Archive for April 26, 2018

Gaia’s Second Data Release!

Posted in The Universe and Stuff with tags , , , on April 26, 2018 by telescoper

It seems like only yesterday that I was blogging excitedly about the first release of data (DR1) from the European Space Agency’s Gaia Mission. In fact it was way back in 2016! Anyway, yesterday came another glut of Gaia goodness in the form of the second release of data, known to its friends as DR2.

In case you weren’t aware, Gaia is an ambitious space mission to chart a three-dimensional map of our Galaxy, the Milky Way, in the process revealing the composition, formation and evolution of the Galaxy. Gaia will provide unprecedented positional and radial velocity measurements with the accuracy needed to produce a stereoscopic and kinematic census of about one billion stars in our Galaxy and throughout the Local Group. This amounts to about 1 per cent of the Galactic stellar population.

You can find links to all the DR2 science papers here, a guide to how to use the data here, and of course a link to the full Gaia Archive here.

Here’s a (brief!) list of the contents of DR2:

  • The five-parameter astrometric solution – positions on the sky (α, δ), parallaxes, and proper motions – for more than 1.3 billion (109) sources, with a limiting magnitude of G = 21 and a bright limit of G ≈ 3. Parallax uncertainties are in the range of up to 0.04 milliarcsecond for sources at G < 15, around 0.1 mas for sources with G=17 and at the faint end, the uncertainty is of the order of 0.7 mas at G = 20. The corresponding uncertainties in the respective proper motion components are up to 0.06 mas yr-1 (for G < 15 mag), 0.2 mas yr-1 (for G = 17 mag) and 1.2 mas yr-1 (for G = 20 mag). The Gaia DR2 parallaxes and proper motions are based only on Gaia data; they do no longer depend on the Tycho-2 Catalogue.
  • Median radial velocities (i.e. the median value over the epochs) for more than 7.2 million stars with a mean G magnitude between about 4 and 13 and an effective temperature (Teff) in the range of about 3550 to 6900 K. This leads to a full six-parameter solution: positions and motions on the sky with parallaxes and radial velocities, all combined with mean G magnitudes. The overall precision of the radial velocities at the bright end is in the order of 200-300 m s-1 while at the faint end the overall precision is approximately 1.2 km s-1 for a Teff of 4750 K and about 2.5 km s-1 for a Teff of 6500 K.
  • An additional set of more than 361 million sources for which a two-parameter solution is available: the positions on the sky (α, δ) combined with the mean G magnitude. These sources have a positional uncertainty at G=20 of about 2 mas, at J2015.5.
    G magnitudes for more than 1.69 billion sources, with precisions varying from around 1 milli-mag at the bright (G<13) end to around 20 milli-mag at G=20. Please be aware that the photometric system for the G band in Gaia DR2 is different from the photometric system as used in Gaia DR1.
  • GBP and GRP magnitudes for more than 1.38 billion sources, with precisions varying from a few milli-mag at the bright (G<13) end to around 200 milli-mag at G=20.
  • Full passband definitions for G, BP and RP. These passbands are now available for download.
  • Epoch astrometry for 14,099 known solar system objects based on more than 1.5 million CCD observations. 96% of the along-scan (AL) residuals are in the range -5 to 5 mas, and 52% of the AL residuals are in the range of -1 to 1 mas. The transit observations are part of Gaia DR2 and have also been delivered to the Minor Planet Center (MPC).
  • Subject to limitations (see below) the effective temperatures Teff for more than 161 million sources brighter than 17th magnitude with effective temperatures in the range 3000 to 10,000 K. For a subset of about 87 million sources also the line-of-sight extinction AG and reddening E(BP-RP) are given and for a part of this subset (around 76 million sources) the luminosity and radius are available as well.
  • Classifications for more than 550,000 variable sources consisting of Cepheids, RR Lyrae, Mira and Semi-Regular Candidates as well as High-Amplitude Delta Scuti, BY Draconis candidates, SX Phoenicis Candidates and short time scale phenomena.
  • Planned cross-matches between Gaia DR2 sources on the one hand and Hipparcos-2, Tycho-2, 2MASS PSC, SDSS DR9, Pan-STARRS1, GSC2.3, PPM-XL, AllWISE, and URAT-1 data on the other hand.

There’s much more to Gaia than pictures, but here’s a map of the stars in our galaxy to give you an idea:

I remember first hearing about Gaia about 17 years ago when I was on a PPARC advisory panel and was immediately amazed  by the ambition of its objectives. As I mentioned above, Gaia is a global space astrometry mission, which will make the largest, most precise three-dimensional map of our Galaxy by surveying more than a billion stars. In some sense Gaia is the descendant of the Hipparcos mission launched in 1989, but it’s very much more than that. Gaia monitors each of its target stars about 70 times over a five-year period. It is expected to discover hundreds of thousands of new celestial objects, such as extra-solar planets and brown dwarfs, and observe hundreds of thousands of asteroids within our own Solar System. The mission is also expected to yield a wide variety of other benefits, including new tests of the  General Theory of Relativity.

For the brighter objects, i.e. those brighter than magnitude 15, Gaia  measures their positions to an accuracy of 24 microarcseconds, comparable to measuring the diameter of a human hair at a distance of 1000 km. Distances of relatively nearby stars are measured to an accuracy of 0.001%. Even stars near the Galactic Centre, some 30,000 light-years away, have their distances measured to within an accuracy of 20%.

It’s an astonishing mission that will leave an unbelievably rich legacy not only for the astronomers working on the front-line operations of Gaia but for generations to come.

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