Adrian Price-Whelan (Flatiron) and I have been trying to use the chemical abundances of stars to constrain the mass model (or gravitational potential, or orbit structure) of the Milky Way. One thing we have noticed is that the abundances are very sensitive to the coordinate system: If you have the velocity or position of the disk wrong, it is clearly visible in the abundances! That's fun, and motivating. But then we have noticed—and we noticed this two summers ago now—that different elements want to put the disk midplane in different places!
What gives? We have various theories. We started with systematics in the data, but the effects are seen in both APOGEE and GALAH. So it seems like it is real. Is it because relaxation times are very long at small vertical heights? (The disk is like a harmonic oscillator at small vertical amplitudes.) Is it because the thinner disk and thicker disk have inconsistent midplanes? Whatever it is, it seems like it is super interesting. We can't solve this problem in our current paper, but we want to comment on it.
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