two kinds of alpha elements?

A few days ago I sent around—to the APOGEE Collaboration—plots of large-scale gradients of chemical-abundance ratios in the Milky Way. One of the comments (made by several collaborators) was “that's odd; the alpha elements don't track each other!” I started looking at this today: Can we show that there are multiple kinds of alpha elements? And is that interesting? I made plots of five different [α/Fe] ratios: [O/Fe], [Mg/Fe], [Si/Fe], [S/Fe], and [Ca/Fe], all as a function of various things, like metallicity, Galactic radius, Galactic height, stellar effective temperature and gravity, and so on. It does look like there are (at least) two different alpha-elements. I discussed with Rix whether this is interesting and how to make the case that the effects are real.

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