Yesterday, in conversation with Andrew Mann (Columbia), Jessica Birky (UCSD) and I decided that she should do a full set of jackknife tests on her Cannon model of APOGEE M-dwarf stars. She did that overnight (I love working with such great people!) and the results indeed show that we don't have much good metallicity information about the M-dwarf stars in the training set we have. This inspired Mann to look for more training-set objects; he found a few dozen more, with a bit more metallicity span. Excellent.
In the afternoon, Kathryn Johnston (Columbia) organized a meeting of the Local Local-Group Group. As it were. There were many interesting things discussed. Megan Bedell (Flatiron) showed her Solar-twin abundances and this got a lot of interesting discussion going about their use to constrain Galactic chemical evolution and radial migration in the Milky Way disk. In particular, they could be very constraining if stellar birth composition is a nearly-unique function of time and Galactocentric radius. There were also questions about whether she can constrain nucleosynthetic yields, which I think she can!
Also in that session Tomer Yavetz spoke about chaos and chaotic orbits, and the properties of stellar streams thereon. He had a nice explanation for why chaos shows up so quickly and clearly in stellar streams: The relevant timescale is not the Lyapunov time, but the time it takes for orbits to wander around their local neighborhood in frequency space, which can be a much shorter time (short reason: because that frequency neighborhood can be small). I hope this is correct, because it has been a puzzle!