rigid disk kinematics, habitability

After a morning of beating our heads against intransigent code, Foreman-Mackey and I spoke with Rix and Lisa Kaltenegger (MPIA) about their project to do a more responsible job of assigning probabilities of habitability to noisily observed tiny planets. The issues include: The observational uncertainties can be bad when projected into the quantities of interest (insolation and size). Some aspects of the problem are essentially unobserved (albedo and existence of an atmosphere and rockiness). Some planets have mass and radius observed, some have only one or the other. We discussed in general, and Foreman-Mackey is going to think about whether to take it on.

At lunch we discussed Bovy's excellent (not yet released) manuscript on the Milky Way disk using SDSS-III APOGEE, which is based on measurements of radial velocities all over the whole disk (APOGEE is an infrared spectroscopic survey, so it sees through a lot of the dust). Even marginalizing out distances to the stars, he can obtain a few-percent measurement of the rotation curve! One fundamental and obvious yet surprising point that came up is that in radial velocity the rotation of a perfectly cold, rigid disk is unobservable! Several of us laughed to realize we had never thought of that previously. Bovy's method therefore relies heavily on the asymmetric drift model—the fact that the lag of a population relative to the cold-population rotation curve is a function of velocity dispersion. But since he can see most of the disk, and since he is marginalizing out troublesome distance estimates (and also giant/dwarf classification), his results are the best ever. We discussed the comparison with future maser studies but I think that the much more precise maser measurements might not help that much: The masers and other young stars have a lag that is not easily understood in terms of the theory of asymmetric drift, as we showed here and here. Rix pointed out that it shouldn't be easily understood, because the masers aren't expected to be an angle-mixed population.