#GaiaDR2 zero-day workshop, day 2

It was a little harder to get up this morning after yesterday's 13-hour day, but I still made it in early for the second day of the Gaia DR2 zero-day workshop. We had about 70 yesterday and still maybe 50 today; the room was at capacity and we had people all over the 3rd floor of the (very generous) Flatiron Institute.

Dustin Lang (Toronto) coined the name "BetterTogether" for a project that Megan Bedell (Flatiron) and I started to find all the comoving pairs that can be confidently identified in the data. This kind of work isn't new: Semyeong Oh (Princeton) had big impact with her comoving-pair work in Gaia DR1. But what's new is the idea of using the co-moving-ness to betterize the parallaxes of both stars, and in particular the less luminous (and hence noisier) star. So pairs that are WD-MS or MS-RGB are most valuable! This project builds conceptually on work I did with Morgan Fouesneau (MPIA) and Hans-Walter Rix (MPIA) in the TGAS–PanSTARRS overlap.

The issue is that you can't trivially look at every pair in a 1.3-billion-star catalog. There are 1e18 pairs! And even deciding not to look at a pair takes time. So Lang started to build us a very nice data structure for doing the two-point work while Bedell looked at the restricted sample that matches the Kepler targets.

In the mid-day check-in, some really impressive things were shown. Lang and David Schiminovich (Columbia) showed a set of UV color–magnitude diagrams that literally caused the audience to gasp. Stars look so different in the UV! And there are stars where there “shouldn't be”, because of binarity or chromospheric activity or something. So much structure! Kohei Hattori (Michigan) showed a hyper-velocity star that looks like it was launched from the disk towards the Galactic Center. Tim Morton (Princeton) showed that the Gaia stellar radii are good enough to bring out the radius gap in Kepler exoplanets. Ana Bonaca (Harvard) and Adrian Price-Whelan (Princeton) showed that the gaps in the GD-1 stellar stream are really there, and also had hints of kinematic offsets that might indicate dark-matter substructure!

On a more astrophysical note, Kareem El-Badry (Berkeley) spent yesterday and today becoming an expert on white-dwarf physics and was able to give a reasonable, quantitative explanation of the (exquisite, surprising) morphology of the white-dwarf part of the color-magnitude diagram, including a generative model! He finds that even if the IMF and star-formation history are monotonic, the white-dwarf mass distribution is not, because of wiggly initial-mass–final-mass relations. That gets much (but not all, I'm interested to note) of the multi-modal structure in the diagram.

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