The SNIa Runaway LP 398-9: Detection of Circumstellar Material and Surface Rotation

TL;DR

This paper reports the discovery of circumstellar material and surface rotation in LP 398-9, supporting its candidacy as a surviving donor star from a Type Ia supernova progenitor in a double-degenerate system.

Contribution

It provides observational evidence of circumstellar material and rotation in LP 398-9, strengthening the double-degenerate supernova progenitor scenario.

Findings

Detection of carbon-rich circumstellar material around LP 398-9.

Identification of a 15.4-hour surface rotation period.

Support for the double-degenerate SNIa progenitor channel.

Abstract

A promising progenitor scenario for Type Ia supernovae (SNeIa) is the thermonuclear detonation of a white dwarf in a close binary system with another white dwarf. After the primary star explodes, the surviving donor can be spontaneously released as a hypervelocity runaway. One such runaway donor candidate is LP 398-9, whose orbital trajectory traces back $\approx 10^5$ years to a known supernova remnant. Here we report the discovery of carbon-rich circumstellar material around LP 398-9, revealed by a strong infrared excess and analyzed with follow-up spectroscopy. The circumstellar material is most plausibly composed of inflated layers from the star itself, mechanically and radioactively heated by the past companion's supernova. We also detect a 15.4 hr periodic signal in the UV and optical light curves of LP 398-9, which we interpret as surface rotation. The rotation rate is consistent with theoretical predictions from this supernova mechanism, and the brightness variations could originate from surface inhomogeneity deposited by the supernova itself. Our observations strengthen the case for this double-degenerate SNIa progenitor channel, and motivate the search for more runaway SNIa donors.