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 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 eff /K < 8000. Remarkably, all secondaries with 𝑇 eff 7000 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. Joint fitting of spectral energy distributions, light curves, radial velocities, and parallaxes constrains system parameters robustly. Most of the proto-WD secondaries have masses near 0.15 𝑀 ; their companions have masses near 0.8 𝑀 . 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 30 within a Hubble time, where they will become AM CVn binaries or merge. This sample triples the known evolved CV population, includes far hotter donors than previous studies, and offers broad opportunities for improving understanding of the compact binary population.

4FGL J1120.0-2204: A Unique Gamma-ray Bright Neutron Star Binary with an Extremely Low Mass Proto-White Dwarf

“…In recent years, ELM white dwarfs have seen increasing study outside of the context of MSP binaries (e.g., Brown et al 2010;Kilic et al 2011;Brown et al 2020), where surveys such as SDSS have allowed the selection of hot, high surface gravity objects with masses too low to have been formed except through close binary evolution. Gaia has allowed the efficient expansion of these studies to include somewhat cooler and more bloated stars (e.g., Pelisoli et al 2019;El-Badry et al 2021) which can be interpreted as "pre-ELM" stars (e.g., Maxted et al 2014), i.e., stripped stars still in the process of contracting to sit on a He white dwarf cooling track.…”