ESA Science & Technology - Publication Archive

The 3D velocities of M31 and M33 are important for understanding the evolution and cosmological context of the Local Group. Their most massive stars are detected by Gaia, and we use Data Release 2 (DR2) to determine the galaxy proper motions (PMs). We select galaxy members based on, e.g., parallax, PM, color–magnitude diagram location, and local stellar density. The PM rotation of both galaxies is confidently detected, consistent with the known line-of-sight rotation curves: V_rot = -206 ± 86 km s^-1 (counterclockwise) for M31, and V_rot = 80 ± 52 km s^-1 (clockwise) for M33. We measure the center-of-mass PM of each galaxy relative to surrounding background quasars in DR2. This yields that (μ_α*,μ_δ) equals (65 ± 18, -57 ± 15) μas yr^-1 for M31 and (31 ± 19, -29 ± 16) μas yr^-1 for M33. In addition to the listed random errors, each component has an additional residual systematic error of 16 μas yr^-1. These results are consistent at 0.8σ and 1.0σ with the (2 and 3 times higher accuracy) measurements already available from Hubble Space Telescope (HST) optical imaging and Very Long Baseline Array water maser observations, respectively. This lends confidence that all these measurements are robust. The new results imply that the M31 orbit toward the Milky Way (MW) is somewhat less radial than previously inferred, V_tan,DR2+HST = 57⁺³⁵_-31 km s^-1, and strengthen arguments that M33 may be on its first infall into M31. The results highlight the future potential of Gaia for PM studies beyond the MW satellite system.

White dwarfs are stellar embers depleted of nuclear energy sources that cool over billions of years. These stars, which are supported by electron degeneracy pressure, reach densities of 10⁷ grams per cubic centimetre in their cores. It has been predicted that a first-order phase transition occurs during white-dwarf cooling, leading to the crystallization of the non-degenerate carbon and oxygen ions in the core, which releases a considerable amount of latent heat and delays the cooling process by about one billion years. However, no direct observational evidence of this effect has been reported so far. Here we report the presence of a pile-up in the cooling sequence of evolving white dwarfs within 100 parsecs of the Sun, determined using photometry and parallax data from the Gaia satellite. Using modelling, we infer that this pile-up arises from the release of latent heat as the cores of the white dwarfs crystallize. In addition to the release of latent heat, we find strong evidence that cooling is further slowed by the liberation of gravitational energy from element sedimentation in the crystallizing cores. Our results describe the energy released by crystallization in strongly coupled Coulomb plasmas, and the measured cooling delays could help to improve the accuracy of methods used to determine the age of stellar populations from white dwarfs.

The assembly of our Galaxy can be reconstructed using the motions and chemistry of individual stars. Chemo-dynamical studies of the stellar halo near the Sun have indicated the presence of multiple components, such as streams and clumps, as well as correlations between the stars' chemical abundances and orbital parameters. Recently, analyses of two large stellar surveys revealed the presence of a well populated elemental abundance sequence, two distinct sequences in the colour–magnitude diagram and a prominent, slightly retrograde kinematic structure in the halo near the Sun, which may trace an important accretion event experienced by the Galaxy. However, the link between these observations and their implications for Galactic history is not well understood. Here we report an analysis of the kinematics, chemistry, age and spatial distribution of stars that are mainly linked to two major Galactic components: the thick disk and the stellar halo. We demonstrate that the inner halo is dominated by debris from an object that at infall was slightly more massive than the Small Magellanic Cloud, and which we refer to as Gaia–Enceladus. The stars that originate in Gaia–Enceladus cover nearly the full sky, and their motions reveal the presence of streams and slightly retrograde and elongated trajectories. With an estimated mass ratio of four to one, the merger of the Milky Way with Gaia–Enceladus must have led to the dynamical heating of the precursor of the Galactic thick disk, thus contributing to the formation of this component approximately ten billion years ago. These findings are in line with the results of galaxy formation simulations, which predict that the inner stellar halo should be dominated by debris from only a few massive progenitors.

We search for the fastest stars in the subset of stars with radial velocity measurements of the second data release (DR2) of the European Space Agency mission Gaia. Starting from the observed positions, parallaxes, proper motions, and radial velocities, we construct the distance and total velocity distribution of more than 7 million stars in our Milky Way, deriving the full 6D phase space information in Galactocentric coordinates. These information are shared in a catalogue, publicly available at http://home.strw.leidenuniv.nl/ marchetti/research.html. To search for unbound stars, we then focus on stars with a probability greater than 50% of being unbound from the Milky Way. This cut results in a clean sample of 125 sources with reliable astrometric parameters and radial velocities. Of these, 20 stars have probabilities greater than 80 % of being unbound from the Galaxy. On this latter sub-sample, we perform orbit integration to characterize the stars’ orbital parameter distributions. As expected given the relatively small sample size of bright stars, we find no hypervelocity star candidates, stars that are moving on orbits consistent with coming from the Galactic Centre. Instead, we find 7 hyper-runaway star candidates, coming from the Galactic disk. Surprisingly, the remaining 13 unbound stars cannot be traced back to the Galaxy, including two of the fastest stars (around 700 km s⁻¹). If conformed, these may constitute the tip of the iceberg of a large extragalactic population or the extreme velocity tail of stellar streams.

The first detected interstellar object 'Oumuamua that passed within 0.25au of the Sun on 2017 September 9 was presumably ejected from a stellar system. We use its newly determined non-Keplerian trajectory together with the reconstructed Galactic orbits of 7 million stars from Gaia DR2 to identify past close encounters. Such an "encounter" could reveal the home system from which 'Oumuamua was ejected. The closest encounter, at 0.60pc (0.53-0.67pc, 90% confidence interval), was with the M2.5 dwarf HIP 3757 at a relative velocity of 24.7km/s, 1Myr ago. A more distant encounter (1.6pc) but with a lower encounter (ejection) velocity of 10.7km/s was with the G5 dwarf HD 292249, 3.8Myr ago. Two more stars have encounter distances and velocities intermediate to these. The encounter parameters are similar across six different non-gravitational trajectories for 'Oumuamua. Ejection of 'Oumuamua by scattering from a giant planet in one of the systems is plausible, but requires a rather unlikely configuration to achieve the high velocities found. A binary star system is more likely to produce the observed velocities. None of the four home candidates have published exoplanets or are known to be binaries. Given that the 7 million stars in Gaia DR2 with 6D phase space information is just a small fraction of all stars for which we can eventually reconstruct orbits, it is a priori unlikely that our current search would find 'Oumuamua's home star system. As 'Oumuamua is expected to pass within 1pc of about 20 stars and brown dwarfs every Myr, the plausibility of a home system depends also on an appropriate (low) encounter velocity.

The evolution of the Milky Way disk, which contains most of the stars in the Galaxy, is affected by several phenomena. For example, the bar and the spiral arms of the Milky Way induce radial migration of stars and can trap or scatter stars close to orbital resonances. External perturbations from satellite galaxies can also have a role, causing dynamical heating of the Galaxy, ring-like structures in the disk and correlations between different components of the stellar velocity. These perturbations can also cause 'phase wrapping' signatures in the disk, such as arched velocity structures in the motions of stars in the Galactic plane. Some manifestations of these dynamical processes have already been detected, including kinematic substructure in samples of nearby stars, density asymmetries and velocities across the Galactic disk that differ from the axisymmetric and equilibrium expectations, especially in the vertical direction, and signatures of incomplete phase mixing in the disk. Here we report an analysis of the motions of six million stars in the Milky Way disk. We show that the phase-space distribution contains different substructures with various morphologies, such as snail shells and ridges, when spatial and velocity coordinates are combined. We infer that the disk must have been perturbed between 300 million and 900 million years ago, consistent with estimates of the previous pericentric passage of the Sagittarius dwarf galaxy. Our findings show that the Galactic disk is dynamically young and that modelling it as time-independent and axisymmetric is incorrect.

The young massive Jupiters discovered with high-contrast imaging provide a unique opportunity to study the formation and early evolution of gas giant planets. A key question is to what extent gravitational energy from accreted gas contributes to the internal energy of a newly formed planet. This has led to a range of formation scenarios from ‘cold’ to ‘hot’ start models. For a planet of a given mass, these initial conditions govern its subsequent evolution in luminosity and radius. Except for upper limits from radial velocity studies, disk modelling and dynamical instability arguments, no mass measurements of young planets are yet available to distinguish between these different models. Here, we report on the detection of the astrometric motion of Beta Pictoris, the ~21-Myr-old host star of an archetypical directly imaged gas giant planet, around the system’s centre of mass. Subtracting the highly accurate Hipparcos and Gaia proper motion from the internal 3 yr Hipparcos astrometric data reveals the reflex motion of the star, giving a model-independent planet mass of 11 ± 2 Jupiter masses. This is consistent with scenarios in which the planet is formed in a high-entropy state as assumed by hot start models. The ongoing data collection by Gaia will soon lead to mass measurements of other young gas giants and form a great asset to further constrain early-evolution scenarios.

This media kit contains background information about the Gaia mission. It has been prepared to accompany Gaia Data Release 2.

Contents:
Gaia – the billion star surveyor
Fast facts
Mapping the Galaxy with Gaia
Gaia's second data release – the Galactic census takes shape
Caveats and future releases
Science with Gaia's new data
Science highlights from Gaia's first data release
Making sense of it all – the role of the Gaia Data Processing and Analysis Consortium
Where is Gaia and why do we need to know?
From ancient star maps to precision astrometry
Appendices: Resources, Information about the press event, Media contacts

File updated 24 April 2018, to correct typo.

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Context. Parallaxes for 331 classical Cepheids, 31 Type II Cepheids, and 364 RR Lyrae stars in common between Gaia and the Hipparcos and Tycho-2 catalogues are published in Gaia Data Release 1 (DR1) as part of the Tycho-Gaia Astrometric Solution (TGAS). Aims: In order to test these first parallax measurements of the primary standard candles of the cosmological distance ladder, which involve astrometry collected by Gaia during the initial 14 months of science operation, we compared them with literature estimates and derived new period-luminosity (PL), period-Wesenheit (PW) relations for classical and Type II Cepheids and infrared PL, PL-metallicity (PLZ), and optical luminosity-metallicity (MV-[Fe/H]) relations for the RR Lyrae stars, with zero points based on TGAS. [Remainder of abstract truncated due to character limitations]

We use the Gaia data release 1 (DR1) to study the proper motion (PM) fields of the Large and Small Magellanic Clouds (LMC, SMC). This uses the Tycho-Gaia Astrometric Solution (TGAS) PMs for 29 Hipparcos stars in the LMC and 8 in the SMC. The LMC PM in the West and North directions is inferred to be ({μ }W,{μ }N) =(-1.872+/- 0.045,0.224+/- 0.054) {mas} {{yr}}-1 , and the SMC PM ({μ }W,{μ }N)=(-0.874+/- 0.066,-1.229 +/- 0.047) {mas} {{yr}}-1 . These results have similar accuracy and agree to within the uncertainties with existing Hubble Space Telescope (HST) PM measurements. Since TGAS uses different methods with different systematics, this provides an external validation of both data sets and their underlying approaches. Residual DR1 systematics may affect the TGAS results, but the HST agreement implies this must be below the random errors. Also in agreement with prior HST studies, the TGAS LMC PM field clearly shows the clockwise rotation of the disk, even though it takes the LMC disk in excess of 108 years to complete one revolution. The implied rotation curve amplitude for young LMC stars is consistent with that inferred from line of sight (LOS) velocity measurements. Comparison of the PM and LOS rotation curves implies a kinematic LMC distance modulus m-M=18.54+/- 0.39, consistent but not yet competitive with photometric methods. These first results from Gaia on the topic of Local Group dynamics provide an indication of how its future data releases will revolutionize this field.

Context. The first Gaia data release (DR1) delivered a catalogue of astrometry and photometry for over a billion astronomical sources. Within the panoply of methods used for data exploration, visualisation is often the starting point and even the guiding reference for scientific thought. However, this is a volume of data that cannot be efficiently explored using traditional tools, techniques, and habits. Aims. We aim to provide a global visual exploration service for the Gaia archive, something that is not possible out of the box for most people. The service has two main goals. The first is to provide a software platform for interactive visual exploration of the archive contents, using common personal computers and mobile devices available to most users. The second aim is to produce intelligible and appealing visual representations of the enormous information content of the archive. Methods. The interactive exploration service follows a client-server design. The server runs close to the data, at the archive, and is responsible for hiding as far as possible the complexity and volume of the Gaia data from the client. This is achieved by serving visual detail on demand. Levels of detail are pre-computed using data aggregation and subsampling techniques. For DR1, the client is a web application that provides an interactive multi-panel visualisation workspace as well as a graphical user interface. [Remainder of abstract truncated due to character limitations]

The three-dimensional motions of stars in small galaxies beyond our own are minute, yet they are crucial for understanding the nature of gravity and dark matter. Even for the dwarf galaxy Sculptor–one of the best-studied systems, which is inferred to be strongly dark matter dominated–there are conflicting reports on its mean motion around the Milky Way, and the three-dimensional internal motions of its stars have never been measured. Here, we present precise proper motions of Sculptor's stars based on data from the Gaia mission and Hubble Space Telescope. Our measurements show that Sculptor moves around the Milky Way on a high-inclination elongated orbit that takes it much further out than previously thought. For Sculptor's internal velocity dispersions, we find σ_R = 11.5 ± 4.3 km s⁻¹ and σ_T = 8.5 ± 3.2 km s^-1 along the projected radial and tangential directions. Thus, the stars in our sample move preferentially on radial orbits as quantified by the anisotropy parameter, which we find to be β~0.86^+0.12_-0.83 at a location beyond the core radius. Taken at face value, this high radial anisotropy requires abandoning conventional models for Sculptor's mass distribution. Our sample is dominated by metal-rich stars and for these we find β^MR~0.95^+0.04_-0.27–a value consistent with multi-component spherical models where Sculptor is embedded in a cuspy dark halo, as might be expected for cold dark matter.

We present UBVRI and CT1T2 photometry for 15 catalogued open clusters of relative high brightness and compact appearance. From these unprecedented photometric data sets, covering wavelengths from the blue up to the near-infrared, we performed a thorough assessment of their reality as stellar aggregates. We statistically assigned to each observed star within the object region a probability of being a fiducial feature of that field in terms of its local luminosity function, colour distribution and stellar density. Likewise, we used accurate parallaxes and proper motions measured by the Gaia satellite to help our decision on the open cluster reality. 10 catalogued aggregates did not show any hint of being real physical systems; three of them had been assumed to be open clusters in previous studies, though. On the other hand, we estimated reliable fundamental parameters for the remaining five studied objects, which were confirmed as real open clusters. They resulted to be clusters distributed in a wide age range, 8.0 ≤ log (t yr⁻1) ≤ 9.4, of solar metal content and placed between 2.0 and 5.5 kpc from the Sun. Their ages and metallicities are in agreement with the presently known picture of the spatial distribution of open clusters in the Galactic disc.

We observed six He-clump stars of the intermediate-age stellar cluster Gaia1 with the MIKE/Magellan spectrograph. A possible extra-galactic origin of this cluster, recently discovered thanks to the first data release of the ESA Gaia mission, has been suggested, based on its orbital parameters. Abundances for Fe, α, proton- and neutron-capture elements have been obtained. We find no evidence of intrinsic abundance spreads. The iron abundance is solar ([FeI/H] = + 0.00 ± 0.01; σ = 0.03 dex). All the other abundance ratios are generally solar-scaled, similar to the Galactic thin disk and open cluster stars of similar metallicity. The chemical composition of Gaia1 does not support an extra-galactic origin for this stellar cluster, which can be considered as a standard Galactic open cluster.

We confirm the reality of the recently discovered Milky Way stellar cluster Gaia 1 using spectra acquired with the HERMES and AAOmega spectrographs of the Anglo-Australian Telescope. This cluster had been previously undiscovered due to its close angular proximity to Sirius, the brightest star in the sky at visual wavelengths. Our observations identified 41 cluster members, and yielded an overall metallicity of [Fe/H]=−0.13±0.13 and barycentric radial velocity of v_r = 58.30 ± 0.22 km s⁻¹. These kinematics provide a dynamical mass estimate of 12.9^+4.6_−3.9×10³M⊙ . Isochrone fits to Gaia, 2MASS, and Pan-STARRS1 photometry indicate that Gaia 1 is an intermediate age (∼3 Gyr) stellar cluster. Combining the spatial and kinematic data we calculate Gaia 1 has a circular orbit with a radius of about 12 kpc, but with a large out of plane motion: z_max=1.1^+0.4_−0.3  kpc. Clusters with such orbits are unlikely to survive long due to the number of plane passages they would experience.

We present the results of the very first search for faint Milky Way satellites in the Gaia data. Using stellar positions only, we are able to re-discover objects detected in much deeper data as recently as the last couple of years. While we do not identify new prominent ultrafaint dwarf galaxies, we report the discovery of two new star clusters, Gaia 1 and Gaia 2. Gaia 1 is particularly curious, as it is a massive (2.2 × 104 M⊙), large (∼9 pc) and nearby (4.6 kpc) cluster, situated 10 arcmin away from the brightest star on the sky, Sirius! Even though this satellite is detected at significance in excess of 10, it was missed by previous sky surveys. We conclude that Gaia possesses powerful and unique capabilities for satellite detection, thanks to its unrivalled angular resolution and highly efficient object classification.

Forthcoming article

We confirm the reality of the recently discovered Milky Way stellar cluster Gaia 1 using spectra acquired with the HERMES and AAOmega spectrographs of the Anglo-Australian Telescope. This cluster had been previously undiscovered due to its close angular proximity to Sirius, the brightest star in the sky at visual wavelengths. Our observations identified 41 cluster members, and yielded an overall metallicity of [Fe/H]=−0.13±0.13 and barycentric radial velocity of v_r = 58.30 ± 0.22 km s^-1. These kinematics provide a dynamical mass estimate of 12.9^+4.6_-3.9×10³ M_⊙. Isochrone fits to Gaia, 2MASS, and Pan-STARRS1 photometry indicate that Gaia 1 is an intermediate age (~3 Gyr) stellar cluster. Combining the spatial and kinematic data we calculate Gaia 1 has a circular orbit with a radius of about 12 kpc, but with a large out of plane motion: z_max=1.1^+0.4_-0.3 kpc. Clusters with such orbits are unlikely to survive long due to the number of plane passages they would experience.

We observed six He-clump stars of the intermediate-age stellar cluster Gaia1 with the MIKE/Magellan spectrograph. A possible extra-galactic origin of this cluster, recently discovered thanks to the first data release of the ESA Gaia mission, has been suggested, based on its orbital parameters. Abundances for Fe, α, proton- and neutron-capture elements have been obtained. We find no evidence of intrinsic abundance spreads. The iron abundance is solar ([FeI/H] = + 0.00 ± 0.01; σ = 0.03 dex). All the other abundance ratios are generally solar-scaled, similar to the Galactic thin disk and open cluster stars of similar metallicity. The chemical composition of Gaia1 does not support an extra-galactic origin for this stellar cluster, which can be considered as a standard Galactic open cluster.

The paucity of hypervelocity stars (HVSs) known to date has severely hampered their potential to investigate the stellar population of the Galactic Centre and the Galactic Potential. The first Gaia data release (DR1, 2016 September 14) gives an opportunity to increase the current sample. The challenge is the disparity between the expected number of hypervelocity stars and that of bound background stars. We have applied a novel data mining algorithm based on machine learning techniques, an artificial neural network, to the Tycho-Gaia astrometric solution (TGAS) catalogue. With no pre-selection of data, we could exclude immediately ∼99% of the stars in the catalogue and find 80 candidates with more than 90% predicted probability to be HVSs, based only on their position, proper motions, and parallax. We have cross-checked our findings with other spectroscopic surveys, determining radial velocities for 30 and spectroscopic distances for 5 candidates. In addition, follow-up observations have been carried out at the Isaac Newton Telescope for 22 stars, for which we obtained radial velocities and distance estimates. We discover 14 stars with a total velocity in the Galactic rest frame >400 km s^-1, and 5 of these have a probability >50% of being unbound from the Milky Way. Tracing back their orbits in different Galactic potential models we find one possible unbound HVS with v ∼ 520 km s^-1, 5 bound HVSs, and, notably, 5 runaway stars with median velocity between 400 and 780 km s^-1. At the moment, uncertainties in the distance estimates and ages are too large to confirm the nature of our candidates by narrowing down their ejection location, and we wait for future Gaia releases to validate the quality of our sample. This test successfully demonstrates the feasibility of our new data mining routine.