ESA Gaia Research Articles

A calibration of the Rossby number from asteroseismology

Stellar activity and rotation are tightly related in a dynamo process. Our understanding of this mechanism is mainly limited by our capability of inferring the properties of stellar turbulent convection. In particular, the convective turnover time is a key ingredient through the estimation of the stellar Rossby number, which is the ratio of the rotation period and the convective turnover time. In this work, we propose a new calibration of the (B − V) color index dependence of the convective turnover time, hence, of the stellar Rossby number. Our new calibration is based on the stellar structure properties inferred through the detailed modeling of solar-like pulsators using asteroseismic observables. We show the impact of this calibration via a stellar activity-Rossby number diagram by applying it to a sample of about 40 000 stars observed with Kepler and for which the values for the photometric activity proxy S​ph and surface rotation periods are available. Additionally, we provide a new calibration for the convective turnover time as function of the (GBP − GRP) color index for allowing applicability in the ESA Gaia photometric passbands.

Asteroid spectral taxonomy using neural networks

Aims. We explore the performance of neural networks in automatically classifying asteroids into their taxonomic spectral classes. We particularly focus on what the methodology could offer the ESA Gaia mission. Methods. We constructed an asteroid dataset that can be limited to simulating Gaia samples. The samples were fed into a custom-designed neural network that learns how to predict the samples’ spectral classes and produces the success rate of the predictions. The performance of the neural network is also evaluated using three real preliminary Gaia asteroid spectra. Results. The overall results show that the neural network can identify taxonomic classes of asteroids in a robust manner. The success in classification is evaluated for spectra from the nominal 0.45–2.45 μm wavelength range used in the Bus-DeMeo taxonomy, and from a limited range of 0.45–1.05 μm following the joint wavelength range of Gaia observations and the Bus-DeMeo taxonomic system. Conclusions. The obtained results indicate that using neural networks to execute automated classification is an appealing solution for maintaining asteroid taxonomies, especially as the size of the available datasets grows larger with missions like Gaia.

The GALAH Survey: No Chemical Evidence of an Extragalactic Origin for the Nyx Stream

Abstract The results from the ESA Gaia astrometric mission and deep photometric surveys have revolutionized our knowledge of the Milky Way. There are many ongoing efforts to search these data for stellar substructure to find evidence of individual accretion events that built up the Milky Way and its halo. One of these newly identified features, called Nyx, was announced as an accreted stellar stream traveling in the plane of the disk. Using a combination of elemental abundances and stellar parameters from the GALAH and Apache Point Observatory Galactic Evolution Experiment (APOGEE) surveys, we find that the abundances of the highest likelihood Nyx members are entirely consistent with membership of the thick disk, and inconsistent with a dwarf galaxy origin. We conclude that the postulated Nyx stream is most probably a high-velocity component of the Milky Way’s thick disk. With the growing availability of large data sets including kinematics, stellar parameters, and detailed abundances, the probability of detecting chance associations increases, and hence new searches for substructure require confirmation across as many data dimensions as possible.

Galactic seismology: the evolving ‘phase spiral’ after the Sagittarius dwarf impact

ABSTRACT In 2018, the ESA Gaia satellite discovered a remarkable spiral pattern (‘phase spiral’) in the z − Vz phase plane throughout the solar neighbourhood, where z and Vz are the displacement and velocity of a star perpendicular to the Galactic disc. In response to Binney & Schönrich’s analytic model of a disc-crossing satellite to explain the Gaia data, we carry out a high-resolution, N-body simulation (N ≈ 108 particles) of an impulsive mass (2 × 1010 M⊙) that interacts with a cold stellar disc at a single transit point. The disc response is complex since the impulse triggers a superposition of two distinct bisymmetric (m = 2) modes − a density wave and a corrugated bending wave − that wrap up at different rates. Stars in the faster density wave wrap up with time T according to ϕD(R, T) = (ΩD(R) + Ωo) T, where ϕD describes the spiral pattern and ΩD = Ω(R) − κ(R)/2, where κ is the epicyclic frequency. While the pattern speed Ωo is small, it is non-zero. The slower bending wave wraps up according to ΩB ≈ ΩD/2 producing a corrugated wave. The bunching effect of the density wave triggers the phase spiral as it rolls up and down on the corrugated wave (‘roller coaster’ model). The phase spiral emerges slowly about ΔT ≈ 400 Myr after impact. It appears to be a long-lived, disc-wide phenomenon that continues to evolve over most of the 2 Gyr simulation. Thus, given Sagittarius’ (Sgr) low total mass today (Mtot ∼ 3 × 108 M⊙ within 10 kpc diameter), we believe that the phase spiral was excited by the disc-crossing dwarf some 1–2 Gyr before the recent transit. For this to be true, Sgr must be losing mass at 0.5–1 dex per orbit loop.

The Parallax of ω Centauri Measured from Gaia EDR3 and a Direct, Geometric Calibration of the Tip of the Red Giant Branch and the Hubble Constant

Abstract We use data from the ESA Gaia mission Early Data Release 3 (EDR3) to measure the trigonometric parallax of ω Cen, the first high-precision parallax measurement for the most massive globular cluster in the Milky Way. We use a combination of positional and high-quality proper motion data from EDR3 to identify over 100,000 cluster members, of which 67,000 are in the magnitude and color range where EDR3 parallaxes are best calibrated. We find the estimated parallax to be robust, demonstrating good control of systematics within the color–magnitude diagram of the cluster. We find a parallax for the cluster of 0.191 ± 0.001 (statistical) ±0.004 (systematic) mas (2.2% total uncertainty) corresponding to a distance of 5.24 ± 0.11 kpc. The parallax of ω Cen provides a unique opportunity to directly and geometrically calibrate the luminosity of the tip of the red giant branch (TRGB) because it is the only cluster with sufficient mass to provide enough red giant stars, more than 100 one magnitude below the tip, for a precise, model-free measurement of the tip. Combined with the preexisting and most widely used measurements of the tip and foreground Milky Way extinction, we find M I,TRGB =−3.97 ± 0.06 mag for the I-band luminosity of the blue edge. Using the TRGB luminosity calibrated from the Gaia EDR3 parallax of ω Cen to calibrate the luminosity of Type Ia supernovae results in a value for the Hubble constant of H 0 = 72.1 ± 2.0 km s−1 Mpc−1. We make the data for the stars in ω Cen available electronically and encourage independent analyses of the results presented here.

Runaway blue main-sequence stars at high Galactic latitudes

Context. The ESA Gaia mission is a remarkable tool for stellar population analysis through its accurate Hertzsprung–Russell diagram. Its precise astrometry has propelled detailed kinematic studies of the Milky Way and the identification of high-velocity outliers. Aims. Motivated by the historical identification of runaway main-sequence (MS) stars of an early spectral type at high Galactic latitudes, we test the capability of Gaia at identifying new such stars. Methods. We selected ≈2300 sources with Gaia magnitudes of GBP − GRP ≤ 0.05, which are compatible with the colors of low-extinction MS stars that are earlier than a mid-A spectral type, and obtained low-resolution optical spectroscopy for 48 such stars. By performing detailed photometric and spectroscopic analyses, we derived their atmospheric and physical parameters (effective temperature, surface gravity, radial velocity, interstellar reddening, spectrophotometric distance, mass, radius, luminosity, and age). The comparison between spectrophotometric and parallax-based distances enabled us to disentangle the MS candidates from older blue horizontal branch (BHB) candidates. Results. We identify 12 runaway MS candidates, with masses between 2 and 6 M⊙. Their trajectories were traced back to the Galactic disk in order to identify their most recent Galactic plane crossings and the corresponding flight times. All 12 candidates are ejected from the Galactic disk within 2 to 16.5 kpc from the Galactic center and possess flight times that are shorter than their evolutionary ages, which is compatible with a runaway hypothesis. Three MS candidates have ejection velocities exceeding 450 km s−1, thus, they appear to challenge the canonical ejection scenarios for late B-type stars. The fastest star of our sample also has a non-negligible Galactic escape probability if its MS nature can be confirmed. We identify 27 BHB candidates, and the two hottest stars in our sample are rare low-mass stars of late O- and early B-types, evolving towards the white dwarf cooling sequence. Conclusions. The combination of Gaia parallaxes and proper motions can lead to the efficient selection of runaway blue MS candidates up to 10 kpc away from the Sun. High resolution spectra are needed to confirm the MS status, via precise measurements of projected rotational velocities and chemical compositions.

HATS-34b and HATS-46b: re-characterization using TESS and Gaia

ABSTRACT We present a revised characterization of the previously discovered transiting planet systems HATS-34 and HATS-46. We make use of the newly available space-based light curves from the NASA TESS mission and high-precision parallax and absolute photometry measurements from the ESA Gaia mission to determine the mass and radius of the planets and host stars with dramatically increased precision and accuracy compared to published values, with the uncertainties in some parameters reduced by as much as a factor of 7. Using an isochrone-based fit, for HATS-34 we measure a revised host star mass and radius of $0.952_{-0.020}^{+0.040}$ $\, \mathrm{M}_\odot$ and 0.9381 ± 0.0080 $\, \mathrm{R}_\odot$, respectively, and a revised mass and radius for the transiting planet of 0.951 ± 0.050 MJ and 1.282 ± 0.064 RJ, respectively. Similarly, for HATS-46 we measure a revised mass and radius for the host star of 0.869 ± 0.023 $\, \mathrm{M}_\odot$ and 0.894 ± 0.010 $\, \mathrm{R}_\odot$, respectively, and a revised mass and radius for the planet of 0.158 ± 0.042 MJ and 0.951 ± 0.029 RJ, respectively. The uncertainties that we determine on the stellar and planetary masses and radii are also substantially lower than re-determinations that incorporate the Gaia results without performing a full re-analysis of the light curves and other observational data. We argue that, in light of Gaia and TESS, a full re-analysis of previously discovered transiting planets is warranted.

Cluster Difference Imaging Photometric Survey. II. TOI 837: A Young Validated Planet in IC 2602

Abstract We report the discovery of TOI 837b and its validation as a transiting planet. We characterize the system using data from the NASA Transiting Exoplanet Survey Satellite mission, the ESA Gaia mission, ground-based photometry from El Sauce and ASTEP400, and spectroscopy from CHIRON, FEROS, and Veloce. We find that TOI 837 is a T = 9.9 mag G0/F9 dwarf in the southern open cluster IC 2602. The star and planet are therefore million years old. Combining the transit photometry with a prior on the stellar parameters derived from the cluster color–magnitude diagram, we find that the planet has an orbital period of and is slightly smaller than Jupiter ( ). From radial velocity monitoring, we limit to less than 1.20 M Jup (3σ). The transits either graze or nearly graze the stellar limb. Grazing transits are a cause for concern, as they are often indicative of astrophysical false-positive scenarios. Our follow-up data show that such scenarios are unlikely. Our combined multicolor photometry, high-resolution imaging, and radial velocities rule out hierarchical eclipsing binary scenarios. Background eclipsing binary scenarios, though limited by speckle imaging, remain a 0.2% possibility. TOI 837b is therefore a validated adolescent exoplanet. The planetary nature of the system can be confirmed or refuted through observations of the stellar obliquity and the planetary mass. Such observations may also improve our understanding of how the physical and orbital properties of exoplanets change in time.

PTFO 8-8695: Two Stars, Two Signals, No Planet

Abstract PTFO 8-8695 (CVSO 30) is a star in the 7–10 million year old Orion OB1a cluster that shows brightness dips that resemble planetary transits. Although strong evidence against the planet hypothesis has been presented, the possibility remains debated in the literature. To obtain further clues, we inspected data from the NASA Transiting Exoplanet Survey Satellite (TESS) and the ESA Gaia mission. The Gaia data suggest that PTFO 8-8695 is a binary: the photometric data show it to be overluminous with respect to members of its kinematic group, and the astrometric data are inconsistent with a single star. The TESS light curve shows two different photometric periods. The variability is dominated by a sinusoidal signal with a period of 11.98 hr, presumably caused by stellar rotation. Also present is a 10.76 hr signal consisting of a not-quite sinusoid interrupted by hour-long dips, the type of signal previously interpreted as planetary transits. The phase of the dips is nearly 180° away from the phase of the originally reported dips. As noted previously, this makes them difficult to explain as planetary transits. Instead, we believe that PTFO 8-8695 is a pair of young and rapidly rotating M dwarfs, one of which shows the same “transient-dipper” behavior that has been seen in at least five other cases. The origin of these transient dips is still unknown but likely involves circumstellar material.

The nearby spiral density-wave structure of the Galaxy: line-of-sight velocities of the Gaia DR2 main-sequence A, F, G, and K stars

ABSTRACT Distances and velocities of $\approx \!2400\, 000$ main-sequence A, F, G, and K stars are collected from the second data release of ESA's Gaia astrometric mission. This material is analysed to find evidence of radial and azimuthal systematic non-circular motions of stars in the solar neighbourhood on the assumption that the system is subject to spiral density waves (those produced by a spontaneous disturbance, a central bar, or an external companion), developing in the Galactic disc. Data analysis of line-of-sight velocities of $\approx \!1500\, 000$ stars selected within 2 kpc from the Sun and 500 pc from the Galactic mid-plane with distance accuracies of <10 per cent makes evident that a radial wavelength of the wave pattern is 1.1–1.6 kpc and a phase of the wave at the Sun’s location in the Galaxy is 55°–95°. Respectively, the Sun is situated at the inner edge of the nearest Orion spiral arm segment. Thus, the local Orion arm is a part of a predominant density-wave structure of the system. The spiral structure of the Galaxy has an oscillating nature corresponding to a concept of the Lin–Shu-type moderately growing in amplitude, tightly wound, and rigidly rotating density waves.

Infrared K-band Photometry of Field RR Lyrae Variable Stars

We present multi-epoch infrared photometry in the $K_s$-band for 74 bright RR Lyrae variable stars tied directly to the 2MASS photometric system. We systematize additional $K$-band photometry from the literature to the 2MASS system and combine it to obtain photometry for 146 RR Lyrae stars on a consistent, modern system. A set of outlier stars in the literature photometry is identified and discussed. Reddening estimates for each star were gathered from the literature and combined to provide an estimate of the interstellar absorption affecting each star, and we find excellent agreement with another source in the literature. We utilize trigonometric parallaxes from the Second Data Release of ESA's Gaia astrometric satellite to determine the absolute magnitude, $M_{K_s}$ for each of these stars, and analyze them using the astrometry based luminosity prescription to obtain a parallax-based calibration of $M_K$(RR). Our period-luminosity-metallicity relationship is $ M_{K_s} = (-2.8\pm 0.2) (\log P +0.27) + (0.12\pm 0.02) ( [\mathrm{Fe/H}] + 1.3) - (0.41\pm 0.03)$ mag. A Gaia global zero-point error of $\pi_{zp} = -0.042\pm 0.013\,$mas is determined for this sample of RR Lyrae stars.

Piercing the Milky Way: an all-sky view of the Orphan Stream

We use astrometry, broad-band photometry and variability information from the Data Release 2 of ESA's Gaia mission (GDR2) to identify members of the Orphan Stream (OS) across the whole sky. The stream is traced above and below the celestial equator and in both Galactic hemispheres, thus increasing its visible length to ~ 210 degrees equivalent to ~150 kpc in physical extent. Taking advantage of the large number of RR Lyrae stars in the OS, we extract accurate distances and proper motions across the entire stretch of the tidal debris studied. As delineated by the GDR2 RR Lyrae, the stream exhibits two prominent twists in its shape on the sky which are accompanied by changes in the tangential motion. We complement the RR Lyrae maps with those created using GDR2 Red Giants and the DECam Legacy Survey Main Sequence Turn-Off stars. The behavior of the OS track on the sky is consistent across all three tracers employed. We detect a strong non-zero motion in the across-stream direction for a substantial portion of the stream. Such a misalignment between the debris track and the streaming velocity cannot be reproduced in a static gravitational potential and signals an interaction with a massive perturber.

The GALAH survey and Gaia DR2: dissecting the stellar disc’s phase space by age, action, chemistry, and location

We use the second data releases of the ESA Gaia astrometric survey and the high-resolution GALAH spectroscopic survey to analyse the structure of our Galaxy's disc components. With GALAH, we separate the alpha-rich and alpha-poor discs (with respect to Fe), which are superposed in both position and velocity space, and examine their distributions in action space. We study the distribution of stars in the zV_z phase plane, for both V_phi and V_R, and recover the remarkable "phase spiral" discovered by Gaia. We identify the anticipated quadrupole signature in zV_z of a tilted velocity ellipsoid for stars above and below the Galactic plane. By connecting our work with earlier studies, we show that the phase spiral is likely to extend well beyond the narrow solar neighbourhood cylinder in which it was found. The phase spiral is a signature of corrugated waves that propagate through the disc, and the associated non-equilibrium phase mixing. The radially asymmetric distribution of stars involved in the phase spiral reveals that the corrugation, which is mostly confined to the alpha-poor disc, grows in z-amplitude with increasing radius. We present new simulations of tidal disturbance of the Galactic disc by the Sagittarius (Sgr) dwarf. The effect on the zV_z phase plane lasts >2 Gyr but a subsequent disc crossing wipes out the coherent structure. We find that the phase spiral was excited < 0.5 Gyr ago by an object like Sgr with total mass 3 x 10^10 Msun (stripped down from 5 x 10^10 Msun when it first entered the halo) passing through the plane.

Astronomical distance scales

Overview of the determination of astronomical distances from a metrological standpoint. Distances are considered from the Solar System (planetary distances) to extragalactic distances, with a special emphasis on the fundamental step of the trigonometric stellar distances and the giant leap recently experienced in this field thanks to the ESA space astrometry missions Hipparcos and Gaia.

The population of hot subdwarf stars studied with Gaia

Based on data from the ESA Gaia Data Release 2 (DR2) and several ground-based, multi-band photometry surveys we have compiled an all-sky catalogue of 39 800 hot subluminous star candidates selected in Gaia DR2 by means of colour, absolute magnitude, and reduced proper motion cuts. We expect the majority of the candidates to be hot subdwarf stars of spectral type B and O, followed by blue horizontal branch stars of late B-type (HBB), hot post-AGB stars, and central stars of planetary nebulae. The contamination by cooler stars should be about 10%. The catalogue is magnitude limited to Gaia G &lt; 19 mag and covers the whole sky. Except within the Galactic plane and LMC/SMC regions, we expect the catalogue to be almost complete up to about 1.5 kpc. The main purpose of this catalogue is to serve as input target list for the large-scale photometric and spectroscopic surveys which are ongoing or scheduled to start in the coming years. In the long run, securing a statistically significant sample of spectroscopically confirmed hot subluminous stars is key to advance towards a more detailed understanding of the latest stages of stellar evolution for single and binary stars.

Transient spiral structure and the disc velocity substructure in Gaia DR2

The second data release from ESA's Gaia mission has revealed many ridge-like structures in the velocity distribution of the Milky Way. We show that these can arise naturally from winding transient spiral structure that is commonly seen in N-body simulations of disk galaxies. We construct test particle models of the winding spiral structure, and compare the resulting distribution of orbits with the observed two-dimensional velocity distribution in the extended solar neighbourhood and with the distribution of rotational velocities over 8 kpc along the Sun--Galactic-centre--Galactic anti-centre line. We show that the ridges in these observations are well reproduced by the winding spiral model. Additionally, we demonstrate that the transient winding spiral potential can create a Hercules-like feature in the kinematics of the solar neighbourhood, either alone, or in combination with a long-slow bar potential.

The Open Cluster Chemical Abundances and Mapping Survey. II. Precision Cluster Abundances for APOGEE Using SDSS DR14

Abstract The Open Cluster Chemical Abundances and Mapping (OCCAM) survey aims to produce a comprehensive, uniform, infrared-based spectroscopic data set for hundreds of open clusters, and to constrain key Galactic dynamical and chemical parameters from this sample. This second contribution from the OCCAM survey presents analysis of 259 member stars with [Fe/H] determinations in 19 open clusters, using Sloan Digital Sky Survey Data Release 14 (SDSS/DR14) data from the Apache Point Observatory Galactic Evolution Experiment and ESA Gaia. This analysis, which includes clusters with R GC ranging from 7 to 13 kpc, measures an [Fe/H] gradient of −0.061 ± 0.004 dex kpc−1. We also confirm evidence of a significant positive gradient in the α-elements ([O/Fe], [Mg/Fe], and [Si/Fe]) and present evidence for a significant negative gradient in iron-peak elements ([Mn/Fe] and [Ni/Fe]).

Gaia Sky: Navigating the Gaia Catalog.

In this paper, we present Gaia Sky, a free and open-source multiplatform 3D Universe system, developed since 2014 in the Data Processing and Analysis Consortium framework of ESA's Gaia mission. Gaia's data release 2 represents the largest catalog of the stars of our Galaxy, comprising 1.3 billion star positions, with parallaxes, proper motions, magnitudes, and colors. In this mission, Gaia Sky is the central tool for off-the-shelf visualization of these data, and for aiding production of outreach material. With its capabilities to effectively handle these data, to enable seamless navigation along the high dynamic range of distances, and at the same time to provide advanced visualization techniques including relativistic aberration and gravitational wave effects, currently no actively maintained cross-platform, modern, and open alternative exists.

High-precision Orbit Fitting and Uncertainty Analysis of (486958) 2014 MU69

Abstract NASA’s New Horizons spacecraft will conduct a close flyby of the cold-classical Kuiper Belt Object (KBO) designated (486958) 2014 MU69 on 2019 January 1. At a heliocentric distance of 44 au, “MU69” will be the most distant object ever visited by a spacecraft. To enable this flyby, we have developed an extremely high-precision orbit fitting and uncertainty processing pipeline, making maximal use of the Hubble Space Telescope’s Wide Field Camera 3 (WFC3) and pre-release versions of the ESA Gaia Data Release 2 (DR2) catalog. This pipeline also enabled successful predictions of a stellar occultation by MU69 in 2017 July. We describe how we process the WFC3 images to match the Gaia DR2 catalog, extract positional uncertainties for this extremely faint target (typically 140 photons per WFC3 exposure), and translate those uncertainties into probability distribution functions for MU69 at any given time. We also describe how we use these uncertainties to guide New Horizons, plan stellar occultions of MU69, and derive MU69's orbital evolution and long-term stability.

Application of time transfer functions toGaia’s global astrometry

A key objective of the ESA Gaia satellite is the realization of a quasi-inertial reference frame at visual wavelengths by means of global astrometric techniques. This requires an accurate mathematical and numerical modeling of relativistic light propagation, as well as double-blind-like procedures for the internal validation of the results, before they are released to the scientific community at large. Aim of this work is to specialize the Time Transfer Functions (TTF) formalism to the case of the Gaia observer and prove its applicability to the task of Global Sphere Reconstruction (GSR), in anticipation of its inclusion in the GSR system, already featuring the suite of RAMOD models, as an additional semi-external validation of the forthcoming Gaia baseline astrometric solutions. We extend the current GSR framework and software infrastructure (GSR2) to include TTF relativistic observation equations compatible with Gaia's operations. We use simulated data generated by the Gaia Data Reduction and Analysis Consortium (DPAC) to obtain different least-squares estimations of the full stellar spheres and gauge results. These are compared to analogous solutions obtained with the current RAMOD model in GSR2 and to the catalog generated with GREM, the model baselined for Gaia and used to generate the DPAC synthetic data. Linearized least-squares TTF solutions are based on spheres of about 132,000 primary stars uniformly distributed on the sky and simulated observations spanning the entire 5-yr range of Gaia's nominal operational lifetime. The statistical properties of the results compare well with those of GREM. Finally, comparisons to RAMOD@GSR2 solutions confirmed the known lower accuracy of that model and allowed us to establish firm limits on the quality of the linearization point outside of which an iteration for non-linearity is required for its proper convergence.