaia EDR3 contains celestial positions and the apparent brightness in G for approximately 1.8 billion sources. For 1.5 billion of those sources, parallaxes, proper motions, and the (GBP − GRP) colour are also available. The passbands for G, GBP, and GRP are provided as part of the release. For ease of use, the 7 million radial velocities from Gaia DR2 are included in this release, after the removal of a small number of spurious values. New radial velocities will appear as part of Gaia DR3. Finally, Gaia EDR3 represents an updated materialisation of the celestial reference frame (CRF) in the optical, the Gaia-CRF3, which is based solely on extragalactic sources. The creation of the source list for Gaia EDR3 includes enhancements that make it more robust with respect to high proper motion stars, and the disturbing effects of spurious and partially resolved sources. The source list is largely the same as that for Gaia DR2, but it does feature new sources and there are some notable changes. The source list will not change for Gaia DR3. Conclusions. Gaia EDR3 represents a significant advance over Gaia DR2, with parallax precisions increased by 30 per cent, proper motion precisions increased by a factor of 2, and the systematic errors in the astrometry suppressed by 30–40% for the parallaxes and by a factor ~2.5 for the proper motions. The photometry also features increased precision, but above all much better homogeneity across colour, magnitude, and celestial position. A single passband for G, GBP, and GRP is valid over the entire magnitude and colour range, with no systematics above the 1% level

Context. Gaia Early Data Release 3 (Gaia EDR3) contains astrometry and photometry results for about 1.8 billion sources based on observations collected by the European Space Agency Gaia satellite during the first 34 months of its operational phase. Aims. In this paper, we focus on the photometric content, describing the input data, the algorithms, the processing, and the validation of the results. Particular attention is given to the quality of the data and to a number of features that users may need to take into account to make the best use of the Gaia EDR3 catalogue. Methods. The processing broadly followed the same procedure as for Gaia DR2, but with significant improvements in several aspects of the blue and red photometer (BP and RP) preprocessing and in the photometric calibration process. In particular, the treatment of the BP and RP background has been updated to include a better estimation of the local background, and the detection of crowding effects has been used to exclude affected data from the calibrations. The photometric calibration models have also been updated to account for flux loss over the whole magnitude range. Significant improvements in the modelling and calibration of the Gaia point and line spread functions have also helped to reduce a number of instrumental effects that were still present in DR2. Results. Gaia EDR3 contains 1.806 billion sources with G-band photometry and 1.540 billion sources with GBP and GRP photometry. The median uncertainty in the G-band photometry, as measured from the standard deviation of the internally calibrated mean photometry for a given source, is 0.2 mmag at magnitude G = 10–14, 0.8 mmag at G ≈ 17, and 2.6 mmag at G ≈ 19. The significant magnitude term found in the Gaia DR2 photometry is no longer visible, and overall there are no trends larger than 1 mmag mag−1. Using one passband over the whole colour and magnitude range leaves no systematics above the 1% level in magnitude in any of the bands, and a larger systematic is present for a very small sample of bright and blue sources. A detailed description of the residual systematic effects is provided. Overall the quality of the calibrated mean photometry in Gaia EDR3 is superior with respect to DR2 for all bands.

We carried out a Bayesian homogeneous determination of the orbital parameters of 231 transiting giant planets (TGPs) that are alone or have distant companions; we employed differential evolution Markov chain Monte Carlo methods to analyse radial-velocity (RV) data from the literature and 782 new high-accuracy RVs obtained with the HARPS-N spectrograph for 45 systems over ∼3 years. Our work yields the largest sample of systems with a transiting giant exoplanet and coherently determined orbital, planetary, and stellar parameters. We found that the orbital parameters of TGPs in non-compact planetary systems are clearly shaped by tides raised by their host stars. Indeed, the most eccentric planets have relatively large orbital separations and/or high mass ratios, as expected from the equilibrium tide theory. This feature would be the outcome of planetary migration from highly eccentric orbits excited by planetplanet scattering, Kozai-Lidov perturbations, or secular chaos. The distribution of α = a/a R , where a and a R are the semi-major axis and the Roche limit, for well-determined circular orbits peaks at 2.5; this agrees with expectations from the high-eccentricity migration (HEM), although it might not be limited to this migration scenario. The few planets of our sample with circular orbits and α > 5 values may have migrated through disc-planet interactions instead of HEM. By comparing circularisation times with stellar ages, we found that hot Jupiters with a < 0.05 au have modified tidal quality factors 10 5 Q p 10 9 , and that stellar Q s 10 6 −10 7 are required to explain the presence of eccentric planets at the same orbital distance. As a by-product of our analysis, we detected a non-zero eccentricity e = 0.104 +0.021 −0.018 for HAT-P-29; we determined that five planets that were previously regarded to be eccentric or to have hints of non-zero eccentricity, namely CoRoT-2b, CoRoT-23b, TrES-3b, HAT-P-23b, and WASP-54b, have circular orbits or undetermined eccentricities; we unveiled curvatures caused by distant companions in the RV time series of HAT-P-2, HAT-P-22, and HAT-P-29; we significantly improved the orbital parameters of the long-period planet HAT-P-17c; and we revised the planetary parameters of CoRoT-1b, which turned out to be considerably more inflated than previously found. of giant planets are still open questions. Among these are the migration of hot Jupiters, the origin of the frequently observed spin-orbit misalignments, and the architecture of planetary systems with closein giant planets. These planets are thought to be formed beyond the water-ice line (a 1−3 au) in the protoplanetary disc, where solid material is abundant because of ice condensation, Article published by EDP Sciences A107, page 1 of 16 A&A 602, A107 (2017)

One of the essential prerequisites for detection of Earth-like extra-solar planets or direct measurements of the cosmological expansion is the accurate and precise wavelength calibration of astronomical spectrometers. It has already been realized that the large number of exactly known optical frequencies provided by laser frequency combs (astrocombs) can significantly surpass conventionally used hollow-cathode lamps as calibration light sources. A remaining challenge, however, is generation of frequency combs with lines resolvable by astronomical spectrometers. Here we demonstrate an astrocomb generated via soliton formation in an on-chip microphotonic resonator (microresonator) with a resolvable line spacing of 23.7 GHz. This comb is providing wavelength calibration on the 10 cm/s radial velocity level on the GIANO-B high-resolution near-infrared spectrometer. As such, microresonator frequency combs have the potential of providing broadband wavelength calibration for the next-generation of astronomical instruments in planet-hunting and cosmological research.The existence of life on other planets and the evolution of our Universe are questions that extend far beyond a purely astronomical context into other domains of science and society. Observational contributions relevant to both questions can be made by measuring minute wavelength shifts of spectral features in astronomical objects. For instance, an Earth-like planet, too faint for a direct observation, can reveal its presence by periodically modifying the radial velocity of its host star and hence Doppler-shifting characteristic features in the stellar spectrum 1,2 . Similarly, it has been suggested that the changing expansion rate of the Universe could be directly measured by observing the cosmological redshift in distant quasars 3,4 . The major challenge for such measurements is the requirement of a precisely and accurately calibrated astronomical spectrometer capable of detecting frequency shifts equivalent to radial velocities of the order of 10 cm/s or smaller. Conventional approaches of spectrometer calibration typically rely on the emission lines of hollow-cathode gas lamps that are used as calibration markers. However, the limited stability over time, the sparsity and different intensities of emission lines as well as the sensitivity to line blending impose limitations that are incompatible with the observational requirements. Over the last decade it has been realized that laser frequency combs (LFCs) 5-11 provide new means of wavelength calibration with unprecedented accuracy and precision 12-15 . Such LFCs are typically derived from mode-locked lasers and consist of large sets of laser lines whose optical frequencies ν n are equidistantly spaced: nu n = n * f rep + f 0 (n is an integer number). The two parameters f rep and f 0 are radio-frequencies (RF) accessible by conventional electronics and refer to the pulse repetition rate and carrier-envelope offset frequency of the mode-locked laser.Via self-referencing and stabilization schemes, f rep and ...

Aims. This work presents the results obtained by CoRoT on HD 50844, the only δ Sct star observed in the CoRoT initial run (57.6 d). The aim of these CoRoT observations was to investigate and characterize for the first time the pulsational behaviour of a δ Sct star, when observed at a level of precision and with a much better duty cycle than from the ground. Methods. The 140 016 datapoints were analysed using independent approaches (SigSpec software and different iterative sine-wave fittings) and several checks performed (splitting of the timeseries in different subsets, investigation of the residual light curves and spectra). A level of 10 −5 mag was reached in the amplitude spectra of the CoRoT timeseries. The space monitoring was complemented by ground-based high-resolution spectroscopy, which allowed the mode identification of 30 terms.Results. The frequency analysis of the CoRoT timeseries revealed hundreds of terms in the frequency range 0-30 d −1 . All the crosschecks confirmed this new result. The initial guess that δ Sct stars have a very rich frequency content is confirmed. The spectroscopic mode identification gives theoretical support since very high-degree modes (up to = 14) are identified. We also prove that cancellation effects are not sufficient in removing the flux variations associated to these modes at the noise level of the CoRoT measurements. The ground-based observations indicate that HD 50844 is an evolved star that is slightly underabundant in heavy elements, located on the Terminal Age Main Sequence. Probably due to this unfavourable evolutionary status, no clear regular distribution is observed in the frequency set. The predominant term ( f 1 = 6.92 d −1 ) has been identified as the fundamental radial mode combining ground-based photometric and spectroscopic data.

Context. OB stars are important building blocks of the Universe, but we have only a limited sample of them well understood enough from an asteroseismological point of view to provide feedback on the current evolutionary models. Our study adds one special case to this sample, with more observational constraints than for most of these stars. Aims. Our goal is to analyse and interpret the pulsational behaviour of the B3 IV star HD 43317 using the CoRoT light curve along with the ground-based spectroscopy gathered by the HARPS instrument. This way we continue our efforts to map the β Cep and SPB instability strips. Methods. We used different techniques to reveal the abundances and fundamental stellar parameters from the newly-obtained highresolution spectra. We used various time-series analysis tools to explore the nature of variations present in the light curve. We calculated the moments and used the pixel-by-pixel method to look for line profile variations in the high-resolution spectra. Results. We find that HD 43317 is a single fast rotator (v rot ≈ 50% v crit ) and hybrid SPB/β Cep-type pulsator with Solar metal abundances. We interpret the variations in photometry and spectroscopy as a result of rotational modulation connected to surface inhomogeneities, combined with the presence of both g and p mode pulsations. We detect a series of ten consecutive frequencies with an almost constant period spacing of 6339 s as well as a second shorter sequence consisting of seven frequencies with a spacing of 6380 s. The dominant frequencies fall in the regime of gravito-inertial modes.

Context. Radial-velocity (RV) signals arising from stellar photospheric phenomena are the main limitation for precise RV measurements Those signals induce RV variations an order of magnitude larger than the signal created by the orbit of Earth-twins, thus preventing their detection. Aims. Different methods have been developed to mitigate the impact of stellar RV signals. The goal of this paper is to compare the efficiency of these different methods to recover extremely low-mass planets despite stellar RV signals. However, because observed RV variations at the meter-per-second precision level or below is a combination of signals induced by unresolved orbiting planets, by the star, and by the instrument, performing such a comparison using real data is extremely challenging. Methods. To circumvent this problem, we generated simulated RV measurements including realistic stellar and planetary signals. Different teams analyzed blindly those simulated RV measurements, using their own method to recover planetary signals despite stellar RV signals. By comparing the results obtained by the different teams with the planetary and stellar parameters used to generate the simulated RVs, it is therefore possible to compare the efficiency of these different methods. Results. The most efficient methods to recover planetary signals take into account the different activity indicators, use red-noise models to account for stellar RV signals and a Bayesian framework to provide model comparison in a robust statistical approach. Using the most efficient methodology, planets can be found down to K/N = K pl /RV rms × √ N obs = 5 with a threshold of K/N = 7.5 at the level of 80-90% recovery rate found for a number of methods. These recovery rates drop dramatically for K/N smaller than this threshold. In addition, for the best teams, no false positives with K/N > 7.5 were detected, while a non-negligible fraction of them appear for smaller K/N. A limit of K/N = 7.5 seems therefore a safe threshold to attest the veracity of planetary signals for RV measurements with similar properties to those of the different RV fitting challenge systems.