normal modes in stars

At lunch Huppenkothen gave the brown-bag talk, on neutron star normal modes and their possible use in constraining neutron-star equation of state (and thus nuclear physics). She was pessimistic in the end, because there are so few modes measured, but in a precision sense, the data (taken at face value) do rule out some models.

After the talk, Andrei Gruzinov and I argued about the relationship between Huppenkothen's normal-mode constraints and spin constraints on neutron stars (mentioned also last week by Kaspi in our Physics Colloquium). He made a nice argument, which I will butcher to this: The speed of sound at the surface of a neutron star (or really any gravitationally bound object) must be on the order of the gravitational orbit velocity at the surface. Why? Because otherwise the object would further compress under gravity! This all flows from the point that the sound speed is related to the compressibility through some kind of modulus. Simple! I should check this for the Sun.

I spent part of the morning prior to all this with Foreman-Mackey, discussing plans for his trip to Tübingen and Heidelberg. We want to work further on noise modeling or calibration, in the context of stellar variability, exoplanet search, and asteroseismology. We discussed Schölkopf's causal arguments and why we get overfitting despite them; I don't yet understand what is the appropriate "large data" limit at which the relevant theorems are going to hold.