Christina Eilers (MIT) got some really nice results today, in which she used a causal argument to self-calibrate element abundances in red-giant stars to a fiducial surface-gravity. Abundances—for red-giant stars—can depend on surface gravity for (at least) two reasons: There can be evolutionary effects, in which surface abundances change as elements are dredged up from the stellar interiors, and there can be systematic errors in abundance measurements as a function of surface gravity (because of model wrongness, convective shifts, and so on). Since we have many stars at different positions in the Galaxy and at different surface gravities, we can take a self-calibration approach. Today, Eilers showed amazing results: The inconsistencies in Galactic abundance gradients along the red-giant branch get resolved (with some exceptions) and precision is greatly improved. I mentioned “causal” above: Self-calibration methods involve assumptions that would be described as causal by statisticians.
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