Abstract
We present new XSHOOTER spectra of NLTT5306, a 0.44 $\pm$ 0.04\msun white dwarf in a short period (101\,min) binary system with a brown dwarf companion that is likely to have previously undergone common envelope evolution. We have confirmed the presence of H$\alpha$ emission and discovered Na I absorption associated with the white dwarf. These observations are indicative of accretion. Accretion is typically evidenced by high energy emission in the UV and X-ray regime. However our \textit{Swift} observations covering the full orbital period in three wavebands (uvw1, uvm2, uvw2) revealed no UV excess or modulation. We used the X-ray non-detection to put an upper limit on the accretion rate of 2$\times$10$^{-15}$\msun yr$^{-1}$. We compare NLTT5306 to similar accreting binaries with brown dwarf donors and suggest the inferred accretion rate could be from wind accretion or accretion from a debris/dust disk. The lack of evidence for a disk implies NLTT5306 is magnetically funnelling a weak wind from a potentially low gravity brown dwarf. The upper limit on the accretion rate suggests a magnetic field as low as 0.45\,kG would be sufficient to achieve this. If confirmed this would constitute the first detection of a brown dwarf wind and could provide useful constraints on mass loss rates.
Highlights
Brown dwarf companions to main sequence stars are rarely detected in orbits closer than 3 AU (Metchev 2006)
White dwarfs with stellar companions that accrete via a wind have accretion rates from 10−19 to 10−16 M⊙ yr−1 (Debes 2006; Kawka et al 2008) and the rate largely depends on the spectral type of the companion
Hα emission and Na I absorption associated with the white dwarf lead us to believe accretion is happening in NLTT5306
Summary
Brown dwarf companions to main sequence stars are rarely detected in orbits closer than 3 AU (Metchev 2006). White dwarf – brown dwarf binaries are rare: Steele et al (2011) estimate only 0.5% of white dwarfs have a brown dwarf companion This scarcity exists in both detached and non-detached binaries. Their formation begins when the white dwarf progenitor evolves off the main sequence and expands up the giant or asymptotic branch. This expansion leads to Roche lobe overflow (RLOF), which causes the companion to become engulfed and the subsequent brief phase of binary evolution to take place in a common envelope (CE). The companion loses orbital angular momentum to the envelope, which is expelled from the system, and the companion spirals in towards the white dwarf progenitor
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