#GaiaSprint, day 4

It's hard work, this full week of sprinting! Especially following a week of hacking in preparation! I was exhausted today (and I can't entirely blame Andy Casey, though I'd like to). Christina Eilers (MPIA) continued with her map-making work. We started the day by trying to find chemical-abundance neighborhoods (that is, regions of element-abundance-space) where the stars lie on a ring in the Milky-Way disk. There should be such rings if we can measure the abundances well enough! But we failed.

In other news, Andy Casey (Monash) and Adrian Price-Whelan (Princeton) asserted to me that they can take the stars in APOGEE with multi-modal posterior pdfs in orbital-companion space (that we produced here) and rule out some modes just with the Gaia DR2 radial-velocity mean and variance (which is all we get!). I hope this is true.

And in yet other news, David Spergel (Flatiron) and Megan Bedell (Flatiron) not only found co-moving stars in the halo, but find that as the separations get large, the velocity vectors point parallel to (or anti-parallel to) the separation vector between the stars. Duh! Disrupted binaries are two-star streams. I pointed out (to some skepticism) that the velocity differences between very wide pairs of nearly-comoving stars could be used to make local acceleration maps of the Milky-Way halo. Stoked!

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