#GaiaSprint, day 2

It is only Tuesday, and yet there are already incredible results flowing in from the Gaia Sprint. I won't do justice in any way to what I saw today, but here are a few very personal highlights from the day. Although everyone spent the day working—the Sprint has almost no formal program—these results are from the morning and evening check-in discussions:

In the morning check-in, Branimir Sesar (MPIA) showed us the results of a hierarchical model of the RR Lyrae stars in TGAS, where he simultaneously fit for the period–luminosity relation parameters, and also parameters of a flexible model for the noise (bias and variance) in the Gaia parallaxes. He confirms the Gaia noise model and gets absolutely beautiful parameter constraints. That was a pretty good result for one day of work!

Sven Buder (MPIA) and Johanna Coronado (MPIA) used dynamical actions computed with the help of Wilma Trick (MPIA) and Jo Bovy (Toronto) to investigate the heating mechanisms in the Milky Way disk. They can clearly show that stars that are older (at least according to spectroscopic parameters and stellar models) have larger vertical actions. This was nice, but they have really beautiful gradients in vertical action across the red clump, consistent with the expected gradient in age across the red clump. This suggests that their stellar labels (from GALAH and LAMOST, respectively) and stellar models and Gaia kinematics are all consistent. Crazy! And beautiful. Is the vertical action the new age estimator?

Semyeong Oh (Princeton) showed her results (with also Price-Whelan and Spergel and me) on co-moving pairs of stars, and their locations in the color-magnitude diagram. This led to a lot of discussion about what can be concluded and what can be predicted. In particular, we expect no old stars (like no red giants, even) for the widest-separation co-moving pairs.

Sergey Koposov (Cambridge) showed us results from an insane and massive project to measure proper motions from the comparison of the SDSS imaging to the Gaia billion-source list. This project involved a complete recalibration of SDSS astrometry! His proper motions look great, and he is using them to search for substructure and analyze Milky Way structure. A simply insane project. All the more insane, because his catalog will be superseded by Gaia at the next data release in a year! And I mean “insane” in the best possible way.

With a tiny bit of consulting from me, Jonathan Bird (Vanderbilt) converted his project to measure the age–velocity relation (the vertical velocity dispersion in the disk as a function of stellar age) into a generative model for the ages. This isn't working yet, but he showed results for the relation when he assumes that the ages are God's truth. This project is one he was working on for a long time with GCS data, but with the TGAS data he obviated all his previous work in one single day. Damn, I love good data.

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