Birth of the ELMs: a ZTF survey for evolved cataclysmic variables turning into extremely low-mass white dwarfs
Abstract
We present a systematic survey for mass-transferring and recently detached cataclysmic variables (CVs) with evolved secondaries, which are progenitors of extremely low mass white dwarfs (ELM WDs), AM CVn systems, and detached ultracompact binaries. We select targets below the main sequence in the Gaia colour–magnitude diagram with ZTF light curves showing large-amplitude ellipsoidal variability and orbital period P_(orb) < 6 h. This yields 51 candidates brighter than G = 18, of which we have obtained many-epoch spectra for 21. We confirm all 21 to be completely or nearly Roche lobe filling close binaries. Thirteen show evidence of ongoing mass transfer, which has likely just ceased in the other eight. Most of the secondaries are hotter than any previously known CV donors, with temperatures 4700 < T_(eff)/K < 8000. Remarkably, all secondaries with T_(eff) ≳ 7000K appear to be detached, while all cooler secondaries are still mass-transferring. This transition likely marks the temperature where magnetic braking becomes inefficient due to loss of the donor's convective envelope. Most of the proto-WD secondaries have masses near 0.15M⊙; their companions have masses near 0.8M⊙. We infer a space density of ∼60 kpc⁻³, roughly 80 times lower than that of normal CVs and three times lower than that of ELM WDs. The implied Galactic birth rate, R ∼ 60 Myr⁻¹, is half that of AM CVn binaries. Most systems are well-described by mesa models for CVs in which mass transfer begins only as the donor leaves the main sequence. All are predicted to reach minimum periods 5 ≲ P_(orb) min⁻¹ ≲ 30 within a Hubble time, where they will become AM CVn binaries or merge. This sample triples the known evolved CV population and offers broad opportunities for improving understanding of the compact binary population.
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 the anonymous referee for constructive comments, Boris Gänsicke, Tom Marsh, Ingrid Pelisoli, and John Thorstensen for helpful discussions, and Geoff Tabin and In-Hei Hahn for their hospitality during the writing of this paper. We thank the staff at Lick observatory for their assistance in obtaining follow-up spectra. Based on observations obtained with the Samuel Oschin 48-inch Telescope at the Palomar Observatory as part of the ZTF project. ZTF is supported by the National Science Foundation under Grant No. AST-1440341 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Centre at Stockholm University, the University of Maryland, the University of Washington, Deutsches Elektronen-Synchrotron and Humboldt University, Los Alamos National Laboratories, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, and Lawrence Berkeley National Laboratories. Operations are conducted by COO, IPAC, and UW. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. DATA AVAILABILITY. The data underlying this article are available upon reasonable request to the corresponding author. For convenience, we include as supplemental data a digital table that combines all data from Tables 1–5.Errata
Due to a typographical error by the authors, the reported G-band apparent magnitudes in table 1 of El-Badry et al. (2021) are incorrect. This has no effect on the paper's conclusions, because the correct values were used in all calculations. Other columns and tables in the paper were not affected. The magnitudes reported in the machine-readable version of the table were also correct. Table 1 lists the corrected magnitudes.Additional details
- Eprint ID
- 118715
- Resolver ID
- CaltechAUTHORS:20230105-895186000.38
- AST-1440341
- NSF
- ZTF partner institutions
- Gaia Multilateral Agreement
- Created
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2023-01-08Created from EPrint's datestamp field
- Updated
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2023-01-08Created from EPrint's last_modified field
- Caltech groups
- Zwicky Transient Facility