A second candidate magnetic helium core white dwarf and 3 other variable white dwarfs in the globular cluster NGC 6397

TL;DR

This study reports the discovery of four variable low-mass white dwarfs in NGC 6397, including a potential magnetic helium core white dwarf with a measured rotation period, emphasizing the value of high-cadence surveys in dense stellar regions.

Contribution

First identification of a candidate magnetic helium core white dwarf with a measured rotation period in NGC 6397, expanding understanding of white dwarf variability in globular clusters.

Findings

Discovery of four variable low-mass white dwarfs in NGC 6397.

Identification of a potential magnetic helium core white dwarf with a 5.21-hour period.

Detection of aperiodic variability in three other white dwarf candidates.

Abstract

Using archival Hubble Space Telescope observations, we report the discovery of four variable low-mass white dwarfs ($0.18 \, M_\odot \leq M \leq 0.5 \,M_\odot$) in the globular cluster NGC 6397. One source exhibits a periodic optical modulation of $5.21 \pm 0.02$ hours, which we interpret as potentially due to the rotation of a magnetic helium core WD (He WD). This makes this candidate the second magnetic He WD in NGC 6397, and one of the few He WDs with a measured rotation period. The other three candidates show aperiodic variability, with a change in magnitude ranging from $\sim 0.11-0.6$. These discoveries highlight the importance of high-cadence photometric surveys in dense stellar environments. Follow-up spectroscopic observations are needed to confirm the nature of the variability of these systems.