A Mass Transferring Brown Dwarf Binary on a 57 Minute Orbit

Abstract

Mass transfer in stellar binaries has been well studied in most stellar mass ranges, with the notable exception of ultracool stars and substellar brown dwarfs. We report the discovery of ZTF J1239+8347 with the Zwicky Transient Facility (ZTF), a brown dwarf binary currently undergoing stable mass transfer with an orbital period of 57.41 minutes. Optical time-series photometry reveals an extremely high amplitude ($> 2$ magnitude peak-to-trough) variability at short wavelengths indicative of an orbiting hot spot slightly buried inside the atmosphere of the accretor. We use parallax measurements from \textit{Gaia} along with optical and near infrared spectra to infer an accretion temperature of $T_\mathrm{eff} = 8904 \pm 54$ K, an atmospheric temperature of the accretor of $T_\mathrm{atmo} \approx 1500$ K, and a slightly inflated accretor radius of $R_{\rm acc} = 1.20^{+0.15}_{-0.11} \, \RJup$. ZTF J1239+8347 is a direct impact accretor, typically only seen in double degenerate white dwarf binaries, which are approximately a million times denser than the components in ZTF J1239+8347. The existence of an accreting brown dwarf binary suggests that angular momentum loss can be strong enough to make ultracool binaries interact in a Hubble time. The observed faintness ($\sim 20$ mag) and relative proximity ($\approx 300$ pc) of ZTF J1239+8347 suggests that many similar systems are likely to be found by the upcoming Rubin Observatory Legacy Survey of Space and Time (LSST).

Figures (4)

• Figure 1: Top: GTC/HiPERCAM light curves of ZTF J1239 in the us, gs, rs, is, and zs bands. Each light curve is normalized with respect to its minimum flux level and offset for visibility. Bottom: Radial velocity (RV) data from Keck/LRIS (see \ref{['sec:optical']}). Light curves are phased such that the maximum of the $u_s$ curve is at $\phi = 0.5$. RV epochs are phased such that the epoch of the brightest LRIS observation is coincident with the phase of the maximum of the $g_s$ light curve.
• Figure 2: Top: Phase resolved LRIS spectra of ZTF J1239+8347. Full resolution spectra are shown behind spectra smoothed to $R \sim 800$ for clarity. The spectra are each offset by 0.25 from the next for clarity and phased such that the brightest spectrum is at $\phi = 0.5$ for consistency with optical photometry. Hydrogen absorption features are prominently visible during bright times, and disappear to a mostly featureless flat spectrum with H-$\alpha$ emission at minimum suggesting a shock. Middle: The normalized spectrum of the brightest LRIS epoch with the normalized BT-Settl model cut to the Balmer region used for RV fitting. Bottom: Residuals of the BT-Settl model subtracted from the normalized LRIS spectrum.
• Figure 3: NIRES spectrum of ZTF J1239+8347 with the best fit scaled blackbody plus single BD atmosphere model overlaid. The shown spectrum is a sum of data over a full orbital period and is smoothed for clarity. The BD model atmosphere chosen is the one with the atmospheric temperature that best fits the data and provides a physically plausible BD radius of $R_{\rm BD} = 1.20^{+0.15}_{-0.11} \, R_\mathrm{Jup}$
• Figure 4: Comparison of ZTF J1239+8347 to double white dwarfs (DWDs) and black widow neutron star -- substellar object binaries (BWs). We note that typical brown dwarf binaries (BDBs) exist at much longer periods than ZTF J1239+8347.