Gaia Data Release 2: Focus on astrophysics
Combined Hertzprung-Russell diagrams of star clusters. Hertzprung-Russell diagram displays the physical properties of stars, and their dependence on the star's mass, age and chemical composition. The intrinsic brightness is plotted vertically, brightest at the top, derived from Gaia's distance and brightness measures. Gaia's colour is plotted horizontally, bluest at the left. Star clusters are dynamically bound groups of hundreds to many thousands of stars sharing the same formation history, and thereby being of the same age and chemical composition. Covering stars with a wide range of masses, they provide invaluable information on stellar evolution through their Hertzsprung-Russell diagram. But for that to be extracted, distances need to be accurately known, and high-accuracy photometric data is needed. Gaia has provided both for a sample of 46 young galactic (or open) clusters, and 75 old globular clusters. The first image (left) shows the combined HRD for 32 open clusters, where the effects of cluster age can clearly be distinguished. Blue is younger, red is older. The second image (right) shows the combined HRD for 14 globular clusters. Here the differences are primarily in chemical composition, and the way this affects stellar evolution. Red contains more heavy elements, blue fewer. (Large versions of images: open clusters HRD, globular clusters HRD.) These images are from one of the science demonstration papers that accompany the second Gaia data release, Gaia Collaboration, Babusiaux, C., van Leeuwen, F., Barstow, M.A. et al. 2018a, A&A (special issue for Gaia DR2). Credit: ESA/Gaia/DPAC/Babusiaux, C., van Leeuwen, F., Barstow, M.A. et al.
On 25 April 2018 astronomers opened one of the last remaining windows on the Universe, publishing the first major 3-dimensional census of over one billion stars in our Milky Way.
With the second Gaia data release by the European Space Agency, the results of the detailed analysis of the first 22 months of mission data are presented. These include over 1.6 billion sources on the sky for which accurate positions and magnitudes are provided, but, most importantly, a sub-sample of 1.3 billion stars for which also the proper motion and parallaxes have been derived at accuracies not seen before for such large volumes of data. Parallaxes provide one of the most crucial elements of information in astronomy: directly measured distances, required to transform observed quantities, such as fluxes and proper motions, into their absolute values.
The Gaia mission was launched in December 2013 for a planned 5 year mission, to do a full sky survey of positional and photometric data for, what was then estimated, a sample of 1 billion stars. The survey was to be supplemented by medium-resolution spectroscopic data, from which radial velocities could be obtained for a smaller sample of a few 100 million stars. The second Gaia data release provides the first exposure of what this mission is capable of, and these early results are already spectacular.
Two of the main science goals of the mission are: 1. to gain a better understanding of our Galaxy, its structure and its formation history, and: 2. to improve significantly on our understanding of stellar evolution and stellar structure. The 2nd Gaia data release is doing exactly that. In galactic structure we now have access to detailed velocity profiles and density distributions within a radius of about 4 kpc around the Sun. Compared to previously available data, this is an increase of more than a factor one thousand in volume covered, allowing for new detection of structures in the density and velocity distributions. These structures will, in future, provide some of the keys to a better understanding of how our Galaxy formed and evolved.
The 2nd Gaia data release has, for the first time, made it possible to do a large-scale survey of accurate motions for globular clusters and dwarf galaxies, which occupy the space outside the galactic disk. The Gaia 2nd data release is also making a highly significant contribution to studies of stellar structure and evolution by combining the parallax information with high-accuracy all-sky photometric measurements. This is most spectacularly shown by the coverage of around 30 000 white dwarfs, showing very well defined relations between temperature and brightness, with various sub-sequences of which the exact nature is still to be determined, but which will undoubtedly lead to better understanding of these enigmatic objects. Measurements on stars in clusters, where we find stars of a wide range of masses sharing the same age and composition, show in great detail the effects of mass, age and composition on the distribution of stars over brightness and temperature. This will further serve as input for theoretical models that aim at predicting these observed distributions. Very important in this respect is that those sequences for stars of the same age and composition have now been observed to be very narrow, showing that, during most of its evolution, the stellar mass, composition and age are in fact highly dominant, determining factors in defining the temperature and brightness of a single star.
The Institute of Astronomy in the University of Cambridge has a major task within the Gaia project, being responsible for the processing of the photometric data. A team of 12 people has been working very hard on ensuring the timely preparation for the 2nd Gaia data release, closely collaborating with similar groups dedicated to other aspects of the same release. The Gaia data releases are all public, there is no proprietary data period. With this release, anyone will be able to extract data and work on those data. There is extensive documentation available for this release, the preparation and presentation of which was led from Cambridge. There will be several papers in a dedicated issue of Astronomy & Astrophysics, to be released on 25 April 2018, and which will include a small number of Science Demonstration papers, showing what can be achieved with the Gaia data by means of actual application examples. Two of those papers, to which significant contributions were made by astronomers at the IoA, are on the Hertzsprung-Russell diagram and on the Globular Clusters and Dwarf Galaxies.
A small team of scientists and developers based in the Wide Field Astronomy Unit of the Institute for Astronomy in the Edinburgh University School of Physics and Astronomy contributes to the Gaia project. The team is responsible for several key calibration subsystems within the on-ground data processing pipelines that have been developed for Gaia in collaboration with hundreds of engineers and scientists spread around Europe. The Edinburgh work includes provision of calibration of the image shapes delivered by the Gaia optics and digital detectors. The team is tasked also with calibration and removal of instrumental background effects. Edinburgh astronomers and developers have played a significant role in the definition of the published catalogue products from the Gaia mission, and the design and implementation of the archive system through which those science-ready data are delivered.