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Published August 2021 | public
Journal Article

LAMOST J0140355 + 392651: an evolved cataclysmic variable donor transitioning to become an extremely low-mass white dwarf


We present LAMOST J0140355 + 392651 (hereafter J0140), a close (P_(orb) = 3.81 h) binary containing a bloated, low-mass (⁠M ≈ 0.15 M_⊙⁠) proto-white dwarf (WD) and a massive (⁠M ≈ 0.95 M_⊙⁠) WD companion. The system's optical light curve is dominated by large-amplitude ellipsoidal variability but also exhibits additional scatter, likely driven by pulsations. The proto-WD is cooler (T_(eff) = 6800 ± 100 K) and more puffy (⁠log[g/(cm s⁻²)] = 4.74 ± 0.07⁠) than any known extremely low-mass (ELM) WD, but hotter than any known cataclysmic variable (CV) donor. It either completely or very nearly fills its Roche lobe (⁠R/R_(Roche lobe) = 0.99 ± 0.01⁠), suggesting ongoing or recently terminated mass transfer. No dwarf nova-like outbursts have been observed. The spectrum is dominated by the proto-WD but shows tentative hints of H α emission, perhaps due to accretion on to the massive WD. The properties of the system are well-matched by mesa binary evolution models of CVs with donors that underwent significant nuclear evolution before the onset of mass transfer. In these models, the bloated proto-WD is either still losing mass via stable Roche lobe overflow or was doing so until very recently. In either case, it is evolving towards higher temperatures at near-constant luminosity to become an ELM WD. If the system is detached, mass transfer likely ended when the donor became too hot for magnetic braking to remain efficient. Evolutionary models predict that the binary will shrink to P_(orb) ≲ 10 min within a few Gyr, when it will either merge or become an AM CVn binary. J0140 provides an observational link between the formation channels of CVs, ELM WDs, detached ultracompact WD binaries, and AM CVn systems.

Additional Information

© 2021 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model). We thank Boris Gänsicke and Tom Marsh for helpful comments. We are grateful to the staff at Lick Observatory for their assistance in obtaining the Kast spectra, and to the developers of pypeit and phoebe for maintaining and making public the codes. KE acknowledges support from an NSF graduate research fellowship and a Hellman fellowship from UC Berkeley. This research benefited from meetings supported by the Gordon and Betty Moore Foundation through grant GBMF5076. EQ was supported in part by a Simons Investigator award from the Simons Foundation. KJS received support for this work from NASA through the Astrophysics Theory Program (NNX17AG28G). We thank Geoff Tabin and In-Hei Hahn for their hospitality during the writing of this manuscript. DATA AVAILABILITY. Data in this paper are available upon reasonable request to the corresponding author.

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August 22, 2023
October 24, 2023