Day 3 of Galactic Archaeology and Stellar Physics opened with talks by Lind and Ness about traditional and new ways of measuring stellar parameters and chemical abundances. Both of them were effectively very critical of the traditional method, where there are large inconsistencies in atomic assumptions between giants and dwarfs, and there are many nuisances that affect the data as strongly as the chemical abundances in question. Lind also compared 1D and 3D models, and LTE and NLTE models and made some general statements about each quadrant. She implicitly suggested that limb darkening (or the spectral version of that) and time-domain spectroscopy might both be filled with information, because some of the 3D effects show up most strongly in the variations of the spectrum with position and time.
These introductions were followed by a set of talks that assess various aspects of the feasibility of chemical tagging—finding pairs or groups of widely separated stars that were born together in the same molecular cloud or association. This subject is dear to my heart! Blanco-Cuaresmo clearly articulated the two questions of chemical tagging, which have also come up here in this forum a few times. My phrasing of these questions would be the following: Two stars that were born together: How different can they be? And: Two stars that were born apart: How similar can they be? He then proceeded to do stuff with PCA and k-means that I didn't love; I don't think vanilla machine learning will solve this problem. However, he did (inadvertently, it seems) show great evidence that chemical tagging is conceivable. Similarly Kos did things with t-SNE that I didn't love, but which also show great evidence for an optimistic view! Carrera showed that open clusters have amazingly uniform chemical abundances. Ting showed argued that we might have to take a more probabilistic approach to chemical tagging than the original hopes. He called this the “pessimistic regime” of chemical tagging; no reason for pessimism there, but I get why he called it that.
In related news (and related to things Price-Whelan and I talk about), Mike Ireland showed an example of running the clock back on Gaia TGAS (plus spectroscopic RVs) to find the ages of disrupting stellar associations. He finds that you get more accurate ages if you take a probabilistic approach, which is music to my ears.
The afternoon was dominated by the Galactic Bulge, which appears to have sub-components formed by monolithic collapse and by long-term evolution out of the disk. The X-shape is primary evidence that the latter is the dominant process, though controversies continue. Unfortunately only one speaker showed the absolutely gorgeous Ness & Lang image, which I have the honor to have elicited with a tweet (tm) a year or so ago.
The day ended with Andy Casey talking about anomalies in the Solar abundances that run systematically with condensation temperature. Tantalizing to think it might have to do with the fact that the Sun hosts rocky exoplanets! These anomalies exist all over, however (or so it seems). They probably have something to do with dust depletion and dust accretion, which can spatially separate the high condensation-temperature elements from the low condensation-temperature elements. The talk was a reminder of how hard it is going to be to get a straightforward interpretation of anything in the high dimensional chemical-abundance space.