All-in on URCA: Bankrupting a White Dwarf
Introduction
The goal of this lab is to understand the Urca process in O/Ne white dwarfs. Urca is a competition between electron-captures and β-decays. At high densities, electron-captures dominate and at lower densities, β-decays dominate. Each process emits a neutrino, which leaves the white dwarf and provides a source of cooling.
The labs begin with a low mass white dwarf that is accreting. You can think of this either as a white dwarf in a binary system that is accreting from the companion (although we neglect the burning of solar material to O/Ne in accretion) or as the core of a 8-10 star that will eventually become an electron-capture supernova.
As we accrete, the density throughout the WD increases, allowing for electron captures. At the center, the conditions become high enough for oxygen-burning (via ).
Note
The question posed by these labs: does the oxygen burning win? or does the Urca cooling dominate leading to an accretion-induced collapse into a neutron star?
Warning
We don’t attempt to follow the oxygen burning beyond ignition in this lab, so the models stop once the temperature exceeds .
Background lectures
Lectures giving the background on white dwarfs, nuclear reactions, and the conditions for Urca will be provided as a set of Jupyter notebooks. You can read through (or run via Jupyter) the collection of lecture notes.
Lab modules
The three labs will walk you through increasing-complex nuclear physics. Lab 1 is expected to take the longest and lab 3 is open-ended.