Lab Goals

Our goal is to model nuclear processes in an accreting O/Ne white dwarf to understand temporal evolution (and ultimately whether the white dwarf can explode).

We need to understand some concepts from astronomy:

• Electron degeneracy and the equation of state

• The structure of white dwarfs

• Nuclear reactions, including strong-mediated and weak-interactions (like electron-capture and \(\beta\)-decay).

We’ll start with a framing of the problem and then discuss the concepts.

Urca intro

We will consider the competition of different nuclear processes. For weak interactions, we have:

• Electron-capture: a nucleus captures an electron, converting a proton to a neutron.

• \(\beta\)-decay: a neutron in a nucleus decays into a proton + electron

Each of these processes emits a neutrino, which carries energy out of the star—this can lead to cooling.

The Urca process is when the electron-capture and \(\beta\)-decay competes, converting a nucleus back and forth, and robbing the star of energy.

The high densities inside a white dwarf can inhibit these processes, so we need to understand how electrons behave at white dwarf densities.

Tip

Why is it called Urca?

Gamow and Schenberg named it after the Cassino da Urca in Urca, Rio de Janeiro.

the Urca Process results in a rapid disappearance of thermal energy from the interior of a star, similar to the rapid disappearance of money from the pockets of the gamblers on the Casino de Urca.

Environments where Urca is at play

We will focus on cooling in massive O/Ne WDs. Some background (and inspiration for these labs) can be found in:

• The importance of Urca-process cooling in accreting ONe white dwarfs (Schwab et al. 2017)

  This is the main inspiration for these labs

• The Formation of Electron-capture Supernovae: A Review (Wang et al. 2025)

  This discusses electron-capture supernova, where Urca in the O/Ne core can also be important.

Beyond the setup in this lab, we can encounter Urca in other contexts:

• Convective Urca in massive C/O WDs (possible SN Ia progenitors?) See:

  • The nuclear diversity of Type Ia supernova explosions (Podsiadlowski et al. 2008)

  • Exploring the Carbon Simmering Phase: Reaction Rates, Mixing, and the Convective Urca Process (Schwab et al. 2017)

  • Some Thoughts on the Convective Urca Process (Schwab 2021)

  • 3D Convective Urca Process in a Simmering White Dwarf (Boyd et al. 2025)

• Neutron star cooling

  • Direct URCA process in neutron stars (Lattimer et al. 1991)

  • Urca Cooling in Neutron Star Crusts and Oceans: Effects of Nuclear Excitations (Wang et al. 2021)