In the morning, I met with Ruth Angus (Columbia) to discuss the ages of stars. We brainstormed all possible age estimates for stars, and listed some limitations and epistemological properties. In addition to the usuals (rotation, activity, isochrone fitting, asteroseismology, and so on), we came up with some amusing options.
For example, the age of the Universe is a (crude, simple, very stringent) age estimate for every single star, no matter what. It is a very low-precision estimate, but it is unassailable (at the present day). Another odd one is the separation of comoving pairs. In prinicple every co-moving pair provides an age estimate given the relative velocity and relative position, with the proviso that the stars might not be co-eval. This is a good age estimate except when it isn't, and we only have probabilistic information about when it isn't.
We then wrote down the basic idea for a project to build up a hierarchical model of all stellar ages, where each star gets a latent true age, and every age indicator gets latent free parameters (if there any). Then we use stars that overlap multiple age indicators to simultaneously infer all free parameters and all ages. The hope—and this is a theme I would like to thread throughout all my research—is that many bad age indicators (and they are all bad for different reasons) will, when combined, produce precise age estimates nonetheless for many stars.
At lunch-time, Glennys Farrar (NYU) gave an energizing black-board talk about a dark-matter candidate that exists within QCD, made up of a highly symmetric state of six quarks. QCD is a brutal theory, so it is hard to compute the properties of this state, or its stability, but Farrar laid out some of the conditions under which it is a viable dark-matter candidate. It is very interesting phenomenologically if it exists, because it has a non-trivial cross-section for scattering off of atomic nuclei, and it could be involved in baryogenesis or the matter–anti-matter asymmetry.