DISCOVERY OF A BROWN DWARF VERY CLOSE TO THE SUN: A METHANE-RICH BROWN DWARF COMPANION TO THE LOW-MASS STAR SCR 1845-6357

摘要

We present VLT/NACO SDI images of the very nearby star SCR 1845-6357 (hereafter SCR 1845). SCR 1845 is a recently discovered M8.5 star just 3.85 pc from the Sun. Using the capabilities of the unique SDI device, we discovered a substellar companion to SCR 1845 at a separation of 4.5 AU (1.170" ± 0.003" on the sky) and fainter by 3.57 ± 0.057 mag in the 1.575 μm SDI filter. This substellar companion has an H magnitude of 13.16_(-0.26)~(+0.31) (absolute H magnitude of 15.30_(-0.26)~(+0.31)), making it likely the brightest mid-T dwarf known. The Simultaneous Differential Imager (SDI) consists of three narrowband filters placed around the 1.6 μm methane absorption feature characteristic of T dwarfs (T_(eff) < 1200 K). The flux of the substellar companion drops by a factor of 2.7 ± 0.1 between the SDI Fl (1.575 μm) and F3 (1.625 μm) filters, consistent with strong methane absorption in a substellar companion. We estimate a spectral type of T5.5 ± 1 for the companion based on the strength of this methane break. The chances that this object is a background T dwarf are vanishingly small— and there is no isolated background T dwarf in this part of the sky, according to 2MASS. Thus, it is a bound companion, hereafter SCR 1845B. For an age range of 100 Myr to 10 Gyr and spectral type range of T4.5-T6.5, we find a mass range of (9-65)M_(Jup) for SCR 1845B from the Baraffe et al. "COND" models. SCR 1845AB is the 24th-closest stellar system to the Sun (at 3.85 pc); the only brown dwarf system closer to the Sun is the binary brown dwarf ε Indi Ba-Bb (at 3.626 pc). In addition, this is the first T dwarf companion discovered around a low-mass star.