ELSA Conference on Gaia, day three

It was spectroscopy all morning, with the Gaia Radial Velocity Spectrograph worked-over by Cropper (UCL), Katz (Paris), and Jasniewicz (Montpellier). Two things that were of great interest to me: The first is that the satellite can only do relative measurements of velocity (really of logarithmic Doppler Shift, I think) so they need absolute velocity standards. They will use a combination of asteroids and team-calibrated (from the ground) radial velocity standards that span the sphere and spectral type. The issue is more fundamental, of course: To compare stars of different types, you need to tie together radial velocity standards that are based on different lines with different gravitational redshifts and convective corrections. But the issue is even more fundamental, and that is the second thing that was of great interest to me: Lindegren (Lund) has a paper defining radial velocity that shows that it is not really a spectroscopically measured quantity: There are many physical effects that affect stellar lines. Indeed, this relates in my mind to an old conversation with Geha (Yale) and Tremaine (IAS) about whether it is possible—even in principle—to measure a velocity dispersion smaller than Geha's smallest measurements. I think it is but it is because God has been kind to us; this was not something we could expect just by dint of hard work.

Freeman (ANU) gave a talk about HERMES, an ambitious project to take detailed enough spectra of about a million stars to do full chemical tagging and separate them into thousands of unique sub-groups by detailed chemical properties. He made an argument that the chemical space will be larger than (though not much larger than) a seven-dimensional space. I have been hearing about chemical tagging for years and seen nothing, but with this it really looks like it might happen. This project is explicitly motivated by the science that it enables in the context of Gaia. Where are the American projects of this kind? Also, when will we have the technology or money to take spectra of a billion stars?

In the afternoon, there were Solar-System and asteroid talks; these are always impressive because precisions in this field are so much higher than anywhere else in astronomy. Oszkiewicz (Helsinki) gave a nice talk about fully Bayesian orbit fitting for asteroids, showing that they could have predicted the asteroid hit in Sudan in 2008 with high confidence (did they predict that?). She also showed that they can model asteroid shapes from Gaia lightcurve data with the same kind of MCMC machinery.