Special TAPIR Seminar

Friday, August 4, 2023
2:00pm to 3:00pm
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Online and In-Person Event
Evolution of hot helium white dwarf binaries
Lucy McNeill, JSPS Fellow, Department of Physics, Kyoto University,

In person: 370 Cahill -- Attendees joining in person must have a valid Caltech UID.

To Join via Zoom: 864 8902 5566

ABSTRACT: The Laser-Interferometer-Space-Antenna (LISA) will be capable of detecting 10's of thousands of Galactic double white dwarf binaries with orbital periods <20 minutes. These binaries will reach Roche contact within the next ~Myr, after which they could merge to form e.g., an R Cor Bor star, or explode. Alternatively, if a binary's mass transfer is stable, it may never merge.

The current catalogue of detached white dwarf binaries containing at least one helium white dwarf (He WD) identified with Zwicky Transient Facility (ZTF) suggests that short-period binary components of these progenitor systems are hot (surface temperatures >15,000 K). Also, their radii can be more than twice as large as the "cold" mass-radius relation for completely degenerate white dwarfs (Eggleton 1983). This is also the case for the recent, first ever temperature measurement of the mass transferring donor white dwarf in a binary (Burdge et al. 2023). Taken together, these observations are not consistent with the assumption that white dwarfs become cold before the onset of mass transfer, used in earlier works to determine the conditions for dynamically unstable mass transfer (leading to mergers).

In this talk, I present a mass-radius-temperature relation for He WD consistent with both stellar modelling and ZTF observations (1). Next, by formulating a model for tidal friction sourcing the increasing surface luminosity, the coupled orbital frequency and temperature evolution for a detached He WD binary subject to energy losses from both gravitational wave emission and tidal heating (2) can be determined.

With both the (1) mass-radius-temperature relation and (2) coupled binary orbital frequency and temperature evolution in hand, possible temperature and radii of Galactic He WDs at the onset of mass transfer can be determined. Here, we derive a temperature dependent mass transfer stability criteria (Soberman et al. 1997) for He WD. Regardless of tidal heating efficiency, by using (1) instead of the cold relation we expect that a larger fraction of Galactic white dwarf binaries should merge.

For more information, please contact JoAnn Boyd by phone at 626-395-4280 or by email at joann@caltech.edu.