[Off the grid for the last few days.]
In Milky Way group meeting, Xue (MPIA) talked about our Kinematic Consensus project, which is starting to show promise: We look for sets of stars with observable properties consistent with having the same orbital integrals of motion. It appears to group stars even when the trial potential (we use for orbit construction) is significantly different from the true potential.
At that same meeting, I described my hopes for modeling or measuring the Milky Way star-formation history, using stellar ages derived from stellar masses on the red-giant branch. This got Dalcanton (UW) asking about the point that the age distribution of red-giant stars is not the age distribution of the disk as a whole, since different stars get onto the red-giant branch at different times and stay on for different times. This led to the question: Why I don't model the mass distribution and then transform it into a star-formation history “in post” as it were? I like it! So I will re-code tomorrow. It is very good to talk out a project early rather than late!
In the middle of the day, Price-Whelan and I pair-coded some of his search for substructure, in a project called King Kong, which is a direct competitor to Kinematic Consensus (and inspired by work by Ibata at Strasbourg). Here we look for orbits that “catch” a large number of stars (within their uncertainties). We struggled with some elementary geometry, but prevailed!
Late in the day, Kopytova swung by and we looked at marginalizing out the calibration (or continuum) vectors in her inference of stellar parameters for substellar companions. It turns out that the linear algebra is trivial, along the lines of Foreman-Mackey and my re-formulation of our K2 project. We will pair-code the matrix inversion lemma tomorrow.