Today Robyn Sanderson (Groningen) arrived at MPIA for three days of sprint on a paper about inferring the Milky Way Halo potential by optimizing the information (negative entropy) in the phase-space distribution function. This project involves the horror of transforming the observables to the actions (something I railed against a few days ago), but the method is promising: It permits identification of potentials given phase-space information without requiring that we identify related stars or specific structures at all. The method only requires that there be structure. And of course we are looking at how it degrades as the observational errors grow.
In addition to working on this, we attended the Rix-group Milky-Way group meeting, where (among other things), Wyn Evans (Cambridge) told us about using the apocenter information we have about the Sagittarius tidal stream to infer Halo potential parameters. He gets a low mass for the Milky Way (in accord with other stellar kinematic methods, but in conflict with Leo I's high velocity; is it bound?). I had a bit of a problem with how Evans and his collaborators join up the stream to the progenitor, but that may just be a detail. Hoping to learn more later this week.