White dwarf eccentricity fluctuation and dissipation by AGB convection

Yair Cohen, Sivan Ginzburg*, Maya Levy, Tal Bar Shalom, Yoav Siman Tov

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

Millisecond pulsars with white dwarf companions have typical eccentricities e ∼10 -6 -10 -3 . The eccentricities of helium white dwarfs are explained well by applying the fluctuation-dissipation theorem to convective eddies in their red giant progenitors. We extend this theory to more massive carbon-oxygen (CO) white dwarfs with asymptotic giant branch (AGB) progenitors. Due to the radiation pressure in AGB stars, the dominant factor in determining the remnant white dwarf's eccentricity is the critical residual hydrogen envelope mass m env required to inflate the star to giant proportions. Using a suite of MESA stellar evolution simulations with Δmc = 10 -3 Mcore-mass intervals, we resolved the AGB thermal pulses and found that the critical m env ∝ m -6 c . The resulting eccentricity e ∼3 ×10 -3 is almost independent of the remnant CO white dwarf's mass mc . Nearly all of the measured eccentricities lie below this robust theoretical limit, indicating that the eccentricity is damped during the common-envelope inspiral that follows the unstable Roche lobe o v erflow of the AGB star. Specifically, we focused on white dwarfs with median masses m c < 0 . 6 M. These massive white dwarfs begin their inspiral with practically identical orbital periods and eccentricities, eliminating any dependence on the initial conditions. For this sub-sample, we find an empirical relation e ∝ P 3 / 2 between the final period and eccentricity that is much tighter than previous studies -moti v ating theoretical work on the eccentricity evolution during the common envelope phase. The eccentricities of lower mass CO white dwarfs may be explained by alternative formation channels.

Original languageEnglish
Pages (from-to)455-464
Number of pages10
JournalMonthly Notices of the Royal Astronomical Society
Volume534
Issue number1
DOIs
StatePublished - 1 Oct 2024

Bibliographical note

Publisher Copyright:
© 2024 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society.

Keywords

  • binaries: general
  • pulsars: general
  • stars: AGB and post-AGB
  • white dwarfs

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