The talk that most interested me was Ashley Ruiter‘s review of binary population synthesis. It convinced me that I need to review Maoz & Badenes’ recent work on delay time distributions [DTDs], because the functional form of the theoretical DTD is basically universal but the normalization is not. I found the impression that you can change any one of a number of knobs in the stellar evolution code, and still get only an overall constant, rather discouraging.
Later in the week I asked Silvia Toonen, whose talk came right after Ashley’s, whether this was really true. Apparently it is mainly because the long-delay systems (i.e., SNe Ia which explode more than about 200 million years after the progenitor system forms) are mostly double-degenerate systems, which have a simple t-1 form to the DTD. One could nevertheless imagine playing with the properties of common-envelope evolution (or introduce a third star) to change the shape of this distribution somewhat. The DTD on timescales shorter than a few hundred million years has information about single-degenerate progenitors and other channels, but the uncertainties here are too large — probably because it’s tough to determine the age of the group of stars with which a given SN Ia progenitor formed to the necessary accuracy. There may nevertheless be new constraints on the short-delay end of the DTD as we continue to find SNe Ia with low-redshift searches.
Ashley also suggested that models of violent double-degenerate mergers, which explode promptly after merging (instead of sitting around for a while cooling off and spinning down), could fit the most recent measurements of the DTD well. I have a feeling that someone’s error bars aren’t quite right, though, because this is the first I’d heard that there wasn’t at least a factor of 10 discrepancy between the predicted rate of some channel and the observed rate.
Ashley, Silvia, and other binary star experts are working on a project called POPCORN, which is a bake-off of different binary population synthesis codes: seeing the extent to which they get the same answer given the same assumptions, and then seeing how the assumptions change the predictions. I can only applaud this as a good thing.
During the poster session I had useful talks with Aaron Jackson (NRL), who is simulating the microphysics of the nuclear flames within SNe Ia, and with Richard Booth (Oxford), who is calculating predicted spectra of single-degenerate SNe Ia from “symbiotic” systems containing a white dwarf and a red giant star. There were some other brief scatterings off of other scientists. Sometime after that I staggered off to a late Indian dinner and then crashed relatively early.