ε Indi Ba,Bb: The nearest binary brown dwarf

Abstract

Abstract.We have carried out high angular resolution near-infrared imaging and low-resolution (R ∼ 1000) spectroscopy of the nearest known brown dwarf, ε Indi B, using the ESO VLT NAOS/CONICA adaptive optics system. We find it to be a close binary (as also noted by Volk et al. 2003), with an angular separation of 0.732 arcsec, corresponding to 2.65 AU at the 3.626 pc distance of the ε Indi system. In our discovery paper (Scholz et al. 2003), we concluded that ε Indi B was a ∼50 M Jup T2.5 dwarf: our revised finding is that the two system components (ε Indi Ba and ε Indi Bb) have spectral types of T1 and T6, respectively, and estimated masses of 47 and 28 M Jup , respectively, assuming an age of 1.3 Gyr. Errors in the masses are ±10 and ±7 M Jup , respectively, dominated by the uncertainty in the age determination (0.8-2 Gyr range). This uniquely well-characterised T dwarf binary system should prove important in the study of low-mass, cool brown dwarfs. The two components are bright and relatively well-resolved: ε Indi B is the only T dwarf binary in which spectra have been obtained for both components. The system has a well-established distance and age. Finally, their orbital motion can be measured on a fairly short timescale (nominal orbital period ∼15 yrs), permitting an accurate determination of the true total system mass, helping to calibrate brown dwarf evolutionary models.

The brown dwarf atmosphere monitoring (BAM) project

“…Bouy et al 2003;McCaughrean et al 2004;Burgasser et al 2003Burgasser et al , 2005Burgasser et al , 2006Reid et al 2006aReid et al , 2008Looper et al 2008). Based on the companion search programs reported in the literature, a total of 12 targets are members of spatially resolved binary pairs, and the results of all the binary searches are reported in Tables 1 and 2.…”

$\mathsf{\varepsilon}$ Indi Ba, Bb: a detailed study of the nearest known brown dwarfs

“…One of our nearest neighbours, ε Indi A has a well-constrained parallax from HIPPARCOS (Perryman & ESA 1997) as refined by van Leeuwen (2007), putting the system at a distance of A&A 510, A99 (2010) 3.6224 ± 0.0037 pc. This was followed by the discovery of the companion's binary nature (McCaughrean et al 2004). The proximity of ε Indi Ba, Bb to the Earth means Ba is more than a magnitude brighter than any other known T dwarf, and allows unprecedented, detailed spectroscopic studies of these important template objects.…”

“…Although there are a number of other T dwarfs in binary systems, such as the M4/T8.5 binary Wolf 940 (Burningham et al 2009) and the T5/T5.5 binary 2MASS 1534−2952 (Liu et al 2008), ε Indi Ba, Bb has a very well-determined distance, a main-sequence primary star with which to constrain age and metallicity, and a short enough orbit (nominally ∼15 years, McCaughrean et al 2004) such that the system and individual dynamical masses can soon be determined (McCaughrean et al; Cardoso et al, in prep.). They are also relatively bright, close enough, and sufficiently separated to allow detailed photometric and spectroscopic studies of both components.…”

“…These were fit with the unified cloud models of Tsuji (2002) and effective temperatures of 1400 K and 1600 K were derived for the two spectral regions. Mid-IR spectroscopy of the unresolved ε Indi Ba, Bb system was also acquired by Roellig et al (2004), who use evolutionary models along with the luminosities of McCaughrean et al (2004) and an assumed age of 0.8−2.0 Gyr to derive effective temperatures and surface gravities and then compared composite spectral models to the observed spectrum. Their predictions were revised by Mainzer et al (2007) who derived effective temperatures of 1210−1250 K and 840 K, for ε Indi Ba and Bb respectively, under the assumption of a system age of ∼1 Gyr.…”

Dynamical masses for the nearest brown dwarf binary: ε Indi Ba,b

“…If a main sequence star is also involved, the age and metallicity for the system can be determined, making it possible to break the sub-stellar mass-age degeneracy. The most suitable benchmark system for intermediate age T dwarfs is ε Indi Ba,b, two T dwarfs (spectral types T1 and T6; McCaughrean et al (2004)) orbiting a K4.5V star, ε Indi A, at a projected separation of 1460AU. At a distance of 3.6224pc (HIPPARCOS distance to ε Indi A; van Leeuwen (2007)), these are the closest brown dwarfs to the Earth, and thus both components are bright and the system is well-resolved.…”