Gaia, EPRV, photons

In our Gaia DR2 prep workshop, Stephen Feeney (Flatiron) led a discussion on the Lutz–Kelker correction to parallaxes, and when we should and shouldn't use it. He began by re-phrasing the original LK paper in terms of modern language about likelihoods and posteriors. Once you put it in modern language, it becomes clear that you should (almost) never use these kinds of corrections. It is especially wrong to use them in the context of Cepheid (or other distance-ladder) cosmology; this is an error in the literature that Feeney has uncovered.

That discussion devolved into a discussion of the Gaia likelihood function. Nowhere in the Gaia papers does it clearly say how to reconstruct a likelihood function for the stellar parallaxes from the catalog, though it does give a suggestion in the nice papers by Astraatmadja, such as this one. Astraatmadja is a Gaia insider, so his suggestion is probably correct, but there isn't an equivalent statement in the official data-release papers (to my knowledge). There is a big set of assumptions underlying this likelihood function (which is the one we use); we unpacked them a bit in the meeting. My position is that this is so important, it might be worth writing a short note for arXiv.

In Stars group meeting, Megan Bedell (Flatiron) showed her current status on measuring extremely precise radial velocities using data-driven models for the star and the tellurics. It is promising that her methods seem to be doing better than standard pipelines; maybe she can beat the world's best current precision?

Chuck Steidel (Caltech) gave a talk in the afternoon about things he can learn about ionizing photons from galaxies at high redshift by stacking spectra. He had a number of interesting conclusions. One is that high-mass-star binaries are important! Another is that escape fraction for ionizing photons goes up with the strength of nebular lines, and down with total UV luminosity. He had some physical intuitions for these results.