Discovery of a double white dwarf in the Galactic globular cluster NGC 6397

TL;DR

This paper reports the discovery of a double white dwarf binary system in the core of globular cluster NGC 6397, providing insights into binary populations and stellar evolution in dense stellar environments.

Contribution

First detection of a double white dwarf binary in NGC 6397 using MUSE spectroscopy, revealing details about its orbit, mass, and potential for triple system configuration.

Findings

Binary system consists of two white dwarfs with a 0.54-day orbit.

The primary white dwarf has an effective temperature of 16000 K.

The system's minimum companion mass is 0.78 solar masses.

Abstract

Binaries in the cores of globular clusters are known to prevent the gravitational collapse of the cluster, and simulations predict that the core of NGC 6397 contains a large number of white dwarfs (WDs), of which many are expected to be part of a binary system. In this work, we report the discovery of a compact binary system consisting of two WDs in the centre of the Galactic globular cluster NGC 6397. The system, known in the literature as NF1, was observed as part of a MUSE radial-velocity survey aiming at characterizing the binary population in the centre of NGC 6397. The spectral analysis of NF1 provides an effective temperature of 16000 K and a surface gravity (log g) of 5.72 (cgs), which is consistent with an extremely low-mass He-core WD nature. This is further supported by the mass of 0.23 +/- 0.03 Msun obtained from fitting the star's spectral energy distribution using its HST magnitude in various filters. The system has a circular orbit with a period of 0.54 days. The radial velocities show a large semi-amplitude of 200 km/s, implying a minimum mass of 0.78 Msun for the invisible companion, which is likely another WD, or a neutron star if the inclination of the system is smaller than about 50 deg. Some significant residuals in radial velocity remain with our best orbital solution and we tested whether a model with a third body can explain these deviations. While this possibility seems promising, additional measurements are needed to confirm whether the star is actually part of a triple system.