carbon stars, regulation of star formation, and so much more

Rix called me to discuss the problem that when we compare the chemical abundances in pairs of stars, we get stars that are more identical than we expect, given our noise model for chemical abundances. That is, we see things with chi-squared (far) less than the number of elements. This means (I think) that our noise estimation is overly conservative: There are (at least some) stars that we are observing at very good precision. Further evidence for my view is that there are more such (very close) pairs within open clusters than across open clusters (or in the field).

In stars group meeting, Jill Knapp (Princeton) spoke about Carbon stars (stars with more carbon than oxygen, and I really mean more in counts of atoms). She discussed dredge-up and accretion origins for these, and how we might distinguish these. She has some results on the abundance of Carbon stars as a function of expected (from stellar models) surface-convection properties, which suggest accretion origins. But it is early days.

Chang-Goo Kim (Princeton) told us about simulations that are designed to understand the regulation of star formation in galaxy disks (kpc scales). He pointed out the importance of gravity in setting the star-formation rate; these arguments are always reminiscent (to me) of the Eddington argument. His simulations include supernovae feedback in the form of mechanical and radiation energy, and magnetic turbulence and cosmic ray pressure. He emphasized that conclusions about feedback-regulated star formation depend strongly on assumptions about spatial correlations and locations (think escape over time) of the supernovae relative to the dense molecular cloud in which the star formation occurs. Fundamentally the thing that sets the star-formation rate is the pressure, which can be hydrostatic or turbulent or both.

Semyeong Oh (Princeton) and I led a discussion on the lowest-hanging fruit for projects that exploit her comoving star (and group) catalog from TGAS. Some of the lowest-hanging include investigations of the locations of the pairs in phase space, to look at heating, age, and formation mechanisms.