Today was a great day at the meeting; many good and lively talks and discussions. This is a healthy field! Here are some personal highlights for the day:
In his opening talk, Wyn Evans (Cambridge) gave credit to Koposov, Hogg, & Rix for fitting the GD-1 stream with a (trivial, wrong) orbit model (that is, not a physically correct stream model): The orbit model seems to return the correct potential. The authors of the paper refused this credit, since it appears to be luck rather than skill that obtained this coincidence. A few talks later, Bovy disagreed with Evans; he showed that as the potential is made more realistic, the stream model departs more from the orbit model.
Sanders (Cambridge), Bovy, Binney (Oxford), and McMillan (Lund) all talked about using actions and angles for modeling streams. The math is beautiful, but the computation is expensive. It is not clear to me that this is the right approach, long term. It also requires a strong constraint on the potential and the populated orbits: Many orbits are likely to be chaotic!
Continuing on that theme, Pearson (Columbia) and Price-Whelan coined the term "stream fanning" for streams on chaotic orbits, which show very different morphologies from those on regular orbits. Price-Whelan showed that the fanning happens earlier and more prominently than you would expect from standard chaos measures like Lyapunov time; he presented an argument in terms of frequency-space diffusion. In the question period it came up that if we take the long streams to be highlights of regular orbits, then this puts some kind of very complex (or even fractal) posterior on potential models. Crazy!
In the discussion between sessions, many great ideas came up. One is to use The Cannon to obtain more precise spectroscopy-informed stellar absolute magnitudes for red-giant stars. Another is to find the fountain of high-velocity stars above the bulge in Gaia DR2; we might be able to find A and F stars (most known high-velocity stars are B stars, I think).