A proto-helium white dwarf stripped by a substellar companion

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A proto-helium white dwarf stripped by a substellar companion via common-envelope ejection - Uncovering the true nature of a candidate hypervelocity B-type star

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In the past, SDSS J160429.12+100002.2 was spectroscopically classified as a blue horizontal branch (BHB) star. Assuming a luminosity that is characteristic of BHB stars, the object's radial velocity and proper motions from Gaia Early Data Release 3 would imply that its Galactic rest-frame velocity exceeds its local escape velocity. Consequently, the object would be considered a hypervelocity star, which would prove particularly interesting because its Galactic trajectory points in our direction. However, based on the spectroscopic analysis of follow-up observations, we show that the object is actually a short-period (P≈3.4 h) single-lined spectroscopic binary system with a visible B-type star (effective temperature Teff=15840±160 K and surface gravity log(g)=4.86±0.04) that is less luminous than typical BHB stars. Accordingly, the distance of the system is lower than originally thought, which renders its Galactic orbit bound to the Galaxy. Nevertheless, it is still an extreme halo object on a highly retrograde orbit. The abundances of He, C, N, O, Ne, Mg, Al, Si, S, and Ca are subsolar by factors from 3 to more than 100, while Fe is enriched by a factor of about 6. This peculiar chemical composition pattern is most likely caused by atomic diffusion processes. Combining constraints from astrometry, orbital motion, photometry, and spectroscopy, we conclude that the visible component is an unevolved proto-helium white dwarf with a thin hydrogen envelope that was stripped by a substellar companion through common-envelope ejection. Its unique configuration renders the binary system an interesting test bed for stellar binary evolution in general and common-envelope evolution in particular.

Mass range for the secondary, 0.03-0.06 M_Sol (=31-63 M_Jup).

Here is another possible substellar (if not truly planetary) companion around a B subdwarf:

Hard X-ray Emission Associated with White Dwarfs. IV. Signs of Accretion from Sub-stellar Companions

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KPD 0005+5106, with an effective temperature of ≃200,000 K, is one of the hottest white dwarfs (WDs). ROSAT unexpectedly detected "hard" (∼1 keV) X-rays from this apparently single WD. We have obtained Chandra observations that confirm the spatial coincidence of this hard X-ray source with KPD 0005+5106. We have also obtained XMM-Newton observations of KPD 0005+5106, as well as PG 1159−035 and WD 0121−756, which are also apparently single and whose hard X-rays were detected by ROSAT at 3σ-4σ levels. The XMM-Newton spectra of the three WDs show remarkably similar shapes that can be fitted by models including a blackbody component for the stellar photospheric emission, a thermal plasma emission component, and a power-law component. Their X-ray luminosities in the 0.6−3.0 keV band range from 4×1029 to 4×1030 erg~s−1. The XMM-Newton EPIC-pn soft-band (0.3−0.5 keV) lightcurve of KPD 0005+5106 is essentially constant, but the hard-band (0.6−3.0 keV) lightcurve shows periodic variations. An analysis of the generalized Lomb-Scargle periodograms for the XMM-Newton and Chandra hard-band lightcurves finds a convincing modulation (false alarm probability of 0.41%) with a period of 4.7±0.3 hr. Assuming that this period corresponds to a binary orbital period, the Roche radii of three viable types of companion have been calculated: M9V star, T brown dwarf, and Jupiter-like planet. Only the planet has a size larger than its Roche radius, although the M9V star and T brown dwarf may be heated by the WD and inflate past the Roche radius. Thus, all three types of companion may be donors to fuel accretion-powered hard X-ray emission.

A substellar nature for the secondary seems to be favoured.

A little old piece of news (2020) but worth of mention: the "vampire star" KSN:BS-C11a (or EPIC 203830112), with a dedicated NASA article

NASA's Kepler Witnesses Vampire Star System Undergoing Super-Outburst

Discovery paper here: Discovery of a new WZ Sagittae-type cataclysmic variable in the Kepler/K2 data

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We identify a new, bright transient in the K2/Kepler Campaign 11 field. Its light curve rises over 7 mag in a day and then declines 3 mag over a month before quickly fading another 2 mag. The transient was still detectable at the end of the campaign. The light curve is consistent with a WZ Sge-type dwarf nova outburst. Early superhumps with a period of 82 min are seen in the first 10 days and suggest that this is the orbital period of the binary, which is typical for the WZ Sge class. Strong superhump oscillations develop 10 days after peak brightness with periods ranging between 83 and 84 min. At 25 days after the peak brightness a bump in the light curve appears to signal a subtle rebrightening phase implying that this was an unusual type-A outburst. This is the only WZ Sge-type system observed by K2/Kepler during an outburst. The early rise of this outburst is well fitted with a broken power law. In first 10 h, the system brightened linearly and then transitioned to a steep rise with a power-law index of 4.8. Looking at archival K2/Kepler data and new TESS observations, a linear rise in the first several hours at the initiation of a superoutburst appears to be common in SU Ursa Majoris stars.

It reminds me the case of EF Eridani. Although WD mass is not reported, with q = 0.07 the companion is certainly substellar even with a massive 1 M_Sol white dwarf primary. The most likely value has got to be around 42-50 M_Jup assuming a canonical 0.6-0.7 M_Sol WD primary.

Note: tongue twisting "KSN:BS-C11a" has easier alternatives: EPIC 203830112 or UCAC3 131-193943.

Close detached white dwarf + brown dwarf binaries: further evidence for low values of the common envelope efficiency

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Common envelope evolution is a fundamental ingredient in our understanding of the formation of close binary stars containing compact objects which includes the progenitors of type Ia supernovae, short gamma ray bursts and most stellar gravitational wave sources. To predict the outcome of common envelope evolution we still rely to a large degree on a simplified energy conservation equation. Unfortunately, this equation contains a theoretically rather poorly constrained efficiency parameter (αCE) and, even worse, it is unclear if energy sources in addition to orbital energy (such as recombination energy) contribute to the envelope ejection process. In previous works we reconstructed the evolution of observed populations of post common envelope binaries (PCEBs) consisting of white dwarfs with main sequence star companions and found indications that the efficiency is rather small (αCE≃0.2−0.3) and that extra energy sources are only required in very few cases. Here we used the same reconstruction tool to investigate the evolutionary history of a sample of observed PCEBs with brown dwarf companions. In contrast to previous works, we found that the evolution of observationally well characterized PCEBs with brown dwarf companions can be understood assuming a low common envelope efficiency (αCE=0.24−0.41), similar to that required to understand PCEBs with main sequence star companions, and that contributions from recombination energy are not required. We conclude that the vast majority of PCEBs form from common envelope evolution that can be parameterized with a small efficiency and without taking into account additional energy sources.

Described objects are the following: SDSS J1411+2009, SDSS J1205-0242, WD 1032+011, ZTF J0038+2030, WD 0137-349, NLTT 5306, SDSS J1557+0916 and EPIC 212235321. Some of these are already listed in EPE, others aren't.

Planetary-mass white dwarfs or very, very strange degenerate, quark-made substellar objects? That is the question... (personally I find the latter option more likely). The "diamond" pulsar planet PSR J1719-14 b is mentioned as possible example. Just to further complicate IAU for the definition of "exoplanet"

Searching for Strange Quark Matter Objects Among White Dwarfs

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The ground state of matter may be strange quark matter (SQM), not hadronic matter. A whole sequence of SQM objects, ranging from strange quark stars and strange quark dwarfs to strange quark planets, can stably exist according to this SQM hypothesis. A strange dwarf has a mass similar to that of a normal white dwarf but could harbor an extremely dense SQM core (with a density as large as ∼4×1014gcm−3) at the center so that its radius can be correspondingly smaller. In this study, we try to search for strange dwarfs among the observed "white dwarfs" by considering their difference in the mass-radius relation. Seven strange dwarf candidates are identified in this way, whose masses are in the range of ∼0.02 -- 0.12M⊙, with the radii narrowly distributed in ∼ 9,000 -- 15,000 km. The seven objects are LSPM J0815+1633, LP 240-30, BD+20 5125B, LP 462-12, WD J1257+5428, 2MASS J13453297+4200437, and SDSS J085557.46+053524.5. Compared with white dwarfs of similar mass, these candidates are obviously smaller in radius. Further observations with large radio/infrared/optical telescopes on these interesting candidates are solicited.

Candidates are LP 462-12, WD J1257+5428, 2MASS J13453297+4200437 and SDSS J085557.46+053524.5, the last one being the least massive (m = 0.02 M_Sol) and binary too (with M4 dwarf companion).