Partly burnt runaway stellar remnants from peculiar thermonuclear supernovae

• Published in: Monthly notices of the Royal Astronomical Society Vol. 489; no. 2; pp. 1489 - 1508
• Main Authors: Raddi, R, Hollands, M A, Koester, D, Hermes, J J, Gänsicke, B T, Heber, U, Shen, K J, Townsley, D M, Pala, A F, Reding, J S, Toloza, O F, Pelisoli, I, Geier, S, Gentile Fusillo, N P, Munari, U, Strader, J
• Format: Journal Article
• Language: English
• Published: United Kingdom Oxford University Press 21-10-2019
• Subjects: subdwarfs; white dwarfs; Galaxy: kinematics and dynamics; supernovae: general; stars: individual: LP 40−365
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stz1618

Abstract
We report the discovery of three stars that, along with the prototype LP 40−365, form a distinct class of chemically peculiar runaway stars that are the survivors of thermonuclear explosions. Spectroscopy of the four confirmed LP 40−365 stars finds ONe-dominated atmospheres enriched with remarkably similar amounts of nuclear ashes of partial O- and Si-burning. Kinematic evidence is consistent with ejection from a binary supernova progenitor; at least two stars have rest-frame velocities indicating they are unbound to the Galaxy. With masses and radii ranging between 0.20 and 0.28 M$\odot$ and between 0.16 and 0.60 R$\odot$, respectively, we speculate these inflated white dwarfs are the partly burnt remnants of either peculiar Type Iax or electron-capture supernovae. Adopting supernova rates from the literature, we estimate that ∼20 LP 40−365 stars brighter than 19 mag should be detectable within 2 kpc from the Sun at the end of the Gaia mission. We suggest that as they cool, these stars will evolve in their spectroscopic appearance, and eventually become peculiar O-rich white dwarfs. Finally, we stress that the discovery of new LP 40−365 stars will be useful to further constrain their evolution, supplying key boundary conditions to the modelling of explosion mechanisms, supernova rates, and nucleosynthetic yields of peculiar thermonuclear explosions.