At Gaia DR2 prep meeting, I discussed comoving stars and related matters with Oh and Price-Whelan. We discussed moving from our work (in DR1) that made use of marginalized likelihoods for catalog generation to a parameter-estimation method. What would that look like? As my loyal reader knows, I prefer parameter-estimation methods, for both pragmatic and philosophical reasons. But once you go to parameter-estimation methods, there are lots of parameters you could in principle estimate. For example: You can look at the space-time event to which the two stars made their closest approach in the past, and how far apart they would be at that point. If the separation is small, then coeval? That might be much more interesting than co-moving, in the long run.
At Stars group meeting, Allyson Sheffield (CUNY) and Jeff Carlin (LSST) showed us results on abundances of M-type giant stars in the Sagittarius tidal streams. They can clearly see that the progenitor of the stream had element-abundance gradients in it prior to tidal stripping. They also show that the stream matches onto the abundances trend of the Sagittarius dwarf body. But the coolest thing they showed is that there are two different types of alpha elements, which they called explosive and hydrostatic, and the two types have different trends. I need to check this in APOGEE! Sheffield also mentioned some (possibly weak) evidence that the bifurcation in the stream is not from multiple wraps of the stream but rather because the object that tidally shredded was a binary galaxy (galaxy and satellite) pair! I hope that's true, because it's super cool.