Simpson's Paradox, Li, self-calibration

On the plane home from the UK, I worked on three things. The first was a very nice paper by Ivan Minchev (AIP) and Gal Matijevic (AIP) about Simpson's Paradox in Milky Way stellar statistics, like chemodynamics. Simpson's paradox is the point that a trend can have a different sign in a subset of a population than it does in the whole population. The classic example is of two baseball players over two season: In the first season, player A has a higher batting average than player B. And in the second season, player A again has a higher batting average than player B. And yet, overall, player B has a higher average! How is that possible? It works if player A bats far more in one season, and player B bats far more in the other, and they both bat higher in that other season. Anyway, the situation is generic in statistics about stars!

The second thing I worked on was a new paper by Andy Casey (Monash) and company about how red-giant stars get Lithium abundance anomalies. He shows that Li anomalies happen all over the RGB, and even on stars descending the branch (as per asteroseismology) and thus he can show that the Li anomalies are not caused by any particular stellar evolutionary phase. That argues for either planet engulfment or binary-induced convection changes. The former is also disfavored because of the stars descending the RGB. The real triumph is the huge sample of Li-enhanced stars he has found, working with The Cannon and Anna Y. Q. Ho (Caltech). It's a really beautiful use of The Cannon as a spectral synthesis tool.

The third thing I worked on was a plan to self-calibrate HARPS (and equivalent spectrograph) pixel offsets (that is, calibration errors at the pixel level) using the science data from the instrument. That is, you don't need arcs or Fabry–Perot to find these offsets; since they matter to data interpretation, they can be seen in the data directly! I have a plan, and I think it is easy to implement.

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