A low-mass triple system with a wide L/T transition brown dwarf component: NLTT 51469AB/SDSS 2131-0119

Abstract

We demonstrate that the previously identified L/T transition brown dwarf SDSS J213154.43-011939.3 (SDSS 2131-0119) is a widely separated (82.3'', $\sim$3830 au) common proper motion companion to the low-mass star NLTT 51469, which we reveal to be a close binary itself, separated by 0.64''$\pm$0.01'' ($\sim$30 au). We find the proper motion of SDSS 2131-0119 of $\hat{I}{¼}_{\hat{I}}\pm \cos \hat{I}´=-100\pm 20$, $\hat{I}{¼}_{\hat{I}}´=-230\pm 20$ mas/yr consistent with the proper motion of the primary provided by Gaia DR2: $\hat{I}{¼}_{\hat{I}}\pm \cos \hat{I}´=-95.49\pm 0.96$, $\hat{I}{¼}_{\hat{I}}´=-239.38\pm 0.96$ mas/yr. Based on optical and near-infrared spectroscopy we classify NLTT 51469A as a M3$\pm$1 dwarf, estimate photometrically the spectral type of its close companion NLTT 51469B at $\sim$M6 and confirm the spectral type of the brown dwarf to be L9$\pm$1. Using radial velocity, proper motion and parallax we derived the $UVW$ space velocities of NLTT 51469A, showing that the system does not belong to any known young stellar moving group. The high $V,W$ velocities, lack of 670.8 nm Li I absorption line, and absence of H$\hat{I}\pm$ emission, detected X-rays or UV excess, indicate that the system is likely a member of the thin disk population and is older than 1 Gyr. For the parallactic distance of $46.6\pm 1.6$ pc from Gaia DR2 we determined luminosities of $-1.50_{-0.04}^{+0.02}$ and $-4.4\pm 0.1$ dex of the M3 and L9, respectively. Considering the spectrophotometric estimation which yields a slightly lower distance of $34_{-13}^{+10}$ pc the obtained luminosities are $-1.78_{-0.04}^{+0.02}$ and $-4.7_{-0.5}^{+0.3}$ dex. We also estimated their effective temperatures and masses, and obtained 3410${}_{-210}^{+140}$ K and $0.42\pm 0.02M_{\odot }$ for the primary, and 1400-1650 K and $0.05-0.07M_{\odot }$ for the wide companion. For the $\sim$M6 component we estimated $T_{eff}=2850\pm 200K$ and $m=0.10_{-0.01}^{+0.06}{M}_{\odot }$.

12 pages, 8 figures, accepted for publication in MNRAS