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White Dwarf Cooling

White dwarfs cool off and become less luminous over time. This makes them useful astrophysical clocks on timescales of billions of years. Here’s a rough example of what that looks like:

The color of the white dwarf here corresponds to what the human eye would perceive at the effective temperature in the top right corner of the plot. (Animation generated using Eva Laplace’s TULIPS.)

Here are a some publications where I have contributed to and used computational methods studying the physics of white dwarf cooling:

  • Skye: A Differentiable Equation of State
    Jermyn, Schwab, Bauer, Timmes, & Potekhin ApJ, 913:72, May 2021
  • Multi-gigayear White Dwarf Cooling Delays from Clustering-enhanced Gravitational Sedimentation
    Bauer, Schwab, Bildsten, & Cheng ApJ, 902:93, October 2020
  • Modules for Experiments in Stellar Astrophysics (MESA): Convective Boundaries, Element Diffusion, and Massive Star Explosions
    Paxton, Schwab, Bauer, et al. ApJS, 234:34, February 2018

Polluted White Dwarfs

Polluted WD Schematic Schematic of white dwarf pollution from an extremely helpful review article by Jura & Young (2014). Here are some of my contributions to this topic:

  • Horizontal spreading of planetary debris accreted by white dwarfs
    Cunningham, Tremblay, Bauer, et al. MNRAS, 503:1646, May 2021
  • Polluted White Dwarfs: Mixing Regions and Diffusion Timescales
    Bauer & Bildsten ApJ, 872:96, February 2019
  • Increases to Inferred Rates of Planetesimal Accretion due to Thermohaline Mixing in Metal-accreting White Dwarfs
    Bauer & Bildsten ApJL, 859:L19, June 2018

I have argued that the above picture of mixing at the surfaces of polluted white dwarfs is incomplete, because it only considers individual heavy atoms sinking through microscopic diffusion. In some cases, these heavy elements should reach concentrations sufficient to excite macroscopic fluid mixing due to the thermohaline instability, as shown below in the schematic from a review article by Pascale Garaud (2018). This extra mixing means that some polluted white dwarfs could be accreting more material at much faster rates than previously inferred.

Thermohaline Schematic

Compact Binaries and Mass Transfer

Artist's Impression of a Binary (image credit: University of Warwick / Mark Garlick)

  • Discovery of a Double-detonation Thermonuclear Supernova Progenitor
    Kupfer, Bauer, van Roestel, et al. ApJL, 925:L12, February 2022
  • Phases of Mass Transfer from Hot Subdwarfs to White Dwarf Companions and Their Photometric Properties
    Bauer & Kupfer ApJ, 922:245, December 2021
  • The Final Fates of Close Hot Subdwarf-White Dwarf Binaries: Mergers Involving He/C/O White Dwarfs and the Formation of Unusual Giant Stars with C/O-Dominated Envelopes
    Schwab & Bauer ApJ, 920:110, October 2021
  • A New Class of Roche Lobe-filling Hot Subdwarf Binaries
    Kupfer, Bauer, Burdge, et al. ApJL, 898:L25, July 2020
  • The First Ultracompact Roche Lobe-Filling Hot Subdwarf Binary
    Kupfer, Bauer, Marsh, et al. ApJ, 891:45, March 2020
  • Electron Captures on 14N as a Trigger for Helium Shell Detonations
    Bauer, Schwab, & Bildsten ApJ, 845:97, August 2017

Runaway Stars from Thermonuclear Supernovae

Animation of a stellar remnant getting blasted by supernova ejecta after donating enough material to its companion to trigger a thermonuclear detonation, made from a simulation by Chris White for this paper that we wrote together. The full repository of animations from that paper can be found here.

  • Masses of White Dwarf Binary Companions to Type Ia Supernovae Measured from Runaway Velocities
    Bauer, Chandra, Shen, & Hermes ApJL, 923:L34, December 2021
  • Remnants of Subdwarf Helium Donor Stars Ejected from Close Binaries with Thermonuclear Supernovae
    Bauer, White, & Bildsten ApJ, 887:68, December 2019