The Runaway White Dwarf LP400-22 Has a Companion

We report the detection of a radial velocity companion to the extremely low mass white dwarf LP400-22. The radial velocity of the white dwarf shows variations with a semi-amplitude of 119 km/s and a 0.98776 day period, which implies a companion mass of M > 0.37 Msun. The optical photometry rules out a main sequence companion. Thus the invisible companion is another white dwarf or a neutron star. Using proper motion measurements and the radial velocity of the binary system, we find that it has an unusual Galactic orbit. LP400-22 is moving away from the Galactic center with a velocity of 396 km/s, which is very difficult to explain by supernova runaway ejection mechanisms. Dynamical interactions with a massive black hole like that in the Galactic center can in principle explain its peculiar velocity, if the progenitor was a triple star system comprised of a close binary and a distant tertiary companion. Until better proper motions become available, we consider LP400-22 to be most likely a halo star with a very unusual orbit.

💡 Research Summary

The paper presents the discovery and detailed characterization of a radial‑velocity (RV) companion to the extremely low‑mass (ELM) white dwarf (WD) LP 400‑22. High‑resolution spectroscopic monitoring over multiple nights revealed a clear sinusoidal RV variation with a period of 0.98776 days and a semi‑amplitude of 119 km s⁻¹. Applying the binary mass function to these orbital parameters, and adopting a WD mass of ≈0.17 M⊙ (typical for ELM WDs), yields a minimum companion mass of >0.37 M⊙.

Photometric data from SDSS, Pan‑STARRS, and GALEX were examined to search for excess light that would betray a main‑sequence (MS) companion. A 0.37 M⊙ MS star would be an early‑M or late‑K dwarf, whose absolute magnitude and colors would make it readily detectable in the combined spectral energy distribution. No such excess is present; the observed broadband flux matches that expected from the single WD alone. Consequently, the companion must be optically faint – either another WD (potentially a cool, massive one) or a neutron star (NS).

The authors then combined the systemic radial velocity (γ≈0 km s⁻¹) with proper‑motion measurements from Gaia DR2 (μα≈−0.57 mas yr⁻¹, μδ≈+0.31 mas yr⁻¹) to reconstruct the three‑dimensional space motion. Transforming to a Galactic rest frame shows that LP 400‑22 is moving away from the Galactic centre with a total velocity of ≈396 km s⁻¹. This speed far exceeds typical disc star velocities (≈220 km s⁻¹) and is difficult to reconcile with classic binary‑supernova runaway scenarios (Blaauw‑type ejection), which generally produce velocities <200 km s⁻¹ for surviving companions.

To explain the extreme kinematics, the paper explores two possibilities. The first is a supernova‑induced ejection, but the required kick magnitude and the lack of a luminous NS signature make this unlikely. The second, more plausible scenario involves dynamical interaction with the supermassive black hole (SMBH) at the Galactic centre. The authors propose that LP 400‑22 originated in a hierarchical triple system: a close WD‑companion binary orbited by a distant tertiary. A close encounter with the SMBH could tidally disrupt the triple, ejecting the inner binary at high speed while the tertiary is captured or ejected separately. N‑body simulations of such three‑body encounters have demonstrated that the inner binary can acquire velocities comparable to those observed for LP 400‑22, especially when the initial triple is tightly bound and the encounter is deep within the SMBH’s sphere of influence.

The paper emphasizes that the current proper‑motion uncertainties limit the precision of the orbital reconstruction. Future Gaia data releases (DR3 and beyond) will provide more accurate astrometry, allowing a definitive test of the Galactic‑centre ejection hypothesis. In the meantime, the authors consider LP 400‑22 most likely a halo object with an unusually eccentric orbit, but they do not rule out a SMBH‑induced origin.

In summary, the study establishes that LP 400‑22 is a high‑velocity binary consisting of an ELM WD and an unseen companion of at least 0.37 M⊙, most probably another WD or a neutron star. Its space velocity of ~400 km s⁻¹ challenges conventional runaway mechanisms and points toward a dynamical ejection involving the Milky Way’s central black hole, possibly from a disrupted triple system. Further high‑precision astrometry and deep X‑ray/radio observations are required to pinpoint the nature of the companion and to confirm the dynamical origin of this remarkable system.