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 color-magnitude diagram with ZTF light curves showing large-amplitude ellipsoidal variability and orbital period $P_{\mathrm{orb}}<6$ hr. 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. 13 show evidence of ongoing mass transfer, which has likely just ceased in the other 8. Most of the secondaries are hotter than any previously known CV donors, with temperatures $4700<T_{\mathrm{eff}}/\mathrm{K}<8000$. Remarkably, all secondaries with $T_{\mathrm{eff}}\gtrsim 7000\mathrm{K}$ 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 $0.15M_{\odot }$; their companions have masses near $0.8M_{\odot }$. We infer a space density of $\sim 60\mathrm{kp}{\mathrm{c}}^{-3}$, roughly 80 times lower than that of normal CVs and three times lower than that of ELM WDs. The implied Galactic birth rate, $\mathcal{R}\sim 60\mathrm{My}{\mathrm{r}}^{-1}$, 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 $5\lesssim P_{\mathrm{orb}}/\min \lesssim 30$ 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.

26 pages, 16 figures, plus appendices. Magnitudes in Table 1 fixed; machine-readable table included in source files. Accepted to MNRAS

References in corpus (32)

• Modules for Experiments in Stellar Astrophysics (MESA)
• The Zwicky Transient Facility: System Overview, Performance, and First Results
• Modules for Experiments in Stellar Astrophysics (MESA): Pulsating Variable Stars, Rotation, Convective Boundaries, and Energy Conservation
• The Zwicky Transient Facility: Data Processing, Products, and Archive
• The All-Sky Automated Survey for Supernovae (ASAS-SN) Light Curve Server v1.0
• The Zwicky Transient Facility: Science Objectives
• The Catalina Surveys Periodic Variable Star Catalog
• A million binaries from Gaia eDR3: sample selection and validation of Gaia parallax uncertainties
• Faint Thermonuclear Supernovae from AM Canum Venaticorum Binaries
• The Donor Stars of Cataclysmic Variables
• unWISE: unblurred coadds of the WISE imaging
• Physics Of Eclipsing Binaries. II. Towards the Increased Model Fidelity
• New white dwarf stars in the Sloan Digital Sky Survey Data Release 10
• The ELM Survey. I. A Complete Sample of Extremely Low Mass White Dwarfs
• Every interacting double white dwarf binary may merge
• The ELM Survey. VIII. 98 Double White Dwarf Binaries
• The short period end of the contact binary period distribution based on the All Sky Automated Survey (ASAS)
• The timescale of low-mass proto-helium white dwarf evolution
• Completeness of the Gaia-verse II: what are the odds that a star is missing from Gaia DR2?
• Gaia Data Release 2 Catalogue of Extremely-low Mass White Dwarf Candidates
• Year 1 of the ZTF high-cadence Galactic Plane Survey: Strategy, goals, and early results on new single-mode hot subdwarf B-star pulsators
• A Gaia early DR3 mock stellar catalog: Galactic prior and selection function
• ULTRACAM observations of SDSS J170213.26+322954.1 - an eclipsing cataclysmic variable in the period gap
• Selection Functions in Astronomical Data Modeling, with the Space Density of White Dwarfs as Worked Example
• A Trip to the Cataclysmic Binary Zoo: Detailed Follow-Up of 35 Recently-Discovered Systems
• LAMOST J0140355+392651: An evolved cataclysmic variable donor transitioning to become an extremely low mass white dwarf
• SDSSJ001153.08-064739.2, a cataclysmic variable with an evolved donor in the period gap
• An evolved donor star in the long-period cataclysmic variable HS 0218+3229
• Hot Subdwarf All Southern Sky Fast Transit Survey with the Evryscope
• BF Eridani: a cataclysmic variable with a massive white dwarf and an evolved secondary
• ASAS-SN 13cl : A Newly-Discovered Cataclysmic Binary with an Anomalously Warm Secondary
• Informing the Cataclysmic Variable Donor Sequence from Gaia DR2 Color-Magnitude and Inferred Variability Metrics

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