As always, the stars group meeting at the CCA was a research paradise. We opened (at the suggestion of Ruth Angus) by acknowledging that a terrible thing had happened in the United States and that we were collectively sorry about this. That was necessary.
We moved on to a presentation by Dun Wang (NYU) of his image modeling for the Kepler K2C9 imaging campaign to measure microlensing events in the Galactic Bulge. He described his radical technique for image differencing, which has produced (I think) the most precise image differences in the history of astronomy! They have requirements, however, that not all future time-domain imaging surveys would successfully meet. He has some really impressive movies, and we stared at them like zombies for quite a long time.
After that, Ruth Angus (Columbia) presented the recent paper by Davenport about stellar rotation, informed by the Gaia DR1 TGAS data (and thanking the Gaia Sprint!). The paper confirms and extends a discovery of a gap in rotation-inferred ages of stars, which either points to some kind of non-linearity in the evolution of stellar rotation, or else points to a really non-trivial event in the star-formation history of the Galaxy. It could also be that there are just different classes of stars (in terms of rotation). We argued about all this. Hawkins (Columbia) recommended that we look at the relevant stars in chemical-abundance space; this should resolve some of the possible explanations.
After group meeting, Tim Morton (Princeton), Adrian Price-Whelan (Princeton), and I sat with Semyeong Oh (Princeton) to go over the draft (and especially figures) of her new paper on co-moving stars in Gaia TGAS. Lots of great stuff in there, and a valuable tool for stellar astrophysics.