A Roche lobe-filling hot subdwarf and white dwarf binary: possible detection of an ejected common envelope

Abstract

Binaries consisting of a hot subdwarf star and an accreting white dwarf (WD) are sources of gravitational wave radiation at low frequencies and possible progenitors of type Ia supernovae if the WD mass is large enough. Here, we report the discovery of the third binary known of this kind: it consists of a hot subdwarf O (sdO) star and a WD with an orbital period of 3.495 hours and an orbital shrinkage of 0.1 s in 6 yr. The sdO star overfills its Roche lobe and likely transfers mass to the WD via an accretion disk. From spectroscopy, we obtain an effective temperature of Teff = 54 240 ± 1 840 K and a surface gravity of log g = 4.841 ± 0.108 for the sdO star. From the light curve analysis, we obtain a sdO mass of MsdO = 0.55 M⊙ and a mass ratio of q = MWD/MsdO = 0.738 ± 0.001. Also, we estimate that the disk has a radius of ∼0.41 R⊙ and a thickness of ∼0.18 R⊙. The origin of this binary is probably a common envelope ejection channel, where the progenitor of the sdO star is either an RGB star or, more likely, an early AGB star; the sdO star will subsequently evolve into a WD and merge with its WD companion, likely resulting in an R CrB star. The outstanding feature in the spectrum of this object is strong Ca H&K lines, which are blueshifted by ∼200 km s−1 and likely originate from the recently ejected common envelope, and we estimated that the remnant CE material in the binary system has a density ∼6 × 10−10 g cm−3.

He-star donor AM CVn stars and their progenitors as LISA sources

“…The most recently discovered system J1920-2001 (Li et al 2022), with M sdB ≈ 0.337 M and M WD ≈ 0.545 M , differs from the above-mentioned systems by a large P orb =3.4946 hr. The position of the subdwarf in the T e f f − log g diagram suggests that it is in the He-shell burning stage.…”

“…The position of the subdwarf in the T e f f − log g diagram suggests that it is in the He-shell burning stage. After completion of this stage, it will turn into a WD and merge with its companion within 1 Gyr, as estimated by Li et al (2022).…”

A Systematic Search for Short-period Close White Dwarf Binary Candidates Based on Gaia EDR3 Catalog and Zwicky Transient Facility Data

“…They are divided into two main spectroscopies: B-type subdwarfs (sdBs; a core helium-burning star and thin hydrogen envelope), and O-type subdwarfs (sdOs). Hot subdwarf binaries with white dwarf companions that exit the CE phase at orbital periods of less than 2 hr will overflow their Roche lobes while the sdB is still burning helium (Kramer et al 2020;Li et al 2022).…”

A New Route to Massive Hot Subdwarfs: Common Envelope Ejection from Asymptotic Giant Branch Stars

“…For example, hot subdwarfs from the RGB channel generally are He-deficient and thus cannot explain the He-rich hot subdwarfs 6 (Heber 2016). The situation has been aggravated due to the recent discovery of SMSS J1920 (Li et al 2022). SMSS J1920 is a Roche-lobe-filling sdO/B and WD binary with an orbital period of 3.495 hr, where the sdO/B star has a mass of ∼0.55 M e .…”

“…Therefore, the scenario conflicts with the deduction of the recent CE ejection event of SMSS J1920. Li et al (2022) then proposed a new formation channel of hot subdwarfs, i.e., a hot subdwarf produced from the CE ejection process with an asymptotic giant branch (AGB) star (the AGB CE channel). Different from the RGB star, an AGB star contains a large CO core, helium-/hydrogen-burning shells, and a hydrogen envelope.…”