2025MNRAS.541.2231H

SDSS J163712.21 + 363155.9 is a candidate hyper-runaway star, first identified from its unusual spectrum in the Sloan Digital Sky Survey, which exhibits oxygen, magnesium, and silicon lines redshifted by several 100 km s$^{-1}$, leading to the suggestion it was ejected from a thermonuclear supernova. We have acquired GTC (Gran Telescopio CANARIAS) OSIRIS (Optical System for Imaging and low-Intermediate-Resolution Integrated Spectroscopy) spectroscopy of SDSS J1637 + 3631 establishing a warm (${T_\mathrm{eff}}=15\, 680\pm 250$ K) carbon+oxygen-dominated atmosphere, that is also abundant in the intermediate-mass elements silicon, sulphur, and calcium. We interpret SDSS J1637 + 3631 as the donor to an accreting white dwarf that exploded in a dynamically driven double-degenerate double-detonation (D$^6$) type Ia supernova, where the current composition is consistent with a CO white dwarf core, enriched with intermediate-mass elements from deposited supernova ejecta. While SDSS J1637 + 3631 has a low-precision Gaia parallax, our spectroscopic surface gravity ($\log g=6.3\pm 0.3$ dex) helps constrain its tangential velocity to $1950_{-530}^{+810}$ km s$^{-1}$, providing additional support to the D$^6$ mechanism. Under the assumption that SDSS J1637 + 3631 is a D$^6$ survivor, we construct a kinematic model combining all astrometric, spectroscopic, and photometric information, but also including the structure and gravitational potential of the Milky Way. Our model localizes the ejection site to the inner few kpc of the Galactic disc (though excluding the Galactic centre), with an ejection speed of $1870_{-300}^{+360}$ km s$^{-1}$, and a $4.5_{-0.5}^{+0.4}$ Myr time of flight.