Today was day 2 of Galactic Archaeology and Stellar Physics. Again, a great day; here only a few highlights:
Else Starkenburg gave a review on first stars. There is no true first star known—nothing with primordial abundances—but there is one at or near −7 (that is, 7 orders of magnitude below Solar. This star, like many extremely metal-poor stars, is low in iron but very high in carbon relative to iron. That is a mystery, with many conceivable solutions. Starkenburg spoke about binary-star (mass transfer) ideas. The talk left me wondering: Do we know what a primordial-abundance star would look like? Afterwards, Schlaufman argued to me that we do, at least pretty accurately.
This was followed by a bunch of other low-metallicity star talks. DaCosta, Venn, and Schlaufman all spoke about searches for extremely metal-poor stars using clever photometric techniques. This ties in with my comment yesterday that we might be able to do a lot of Galactic Archaeology science with photometric surveys (possibly backed up by spectroscopy for calibration or training). It also bodes extremely well for Gaia Bp–Rp narrow-band photometry, which will be laden with stellar information.
Stello and Huber gave talks about asteroseismology. In Stello's review, I learned that the brightness variations are temperature variations (or really temperature–size variations), not pure size variations. This surprised me, and then was immediately obvious: The atmosphere reacts adiabatically to fast changes. He also very clearly connected the mode properties to the stellar properties, and explained the important point that dwarfs, subgiants, and red giants have different physics connecting their seismic modes to their masses, ages, and bolometric luminosities. Huber compared existing asteroseismology to Gaia data, showing that there is a consistent story, but also showing that for almost all asteroseismic targets, the asteroseismology will provide more precise distances than even end-of-mission parallaxes.
There were a set of nucleosynthesis and supernovae yield talks. My personal highlight here was a talk by Hampel about neutron capture physics. She starts with the observation that between s-process and r-process there is a whole range of neutron densities, and at different densities, you get different abundance yields. She then used real stellar data to measure the neutron density for an intermediate neutron density between s and r, calling it i. This talk stood out among the nucleosynthesis talks for its containing (like Stello's talk on asteroseismology), clearly explained fundamental physics.