exoplanet false positives; emcee3

In a low-research day, the highlight was a long discussion of current projects with Foreman-Mackey, who is now settled in Seattle. We discussed exoplanet search and populations inference with K2 data. One test of the false-positive rate (the part not generated by easy-to-model astrophysical sources) is to invert the lightcurve and search again. And when you do, sure enough, you find small, long-period exoplanets! That is the beginning of an empirical approach to understanding stellar-variability-induced and instrumental false positives. We have also talked about reordering the time-series data, but for long-period planets, this doesn't cover all bases. There are many improvements to Foreman-Mackey's search since his first K2 catalog paper: New basis for the systematic effects, built with the PCP (the robust PCA); new prior on principal component amplitudes for more informative marginalization; automated vetting that makes use of probabilistic models for model comparison. It all looks like it is working.

We also talked about the next generation of the emcee code, dubbed emcee3. The idea is that it is an ensemble sampler, but the stretch move (the basis of emcee now) becomes just one of a huge menu of update steps. We discussed schedules of update methods in mixed schemes. We discussed the problem, after a run with a non-trivial schedule, of figuring out which updates worked best. We discussed what statistics you want to keep for an ensemble sampler to assess performance in general. Homework was assigned.