@article{9f6b9df8c83a453ca0e14d588841d047,
title = "The importance of Urca-process cooling in accreting ONe white dwarfs",
abstract = "We study the evolution of accreting oxygen-neon (ONe) white dwarfs (WDs), with a particular emphasis on the effects of the presence of the carbon-burning products 23Na and 25Mg. These isotopes lead to substantial cooling of the WD via the 25Mg-25Na, 23Na-23Ne and 25Na- 25Ne Urca pairs. We derive an analytic formula for the peak Urca-process cooling rate and use it to obtain a simple expression for the temperature to which the Urca process cools the WD. Our estimates are equally applicable to accreting carbon-oxygen WDs. We use the Modules for Experiments in Stellar Astrophysics (MESA) stellar evolution code to evolve a suite of models that confirm these analytic results and demonstrate that Urca-process cooling substantially modifies the thermal evolution of accreting ONe WDs. Most importantly, we show that MESA models with lower temperatures at the onset of the 24Mg and 24Na electron captures develop convectively unstable regions, even when using the Ledoux criterion. We discuss the difficulties that we encounter in modelling these convective regions and outline the potential effects of this convection on the subsequent WD evolution. For models in which we do not allow convection to operate, we find that oxygen ignites around a density of log(ρc/g cm-3) ≈ 9.95, very similar to the value without Urca cooling. Nonetheless, the inclusion of the effects of Urca-process cooling is an important step in producing progenitor models with more realistic temperature and composition profiles which are needed for the evolution of the subsequent oxygen deflagration and hence for studies of the signature of accretion-induced collapse.",
keywords = "Stars: evolution, White dwarfs",
author = "Josiah Schwab and Lars Bildsten and Eliot Quataert",
note = "Funding Information: We thank Evan Bauer, Jared Brooks, Rob Farmer, Daniel Lecoanet, Ken{\textquoteright}ichi Nomoto, Bill Paxton, Philipp Podsiadlowski, Frank Timmes and Bill Wolf for useful discussions. We thank Toshio Suzuki for providing machine-readable versions of the tables from Suzuki et al. (2016). We thank the anonymous referee for a helpful report. We acknowledge stimulating workshops at Sky House where these ideas germinated. Support for this work was provided by the National Aeronautics and Space Administration (NASA) through Hubble Fellowship grant # HST-HF2-51382.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. JS was also supported by the National Science Foundation (NSF) Graduate Research Fellowship Program under grant DGE-1106400 and by NSF grant AST-1205732. LB is supported by the NSF under grant PHY 11-25915. This research is funded in part by the Gordon and Betty Moore Foundation through Grant GBMF5076 to LB and EQ. EQ is supported in part by a Simons Investigator award from the Simons Foundation and the David and Lucile Packard Foundation. This research used the Savio computational cluster resource provided by the Berkeley Research Computing program at the University of California, Berkeley (supported by the UC Berkeley Chancellor, Vice Chancellor for Research and Chief Information Officer). This research has made use of NASA{\textquoteright}s Astrophysics Data System. Funding Information: We thank Evan Bauer, Jared Brooks, Rob Farmer, Daniel Lecoanet, Ken'ichi Nomoto, Bill Paxton, Philipp Podsiadlowski, Frank Timmes and Bill Wolf for useful discussions. We thank Toshio Suzuki for providing machine-readable versions of the tables from Suzuki et al. (2016). We thank the anonymous referee for a helpful report.We acknowledge stimulatingworkshops at SkyHousewhere these ideas germinated. Support for this work was provided by the National Aeronautics and Space Administration (NASA) through Hubble Fellowship grant # HST-HF2-51382.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. JS was also supported by the National Science Foundation (NSF) Graduate Research Fellowship Program under grant DGE-1106400 and by NSF grant AST-1205732. LB is supported by the NSF under grant PHY 11-25915. This research is funded in part by the Gordon and Betty Moore Foundation through Grant GBMF5076 to LB and EQ. EQ is supported in part by a Simons Investigator award from the Simons Foundation and the David and Lucile Packard Foundation. This research used the Savio computational cluster resource provided by the Berkeley Research Computing program at the University of California, Berkeley (supported by the UC Berkeley Chancellor, Vice Chancellor for Research and Chief Information Officer). This research has made use of NASA's Astrophysics Data System. Publisher Copyright: {\textcopyright} 2017 The Authors.",
year = "2017",
doi = "10.1093/MNRAS/STX2169",
language = "English (US)",
volume = "472",
pages = "3390--3406",
journal = "Monthly Notices of the Royal Astronomical Society",
issn = "0035-8711",
publisher = "Oxford University Press",
number = "3",
}