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

TL;DR

This study identifies NF1 AB as a compact double-white-dwarf binary at the core of the globular cluster NGC 6397. By combining 19 MUSE spectra with HST photometry, the visible component NF1 B is constrained as an extremely-low-mass He-core WD with $T_{ m eff} \approx 1.6\times 10^4$ K, $\log g \approx 5.72$, and $M \approx 0.23\,M_\odot$, while the unseen companion NF1 A has a minimum mass $M_2 \sin i \approx 0.78\,M_\odot$, consistent with a second WD (or, for low inclination, a neutron star). The orbit is short and nearly circular, with $p \approx 0.5435$ days and $K \approx 202$ km s$^{-1}$, implying a WD companion of substantial mass. The results align NF1 AB with Roche-lobe overflow–formed ELM binaries, highlighting a rich, previously hidden population of WD binaries in globular cluster cores and their role in cluster dynamics and gravitational-wave source populations. Additional data are needed to confirm a possible third body as a source of RV residuals.

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.

Figures (10)

• Figure 1: Best fit of the combined MUSE spectrum of NF1 AB. The observed spectrum is combined from 19 single spectra with a total exposure time of 4.2 hours.
• Figure 2: Radial velocities of NF1 B observed in 2023 and predicted radial velocity curves computed from 100 parameter sets randomly drawn from the posterior. The residuals are computed relative to the median prediction. The first observation started on JD 2460082.68297265 (May 18 2023 4:23 UTC).
• Figure 3: SED fit of NF1 B. On the y-axis, we plot the flux $f_{\lambda}$ multiplied by $\lambda^2$. The best-fit model is shown in grey and the observed magnitudes in the various filters are indicated along with their central wavelength. The wavelength coverage of each filter is indicated with gray horizontal bars. The bottom panel shows the uncertainty-weighted residuals $\chi$ = (mag$_{\rm model}$-mag$_{\rm observed}$) / uncertainty.
• Figure 4: Position of NF1 B in the log $g$$-$$T_\text{eff}$ diagram (top) and log $R$$-$$T_\text{eff}$ diagram (bottom) compared to selected model tracks including rotation and diffusion from istrate_models_2016. The labels along the tracks give the stellar mass in solar masses. The grey points are Milky Way ELMs from the survey of brown_elm_2020.
• Figure 5: Mass of NF1 B versus its orbital period compared to those of ELM binaries in the Galactic field brown_elm_2020.