8.9-hr Rotation in the Partly Burnt Runaway Stellar Remnant LP 40-365 (GD 492)

TL;DR

This paper reports the discovery of an 8.914-hour rotation period in the stellar remnant LP 40-365, providing insights into its surface inhomogeneity, magnetic field, and possible origin from a Type Iax supernova explosion.

Contribution

It is the first to identify the rotation period of LP 40-365 and links this to its progenitor scenario, suggesting it is a remnant of a disrupted white dwarf from an underluminous supernova.

Findings

Detected 8.914-hour variability in optical and ultraviolet light curves.

Surface inhomogeneity likely causes the observed variability.

Magnetic field strength is constrained to less than 20 kG.

Abstract

We report the detection of 8.914-hr variability in both optical and ultraviolet light curves of LP 40-365 (also known as GD 492), the prototype for a class of partly burnt runaway stars that have been ejected from a binary due to a thermonuclear supernova event. We first detected this 1.0% amplitude variation in optical photometry collected by the Transiting Exoplanet Survey Satellite. Re-analysis of observations from the Hubble Space Telescope at the TESS period and ephemeris reveal a 5.8% variation in the ultraviolet of this 9800 K stellar remnant. We propose that this 8.914-hr photometric variation reveals the current surface rotation rate of LP 40-365, and is caused by some kind of surface inhomogeneity rotating in and out of view, though a lack of observed Zeeman splitting puts an upper limit on the magnetic field of <20 kG. We explore ways in which the present rotation period can constrain progenitor scenarios if angular momentum was mostly conserved, which suggests that the survivor LP 40-365 was not the donor star but was most likely the bound remnant of a mostly disrupted white dwarf that underwent advanced burning from an underluminous (Type Iax) supernova.