First physical proof of a new population of transitional binary stars:

[beecee]

https://phys.org/news/2021-12-astronomers-binary-star.html

Astronomers observe a new type of binary star long predicted to exist:

Researchers at the Center for Astrophysics | Harvard & Smithsonian have observed a new type of binary star that has long been theorized to exist. The discovery finally confirms how a rare type of star in the universe forms and evolves.

The new class of stars, described in this month's issue of the Monthly Notices of the Royal Astronomical Society, was discovered by postdoctoral fellow Kareem El-Badry using the Shane Telescope at Lick Observatory in California and data from several astronomical surveys.

"We have observed the first physical proof of a new population of transitional binary stars," says El-Badry. "This is exciting; it's a missing evolutionary link in binary star formation models that we've been looking for."

MORE AT LINK..............

the paper:

https://academic.oup.com/mnras/article-abstract/508/3/4106/6368873?redirectedFrom=fulltext

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 Porb < 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 < Teff/K < 8000. Remarkably, all secondaries with Teff≳7000KTeff≳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⊙0.15M⊙⁠; their companions have masses near 0.8M⊙0.8M⊙⁠. We infer a space density of ∼60kpc−3∼60kpc−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∼60Myr−1R∼60Myr−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 ≲ Porb 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.