At lunch we had a discussion (inspired by Bertrand Goldman, MPIA) about the expected shapes of open clusters. I think they should be elongated along their orbits. There was some back and forth but this made me more confident: Once the clusters start to disperse, they should distort through orbital phase-frequency differences. I proposed a simple test of this. But I'm more interested in the point that this should help us find new kinds of (maybe older) clusters!
In the afternoon, Amina Helmi (Kapteyn) showed up and Bonaca (Harvard), Price-Whelan (Princeton), and I discussed many things with her. We discussed the question of when and how stellar streams in the Milky Way halo constrain purely local properties of the Galaxy. Does this result (from Bonaca) depend on the potential being time-dependent? I think it does. Helmi didn't disagree but is optimistic that we can handle the time dependence.
We also discussed the lack of tidal tails around globular clusters: Is it surprising that only Palomar 5 has these tails? Price-Whelan has looked at a few of the most likely clusters in Gaia, and nada. This led to (or was part of) a longer discussion of the statistics of streams: How many will there be and how many do we expect?