Showing papers by "Mariateresa Crosta published in 2022"

Gaia Data Release 3. Summary of the content and survey properties

Abstract: We present the third data release of the European Space Agency's Gaia mission, GDR3. The GDR3 catalogue is the outcome of the processing of raw data collected with the Gaia instruments during the first 34 months of the mission by the Gaia Data Processing and Analysis Consortium. The GDR3 catalogue contains the same source list, celestial positions, proper motions, parallaxes, and broad band photometry in the G, G$_{BP}$, and G$_{RP}$ pass-bands already present in the Early Third Data Release. GDR3 introduces an impressive wealth of new data products. More than 33 million objects in the ranges $G_{rvs}<14$ and $3100

Gaia Data Release 3. Chemical cartography of the Milky Way

Abstract: Gaia DR3 opens a new era of all-sky spectral analysis of stellar populations thanks to the nearly 5.6 million stars observed by the RVS and parametrised by the GSP-spec module. The all-sky Gaia chemical cartography allows a powerful and precise chemo-dynamical view of the Milky Way with unprecedented spatial coverage and statistical robustness. First, it reveals the strong vertical symmetry of the Galaxy and the flared structure of the disc. Second, the observed kinematic disturbances of the disc -- seen as phase space correlations -- and kinematic or orbital substructures are associated with chemical patterns that favour stars with enhanced metallicities and lower [alpha/Fe] abundance ratios compared to the median values in the radial distributions. This is detected both for young objects that trace the spiral arms and older populations. Several alpha, iron-peak elements and at least one heavy element trace the thin and thick disc properties in the solar cylinder. Third, young disc stars show a recent chemical impoverishment in several elements. Fourth, the largest chemo-dynamical sample of open clusters analysed so far shows a steepening of the radial metallicity gradient with age, which is also observed in the young field population. Finally, the Gaia chemical data have the required coverage and precision to unveil galaxy accretion debris and heated disc stars on halo orbits through their [alpha/Fe] ratio, and to allow the study of the chemo-dynamical properties of globular clusters. Gaia DR3 chemo-dynamical diagnostics open new horizons before the era of ground-based wide-field spectroscopic surveys. They unveil a complex Milky Way that is the outcome of an eventful evolution, shaping it to the present day (abridged).

Gaia Data Release 3: Stellar multiplicity, a teaser for the hidden treasure

Abstract: The Gaia DR3 Catalogue contains for the first time about eight hundred thousand solutions with either orbital elements or trend parameters for astrometric, spectroscopic and eclipsing binaries, and combinations of them. This paper aims to illustrate the huge potential of this large non-single star catalogue. Using the orbital solutions together with models of the binaries, a catalogue of tens of thousands of stellar masses, or lower limits, partly together with consistent flux ratios, has been built. Properties concerning the completeness of the binary catalogues are discussed, statistical features of the orbital elements are explained and a comparison with other catalogues is performed. Illustrative applications are proposed for binaries across the H-R diagram. The binarity is studied in the RGB/AGB and a search for genuine SB1 among long-period variables is performed. The discovery of new EL CVn systems illustrates the potential of combining variability and binarity catalogues. Potential compact object companions are presented, mainly white dwarf companions or double degenerates, but one candidate neutron star is also presented. Towards the bottom of the main sequence, the orbits of previously-suspected binary ultracool dwarfs are determined and new candidate binaries are discovered. The long awaited contribution of Gaia to the analysis of the substellar regime shows the brown dwarf desert around solar-type stars using true, rather than minimum, masses, and provides new important constraints on the occurrence rates of substellar companions to M dwarfs. Several dozen new exoplanets are proposed, including two with validated orbital solutions and one super-Jupiter orbiting a white dwarf, all being candidates requiring confirmation. Beside binarity, higher order multiple systems are also found.

Gaia Data Release 3. Mapping the asymmetric disc of the Milky Way

Abstract: With the most recent Gaia data release the number of sources with complete 6D phase space information (position and velocity) has increased to well over 33 million stars, while stellar astrophysical parameters are provided for more than 470 million sources, in addition to the identification of over 11 million variable stars. Using the astrophysical parameters and variability classifications provided in Gaia DR3, we select various stellar populations to explore and identify non-axisymmetric features in the disc of the Milky Way in both configuration and velocity space. Using more about 580 thousand sources identified as hot OB stars, together with 988 known open clusters younger than 100 million years, we map the spiral structure associated with star formation 4-5 kpc from the Sun. We select over 2800 Classical Cepheids younger than 200 million years, which show spiral features extending as far as 10 kpc from the Sun in the outer disc. We also identify more than 8.7 million sources on the red giant branch (RGB), of which 5.7 million have line-of-sight velocities, allowing the velocity field of the Milky Way to be mapped as far as 8 kpc from the Sun, including the inner disc. The spiral structure revealed by the young populations is consistent with recent results using Gaia EDR3 astrometry and source lists based on near infrared photometry, showing the Local (Orion) arm to be at least 8 kpc long, and an outer arm consistent with what is seen in HI surveys, which seems to be a continuation of the Perseus arm into the third quadrant. Meanwhile, the subset of RGB stars with velocities clearly reveals the large scale kinematic signature of the bar in the inner disc, as well as evidence of streaming motions in the outer disc that might be associated with spiral arms or bar resonances. (abridged)

Gaia Early Data Release 3. The celestial reference frame (Gaia-CRF3)

Abstract: Gaia-CRF3 is the celestial reference frame for positions and proper motions in the third release of data from the Gaia mission, Gaia DR3 (and for the early third release, Gaia EDR3, which contains identical astrometric results). The reference frame is defined by the positions and proper motions at epoch 2016.0 for a specific set of extragalactic sources in the (E)DR3 catalogue. We describe the construction of Gaia-CRF3, and its properties in terms of the distributions in magnitude, colour, and astrometric quality. Compact extragalactic sources in Gaia DR3 were identified by positional cross-matching with 17 external catalogues of quasars (QSO) and active galactic nuclei (AGN), followed by astrometric filtering designed to remove stellar contaminants. Selecting a clean sample was favoured over including a higher number of extragalactic sources. For the final sample, the random and systematic errors in the proper motions are analysed, as well as the radio-optical offsets in position for sources in the third realisation of the International Celestial Reference Frame (ICRF3). The Gaia-CRF3 comprises about 1.6 million QSO-like sources, of which 1.2 million have five-parameter astrometric solutions in Gaia DR3 and 0.4 million have six-parameter solutions. The sources span the magnitude range G = 13 to 21 with a peak density at 20.6 mag, at which the typical positional uncertainty is about 1 mas. The proper motions show systematic errors on the level of 12 ${\mu}$as yr${}^{-1}$ on angular scales greater than 15 deg. For the 3142 optical counterparts of ICRF3 sources in the S/X frequency bands, the median offset from the radio positions is about 0.5 mas, but exceeds 4 mas in either coordinate for 127 sources. We outline the future of the Gaia-CRF in the next Gaia data releases.

Gaia Data Release 3. The extragalactic content

Abstract: The Gaia Galactic survey mission is designed and optimized to obtain astrometry, photometry, and spectroscopy of nearly two billion stars in our Galaxy. Yet as an all-sky multi-epoch survey, Gaia also observes several million extragalactic objects down to a magnitude of G~21 mag. Due to the nature of the Gaia onboard selection algorithms, these are mostly point-source-like objects. Using data provided by the satellite, we have identified quasar and galaxy candidates via supervised machine learning methods, and estimate their redshifts using the low resolution BP/RP spectra. We further characterise the surface brightness profiles of host galaxies of quasars and of galaxies from pre-defined input lists. Here we give an overview of the processing of extragalactic objects, describe the data products in Gaia DR3, and analyse their properties. Two integrated tables contain the main results for a high completeness, but low purity (50-70%), set of 6.6 million candidate quasars and 4.8 million candidate galaxies. We provide queries that select purer sub-samples of these containing 1.9 million probable quasars and 2.9 million probable galaxies (both 95% purity). We also use high quality BP/RP spectra of 43 thousand high probability quasars over the redshift range 0.05-4.36 to construct a composite quasar spectrum spanning restframe wavelengths from 72-100 nm.

Gaia Data Release 3: A golden sample of astrophysical parameters

Abstract: Gaia Data Release 3 (DR3) provides a wealth of new data products for the astronomical community to exploit, including astrophysical parameters for a half billion stars. In this work we demonstrate the high quality of these data products and illustrate their use in different astrophysical contexts. We query the astrophysical parameter tables along with other tables in Gaia DR3 to derive the samples of the stars of interest. We validate our results by using the Gaia catalogue itself and by comparison with external data. We have produced six homogeneous samples of stars with high quality astrophysical parameters across the HR diagram for the community to exploit. We first focus on three samples that span a large parameter space: young massive disk stars (~3M), FGKM spectral type stars (~3M), and UCDs (~20K). We provide these sources along with additional information (either a flag or complementary parameters) as tables that are made available in the Gaia archive. We furthermore identify 15740 bone fide carbon stars, 5863 solar-analogues, and provide the first homogeneous set of stellar parameters of the Spectro Photometric Standard Stars. We use a subset of the OBA sample to illustrate its usefulness to analyse the Milky Way rotation curve. We then use the properties of the FGKM stars to analyse known exoplanet systems. We also analyse the ages of some unseen UCD-companions to the FGKM stars. We additionally predict the colours of the Sun in various passbands (Gaia, 2MASS, WISE) using the solar-analogue sample.

Gaia Data Release 3: Reflectance spectra of Solar System small bodies

Abstract: The Gaia mission of the European Space Agency (ESA) has been routinely observing Solar System objects (SSOs) since the beginning of its operations in August 2014. The Gaia data release three (DR3) includes, for the first time, the mean reflectance spectra of a selected sample of 60 518 SSOs, primarily asteroids, observed between August 5, 2014, and May 28, 2017. Each reflectance spectrum was derived from measurements obtained by means of the Blue and Red photometers (BP/RP), which were binned in 16 discrete wavelength bands. We describe the processing of the Gaia spectral data of SSOs, explaining both the criteria used to select the subset of asteroid spectra published in Gaia DR3, and the different steps of our internal validation procedures. In order to further assess the quality of Gaia SSO reflectance spectra, we carried out external validation against SSO reflectance spectra obtained from ground-based and space-borne telescopes and available in the literature. For each selected SSO, an epoch reflectance was computed by dividing the calibrated spectrum observed by the BP/RP at each transit on the focal plane by the mean spectrum of a solar analogue. The latter was obtained by averaging the Gaia spectral measurements of a selected sample of stars known to have very similar spectra to that of the Sun. Finally, a mean of the epoch reflectance spectra was calculated in 16 spectral bands for each SSO. The agreement between Gaia mean reflectance spectra and those available in the literature is good for bright SSOs, regardless of their taxonomic spectral class. We identify an increase in the spectral slope of S-type SSOs with increasing phase angle. Moreover, we show that the spectral slope increases and the depth of the 1 um absorption band decreases for increasing ages of S-type asteroid families.

Gaia Data Release 3. Pulsations in main-sequence OBAF-type stars

Abstract: The third Gaia data release provides photometric time series covering 34 months for about 10 million stars. For many of those stars, a characterisation in Fourier space and their variability classification are also provided. This paper focuses on intermediate- to high-mass (IHM) main sequence pulsators M>= 1.3 Msun) of spectral types O, B, A, or F, known as beta Cep, slowly pulsating B (SPB), delta Sct, and gamma Dor stars. These stars are often multi-periodic and display low amplitudes, making them challenging targets to analyse with sparse time series. All datasets used in this analysis are part of the Gaia DR3 data release. The photometric time series were used to perform a Fourier analysis, while the global astrophysical parameters necessary for the empirical instability strips were taken from the Gaia DR3 gspphot tables, and the vsini data were taken from the Gaia DR3 esphs tables. We show that for nearby OBAF-type pulsators, the Gaia DR3 data are precise and accurate enough to pinpoint them in the Hertzsprung-Russell diagram. We find empirical instability strips covering broader regions than theoretically predicted. In particular, our study reveals the presence of fast rotating gravity-mode pulsators outside the strips, as well as the co-existence of rotationally modulated variables inside the strips as reported before in the literature. We derive an extensive period-luminosity relation for delta Sct stars and provide evidence that the relation features different regimes depending on the oscillation period. Finally, we demonstrate how stellar rotation attenuates the amplitude of the dominant oscillation mode of delta Sct stars.

Astrometric Precision Tests on TESS Data

Abstract: Astrometry at or below the microarcsec level with an imaging telescope assumes that the uncertainty on the location of an unresolved source can be an arbitrarily small fraction of the detector pixel, given a sufficient photon budget. This paper investigates the geometric limiting precision, in terms of CCD pixel fraction, achieved by a large set of star field images, selected among the publicly available science data of the Transiting Exoplanet Survey Satellite (TESS) mission. The statistics of the distance between selected bright stars (G ≃ 5 mag), in pixel units, is evaluated, using the position estimate provided in the TESS light curve files. The dispersion of coordinate differences appears to be affected by long term variation and noisy periods, at the level of 0.01 pixel. The residuals with respect to low-pass filtered data (tracing the secular evolution), which are interpreted as the experimental astrometric noise, reach the level of a few milli-pixel or below, down to 1/5900 pixel. Saturated images are present, evidencing that the astrometric precision is mostly preserved across the CCD columns, whereas it features a graceful degradation in the along column direction. The cumulative performance of the image set is a few micropixel across columns, or a few 10 μpx along columns. The idea of astrometric precision down to a small fraction of a CCD pixel, given sufficient signal to noise ratio, is confirmed by real data from an in-flight science instrument to the 10−6 pixel level. Implications for future high precision astrometry missions are briefly discussed.

Triple line of sight telescopes for space astrometry

Abstract: The concept of a 3 lines of sight telescope for space astrometry is evaluated in terms of implementation feasibility and some science applications. The beam combination by pupil splitting exploits the Hipparcos concept, extending it to true large bidimensional angle. The PSF symmetry provides equivalent astrometric performance and immediate field identification on the common detector. Operation aspects are investigated in terms of field identification and observing strategy, in particular pointing constraints and sky density of reference bright stars. The approach, based on flat mirrors mounted on a pyramidal structure, provides a simple geometric framework, with immediate mechanical materialization and clear interface to metrology. The geometric arrangements considered place the lines of sight either along the (x, y, z) Cartesian axes, or in a planar layout for a Gaia-like astrometric mission. The application to a single head, multiple boresight star tracker is also outlined.

Experimenting techniques of global astrometry from space

Abstract: Since the publication of the HIPPARCOS catalog in 1997, and with its successor Gaia in operational phase since 2013 having already published 3 data releases, global astrometry has earned itself a prominent role in the field of astrophysics. Both missions rest on the well-established concepts of one-dimensional (or nearly one-dimensional) observations performed by a telescope that is continuously scanning the sky following a predefined scanning law. Moreover, Gaia heavily relies also on the concept of self-calibrating instrument. In this work we explore the possibility of exploiting two-dimensional observations performed by a pointed mission, discussing the feasibility of this concept, and its possible advantages.

Post-Newtonian gravity and Gaia-like astrometry. Effect of PPN gamma uncertainty on parallaxes

Abstract: Context. Relativistic models of light propagation adopted for high-precision astrometry are based on the parametrised post-Newtonian formalism, which provides a framework for examining the e ff ects of a hypothetical violation of general relativity on astrometric data. Astrometric observations are strongly a ff ected by the post-Newtonian parameter γ describing the strength of gravitational light deflection. Aims. We study both analytically and numerically how a deviation in the PPN parameter γ from unity, which is the value predicted by general relativity, a ff ects the parallax estimations in Gaia -like astrometry. Methods. Changes in the observable quantities produced by a small variation in PPN γ were calculated analytically. We then con- sidered how such variations of the observables are reflected in the parallax estimations, and we performed numerical simulations to check the theoretical predictions. Results. A variation in the PPN γ results in a global shift of parallaxes and we present a formula describing the parallax bias in terms of the satellite barycentric distance, the angle between the spin axis and the direction to the Sun, and the PPN γ uncertainty. Numerical simulations of the astrometric solutions confirm the theoretical result. The up-to-date estimation of PPN γ suggests that a corresponding contribution to the Gaia parallax zero point unlikely exceeds 0.2 µ as. The numerical simulations indicate that the parallax shift is strongly dependent on ecliptic latitude. It is argued that this e ff ect is due to an asymmetry in the Gaia scanning law and this conclusion is fully validated by additional simulations with a reversed direction of the precession of the spin axis around the direction to the Sun.

Differential astrometry with Gaia. Investigating relativistic light deflection close to Jupiter

Abstract: Aims. In this paper, we develop a differential astrometric framework that is appropriate for a scanning space satellite such as Gaia . We apply it to the first of the GAREQ fields – the Gaia Relativistic Experiment on Jupiter’s quadrupole – which is the fruit of dedicated efforts within the Gaia project focused on measuring the relativistic deflection of light close to Jupiter’s limb. This provides a prelim- inary assessment of the following: a) the observability of the relativistic deflection of light close to Jupiter and b) Gaia ’s astrometric capabilities under extremely difficult conditions such as those around a bright extended object. Methods. Inputs to our differential astrometric model are the charge-coupled device (CCD) transit times as measured by the interme- diate data update (IDU) system, transformed to field angles via astrometric global iterative solution (AGIS) geometric calibrations, and the commanded or nominal spacecraft attitude. Actual attitude rates, including medium and high-frequency effects, were estimated from successive CCD pair observations and used to transfer the field angles onto intermediate tangent planes, finally anchored to a common reference frame by fitting a six-parameter model to a set of suitable reference stars. The best-fit parameters provide the target star’s deflection as a time-varying systematic effect. To illustrate the model, we analyzed Gaia astrometric measurements after their calibration through the latest cyclic early data release EDR3/DR3 processing of the GAREQ event in February 2017. We used observations of the closest bright target star successfully observed several times by Gaia in close proximity to Jupiter and surrounding reference stars brighter than G < 13 mag in transits leading up to the time of closest approach and on subsequent transits. Results. The relativistic deflection signal is detected with a signal-to-noise ratio (S/N) of 50 at closest approach by the target star. This signal is the combined effect due to Jupiter and the Sun, mainly dominated by Jupiter’s monopole, demonstrating Gaia ’s scientific performance under extreme observational conditions. It is an unprecedented detection for the following reasons: a) it is the closest ever to Jupiter’s limb ( ∼ 7 (cid:48)(cid:48) ) in the optical and b) the highest S/N at any wavelength. Finally, this work sets the stage for investigations into disentangling the relativistic quadrupole deflection due to Jupiter with future Gaia astrometric measurements.

Post-Newtonian gravity and Gaia -like astrometry Effect of PPN γ uncertainty on parallaxes

Abstract: Context. Relativistic models of light propagation adopted for high-precision astrometry are based on the parametrised post-Newtonian formalism, which provides a framework for examining the effects of a hypothetical violation of general relativity on astrometric data. Astrometric observations are strongly affected by the post-Newtonian (PPN) parameter γ describing the strength of gravitational light deflection. Aims. We study both analytically and numerically how a deviation in the PPN parameter γ from unity, which is the value predicted by general relativity, affects the parallax estimations in Gaia -like astrometry. Methods. Changes in the observable quantities produced by a small variation in PPN γ were calculated analytically. We then considered how such variations of the observables are reflected in the parallax estimations, and we performed numerical simulations to check the theoretical predictions. Results. A variation in the PPN γ results in a global shift of parallaxes and we present a formula describing the parallax bias in terms of the satellite barycentric distance, the angle between the spin axis and the direction to the Sun, and the PPN γ uncertainty. Numerical simulations of the astrometric solutions confirm the theoretical result. The up-to-date estimation of PPN γ suggests that a corresponding contribution to the Gaia parallax zero point unlikely exceeds 0.2 µ as. The numerical simulations indicate that the parallax shift is strongly dependent on ecliptic latitude. It is argued that this effect is due to an asymmetry in the Gaia scanning law and this conclusion is fully validated by additional simulations with a reversed direction of the precession of the spin axis around the direction to the Sun.

On the principle of Astrometric Gravitational Wave Antenna

Abstract: The direct detection of gravitational waves by ground-based optical interferometers has opened a new window in astronomy. Nevertheless, as these detectors are a combination of two Michelson-Morley like baselines, their sensitivity for determining the incident direction of a gravitational wave is quite weak compared to current high-precision of space astrometry. We therefore present a novel concept for a gravitational wave antenna in space that uses close pairs of point-like sources as natural sensors to record and characterize the very tiny variations in angular separations induced by a passing gravitational wave, thus operating com-plementarily to linear arm detectors and enabling to gain informations on the gravitational wave incoming direction. Indeed, the proposed astrometric gravitational wave observable builds on methods of relativistic astrometry that can substantially enhance the strength of gravitational wave signals by exploiting to the fullest the telescope optical resolution and, at same time, provide a powerful tool for identifying the direction to the sources that originated the gravitational wave by monitoring close pairs of stars. Furthermore, the use of two local-line-of-sights in a differential formulation avoids issues related to high order modeling of the local (Solar System) background geometry and the need for accurate satellite ephemeris and attitude.