EDP Sciences
Free access
Volume 413, Number 3, January III 2004
Page(s) 1029 - 1036
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20034292

A&A 413, 1029-1036 (2004)
DOI: 10.1051/0004-6361:20034292

$\varepsilon$ Indi Ba,Bb: The nearest binary brown dwarf

M. J. McCaughrean1, L. M. Close2, R.-D. Scholz1, R. Lenzen3, B. Biller2, W. Brandner3, M. Hartung4 and N. Lodieu1

1  Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
2  Steward Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721-0065, USA
3  Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
4  European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile

(Received 9 September 2003 / Accepted 2 October 2003)

We have carried out high angular resolution near-infrared imaging and low-resolution ( $R\sim1000$) spectroscopy of the nearest known brown dwarf, $\varepsilon$ 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 $\varepsilon$ Indi system. In our discovery paper (Scholz et al. 2003), we concluded that $\varepsilon$ Indi B was a ~50 $M_{\rm Jup}$ T2.5 dwarf: our revised finding is that the two system components ( $\varepsilon$ Indi Ba and $\varepsilon$ Indi Bb) have spectral types of T1 and T6, respectively, and estimated masses of 47 and 28 $M_{\rm Jup}$ , respectively, assuming an age of 1.3 Gyr. Errors in the masses are $\pm$10 and $\pm$7 $M_{\rm Jup}$ , 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: $\varepsilon$ 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.

Key words: astrometry -- surveys -- stars: late-type -- stars: low mass, brown dwarfs -- stars: binaries: general

Offprint request: M. J. McCaughrean, mjm@aip.de

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© ESO 2004