#GaiaDR2 zero-day workshop, day 3

Today was the third and final day of our Gaia DR2 zero-day workshop. My goodness it was a fun week. Many of the participants told me that they would remember this week for the rest of their lives! Now that's not something I hear every day. And the participants here in NYC were very much focused on learning what is in the data, and exploring the data. There was no sense of trying to rush out publications or results. I loved the atmosphere.

In my own research today, I worked with Dustin Lang (Toronto) to understand the SDSS-III spectra that overlap the white-dwarf parts of the UV color–magnitude diagram that David Schiminovich (Columbia) and Lang showed yesterday. It wasn't obviously simple, but I have ideas about making a latent-variable model for it: Predicting spectra from photometry!

In the lunch-time check-in session there were some really impressive results. One was a big model for stellar physical parameters, and extinctions in a two-component model by Eddie Schlafly (LBNL). He pointed out that since Gaia gives distances and colors, it is sensitive to even fully gray extinction. So it provides a new window into extinction. Since his model involves simultaneously modeling stellar multi-band photometry (combined from many missions) along with the intrinsic properties of every star, it got big fast. I think it was at 800,000 parameters today. Optimized! That's pretty good for day three.

Another beautiful set of results at the check-in were visualizations of tidal features: Sarah Pearson (Columbia) visualized the tidal tails of Palomar 5, hoping to find them extend further than ever before. Chervin Laporte (UVic) visualized the anti-center stream and made the case that all of its kinematic properties are consistent with it being a tidal arm coming off the Milky Way from an interaction (with Sagittarius, I guess?). The morphology of the anti-center stream really is sharp, like a fold caustic.

In more general data-understanding categories, Sergey Koposov (CMU) scanned through proper-motion space, showing us low-parallax (that is, non-close) stars in different proper-motion bins. That highlighted a lot of streams, clusters, and anisotropies. And Andy Cassey (Monash) showed us how good (or bad) astrometric excess variance (and also radial-velocity excess variance) is at detecting binary stars. The answer is: Promising, but not calibrated usefully yet. In an ideal world we would build a self-calibrated model of what causes the variance and then use the residuals to detect binarity.

There were many more impressive things today, about the nearby volume, about comoving stars, about detailed chemical abundances, about the GD-1 stream (and possible progenitor!), and about kinematics of the disk and kinematics of the bar; too many things to mention here. Thank you, Gaia Collaboration.

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