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Title: Systematic Error of the Gaia DR1 TGAS Parallaxes from Data for the Red Giant Clump
Author: George Gontcharov

Based on the Gaia DR1 TGAS parallaxes and photometry from the Tycho-2, Gaia, 2MASS, and WISE catalogues, we have produced a sample of about 100 000 clump red giants within about 800 pc of the Sun. The systematic variations of the mode of their absolute magnitude as a function of the distance, magnitude, and other parameters have been analysed. We show that these variations reach 0.7 mag and cannot be explained by variations in the interstellar extinction or intrinsic properties of stars and by selection. The only explanation seems to be a systematic error of the Gaia DR1 TGAS parallax dependent on the square of the observed distance in kpc: 0.18R² mas. Allowance for this error reduces significantly the systematic dependences of the absolute magnitude mode on all parameters. This error reaches 0.1 mas within 800 pc of the Sun and allows an upper limit for the accuracy of the TGAS parallaxes to be estimated as 0.2 mas. A careful allowance for such errors is needed to use clump red giants as "standard candles". This eliminates all discrepancies between the theoretical and empirical estimates of the characteristics of these stars and allows us to obtain the first estimates of the modes of their absolute magnitudes from the Gaia parallaxes:
mode(MH)=-1.49m±0.04m,
mode(MKs)=-1.63m±0.03m,
mode(MW1)=-1.67m±0.05m,
mode(MW2)=-1.67m±0.05m,
mode(MW3)=-1.66m±0.02m,
mode(MW4)=-1.73m±0.03m, as well as the corresponding estimates of their de-reddened colours.

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Two million stars on the move

The changing face of our Galaxy is revealed in a new video from ESA's Gaia mission. The motion of two million stars is traced 5 million years into the future using data from the Tycho-Gaia Astrometric Solution, one of the products of the first Gaia data release. This provides a preview of the stellar motions that will be revealed in Gaia's future data releases, which will enable scientists to investigate the formation history of our Galaxy.
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Gaia Data Release 1
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Title: Gaia Data Release 1: The variability processing & analysis and its application to the south ecliptic pole region
Author: L. Eyer, N. Mowlavi, D.W. Evans, K. Nienartowicz, D. Ordonez, B. Holl, I. Lecoeur-Taibi, M. Riello, G. Clementini, J. Cuypers, J. De Ridder, A.C. Lanzafame, L.M. Sarro, J. Charnas, L.P. Guy, G. Jevardat de Fombelle, L. Rimoldini, M. Süveges, F. Mignard, G. Busso, F. De Angeli, F. van Leeuwen, P. Dubath, M. Beck, J.J. Aguado, J. Debosscher, E. Distefano, J. Fuchs, P. Koubsky, T. Lebzelter, S. Leccia, M. Lopez, A. Moitinho, S. Regibo, V. Ripepi, M. Roelens, L. Szabados, B. Tingley, V. Votruba, S. Zucker, C. Aerts, F. Barblan, S. Blanco-Cuaresma, M. Grenon, A. Jan, D. Lorenz, B. Miranda, S. Morgenthaler, C. Ordenovic, L. Palaversa, A. Prsa, M.I. Ruiz-Fuertes, R.I. Anderson, H.E. Delgado, Y. Dzigan, R. Hudec, A. Jonckheere, P. Klagyivik, A. Kutka, M. Moniez, J.-M. Nicoletti, P. Park, E. Van Hemelryck, et al. (10 additional authors not shown)

The ESA Gaia mission provides a unique time-domain survey for more than one billion sources brighter than G=20.7 mag. Gaia offers the unprecedented opportunity to study variability phenomena in the Universe thanks to multi-epoch G-magnitude photometry in addition to astrometry, blue and red spectro-photometry, and spectroscopy. Within the Gaia Consortium, Coordination Unit 7 has the responsibility to detect variable objects, classify them, derive characteristic parameters for specific variability classes, and provide global descriptions of variable phenomena.
We describe the variability processing and analysis that we plan to apply to the successive data releases, and we present its application to the G-band photometry results of the first 14 months of Gaia operations that comprises 28 days of Ecliptic Pole Scanning Law and 13 months of Nominal Scanning Law.
Out of the 694 million, all-sky, sources that have calibrated G-band photometry in this first stage of the mission, about 2.3 million sources that have at least 20 observations are located within 38 degrees from the South Ecliptic Pole. We detect about 14% of them as variable candidates, among which the automated classification identified 9347 Cepheid and RR Lyrae candidates. Additional visual inspections and selection criteria led to the publication of 3194 Cepheid and RR Lyrae stars, described in Clementini et al. (2016). Under the restrictive conditions for DR1, the completenesses of Cepheids and RR Lyrae stars are estimated at 67% and 58%, respectively, numbers that will significantly increase with subsequent Gaia data releases.
Data processing within the Gaia Consortium is iterative, the quality of the data and the results being improved at each iteration. The results presented in this article show a glimpse of the exceptional harvest that is to be expected from the Gaia mission for variability phenomena.

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Gaia-DR1
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Title: Hot Stuff for One Year (HSOY) - A 580 million star proper motion catalogue derived from Gaia DR1 and PPMXL
Author: Martin Altmann, Siegfried Roeser, Markus Demleitner, Ulrich Bastian, Elena Schilbach

Recently, the first installment of data from ESA's Gaia astrometric satellite mission (Gaia-DR1) was released, containing positions of more than 1 billion stars with unprecedented precision, as well as only proper motions and parallaxes, however only for a subset of 2 million objects. The second release, due in late 2017 or early 2018, will include those quantities for most objects. In order to provide a dataset that bridges the time gap between the Gaia-DR1 and Gaia-DR2 releases and partly remedies the lack of proper motions in the former, HSOY ("Hot Stuff for One Year") was created as a hybrid catalogue between Gaia-DR1 and ground-based astrometry, featuring proper motions (but no parallaxes) for a large fraction of the DR1 objects. While not attempting to compete with future Gaia releases in terms of data quality or number of objects, the aim of HSOY is to provide improved proper motions partly based on Gaia data, allowing some studies to be carried out just now or as pilot studies for later larger projects requiring higher-precision data. The HSOY catalogue was compiled using the positions taken from Gaia-DR1 combined with the input data from the PPMXL catalogue, employing the same weighted least-squares technique that was used to assemble the PPMXL catalogue itself. Results. This effort resulted in a four-parameter astrometric catalogue containing 583,000,000 objects, with Gaia-DR1 quality positions and proper motions with precisions from significantly less than 1 mas/yr to 5 mas/yr, depending on the object's brightness and location on the sky.

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Virtual Milky Way

ESA's Gaia is surveying stars in our Galaxy and local galactic neighbourhood in order to build the most precise 3D map of the Milky Way and answer questions about its structure, origin and evolution.
Launched in 2013, Gaia has already generated its first catalogue of more than a billion stars - the largest all-sky survey of celestial objects to date.

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Title: Gaia Data Release 1: Astrometry - one billion positions, two million proper motions and parallaxes
Author: L. Lindegren, U. Lammers, U. Bastian, J. Hernández, S. Klioner, D. Hobbs, A. Bombrun, D. Michalik, M. Ramos-Lerate, A. Butkevich, G. Comoretto, E. Joliet, B. Holl, A. Hutton, P. Parsons, H. Steidelmüller, U. Abbas, M. Altmann, A. Andrei, S. Anton, N. Bach, C. Barache, U. Becciani, J. Berthier, L. Bianchi, M. Biermann, S. Bouquillon, G. Bourda, T. Brüsemeister, B. Bucciarelli, D. Busonero, T. Carlucci, J. Castañeda, P. Charlot, M. Clotet, M. Crosta, M. Davidson, F. de Felice, R. Drimmel, C. Fabricius, A. Fienga, F. Figueras, E. Fraile, M. Gai, N. Garralda, R. Geyer, J.J. González-Vidal, R. Guerra, N.C. Hambly, M. Hauser, S. Jordan, M.G. Lattanzi, H. Lenhardt, S. Liao, W. Löffler, P.J. McMillan, F. Mignard, A. Mora, R. Morbidelli, J. Portell, A. Riva, M. Sarasso, et al. (21 additional authors not shown)

Gaia Data Release 1 (Gaia DR1) contains astrometric results for more than 1 billion stars brighter than magnitude 20.7 based on observations collected by the Gaia satellite during the first 14 months of its operational phase. We give a brief overview of the astrometric content of the data release and of the model assumptions, data processing, and validation of the results. For stars in common with the Hipparcos and Tycho-2 catalogues, complete astrometric single-star solutions are obtained by incorporating positional information from the earlier catalogues. For other stars only their positions are obtained by neglecting their proper motions and parallaxes. The results are validated by an analysis of the residuals, through special validation runs, and by comparison with external data. Results. For about two million of the brighter stars (down to magnitude ~11.5) we obtain positions, parallaxes, and proper motions to Hipparcos-type precision or better. For these stars, systematic errors depending e.g. on position and colour are at a level of 0.3 milliarcsecond (mas). For the remaining stars we obtain positions at epoch J2015.0 accurate to ~10 mas. Positions and proper motions are given in a reference frame that is aligned with the International Celestial Reference Frame (ICRF) to better than 0.1 mas at epoch J2015.0, and non-rotating with respect to ICRF to within 0.03 mas/yr. The Hipparcos reference frame is found to rotate with respect to the Gaia DR1 frame at a rate of 0.24 mas/yr. Based on less than a quarter of the nominal mission length and on very provisional and incomplete calibrations, the quality and completeness of the astrometric data in Gaia DR1 are far from what is expected for the final mission products. The results nevertheless represent a huge improvement in the available fundamental stellar data and practical definition of the optical reference frame.

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From the Solar System to the Hyades cluster

Spoiler

A virtual journey, from our Solar System through the Milky Way, based on data from the first release of ESA's Gaia satellite.
The journey starts by looking back at the Sun, surrounded by its eight planets. We then move away from the Sun and travel towards and around the Hyades star cluster, the closest open cluster to the Solar System, some 150 light-years away.



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Gaia sizes up 1.1 billion stars

Gaia, a European Space Agency satellite designed to unlock the secrets of the birth and evolution of the Milky Way, has released its first wave of data on the positions and brightness for about one billion stars.
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Gaia space telescope plots a billion stars

The most precise map of the night sky ever assembled is taking shape.
Astronomers working on the Gaia space telescope are releasing a first tranche of data recording the position and brightness of over a billion stars.
And for some two million of these objects, their distance and sideways motion across the heavens has also been accurately plotted.

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Gaia's billion-star map hints at treasures to come

The first catalogue of more than a billion stars from ESA's Gaia satellite was published today - the largest all-sky survey of celestial objects to date.
On its way to assembling the most detailed 3D map ever made of our Milky Way galaxy, Gaia has pinned down the precise position on the sky and the brightness of 1142 million stars.

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