Hipparcos binaries and Gaia velocity field

Now that Gaia DR2 has happened, we still organize parallel-working time at Flatiron for people to get together and hack on their Gaia projects. I worked on two things in this meeting today. In the first, I wrote to Floor van Leeuwen (Cambridge) about his Hipparcos data, because I learned last week that he released individual-visit astrometry for every star (hooray!), and that the visit astrometry has never really been comprehensively searched for arbitrary binary stars. I'm not sure I will do this! It's hard. But a project along these lines would be a great preparation for future Gaia data.

My second project in Gaia workshop was to talk out something with Adrian Price-Whelan (Princeton) that connects to things I have been talking about with Rix and others: In a very, very local sample (very near the Sun), the stars in velocity-space show a lot of informative structure. That is, some disk orbits are over-populated, and some under-populated. As you grow the region around the Sun, the structure in velocity space gets fuzzier, because the local velocity structure is a function of local position. Rix's view (which is sensible) is that we should look at this structure not in velocity space but in action space. That's a good idea! But if the structure is caused by non-axisymmetry in the disk, the actions computed in an axisymmetric potential won't be the clearest space. Let's find that space using the data themselves, and then interpret the transformation from more naive coordinates in terms of the dynamics. Price-Whelan and I came up with a first-step project, and an objective function to optimize. It looks do-able.

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