Astronomy & Astrophysics

Free access article

Issue		A&A Volume 384, Number 3, March IV 2002
Page(s)		999 - 1011
Section		Formation, structure and evolution of stars
DOI		10.1051/0004-6361:20020060

A&A 384, 999-1011 (2002)
DOI: 10.1051/0004-6361:20020060

HST, VLT, and NTT imaging search for wide companions to bona-fide and candidate brown dwarfs in the Cha I dark cloud

R. Neuhäuser¹, W. Brandner², J. Alves², V. Joergens¹ and F. Comerón²

¹ MPI für extraterrestrische Physik, Giessenbachstraße 1, 85740 Garching, Germany
² European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany

(Received 6 November 2001 / Accepted 14 January 2002)

Abstract
We present results from a deep imaging search for companions around the young bona-fide and candidate brown dwarfs Cha H $\alpha$ 1 to 12 in the Cha I dark cloud, performed with HST WFPC2 ( R, I, H $\alpha$ ), VLT FORS1 ( VRI), and NTT SofI ( $JHK_{\rm s}$ ). We find 16 faint companion candidates around five primaries with separations between 1.5 $^{\prime \prime}$ and 7 $^{\prime \prime}$ and magnitudes in R & I from 19 to 25 mag, i.e. up to 8 mag fainter than the primaries. While most of these companion candidates are probably unrelated background objects, there is one promising candidate, namely $1.5^{\prime \prime}$ SW off the M6-dwarf Cha H $\alpha$ 5. This candidate is 3.8 to 4.7 mag fainter than the primary and its colors are consistent with an early- to mid-L spectral type. Assuming the same distance (140 pc) and absorption ( A_I = 0.47 mag) as towards the primary, the companion candidate has $\log$ ( $L_{\rm bol}/L_{\odot}) = -3.0 \pm 0.3$ . At the age of the primary (1 to 5 Myrs), the faint object would have a mass of 3 to 15 Jupiter masses according to Burrows et al. (1997) and Chabrier & Baraffe (2000) models. The probability for this companion candidate to be an unrelated fore- or background object is $\le$ $0.7\%$ , its colors are marginally consistent with a strongly reddened background K giant. One other companion candidate has infrared colors consistent with an early T-dwarf. In addition, we present indications for Cha H $\alpha$ 2 being a close (~ $0.2^{\prime \prime}$ ) binary with both components very close to the sub-stellar limit. Our detection limits are such that we should have detected all companions above $\sim$ 1 $M_{\rm jup}$ with separations $\ge$ $2^{\prime \prime}$ ( $\ge$ 320 AU) and all above ~ 5 $M_{\rm jup}$ at $\ge$ $0.35^{\prime \prime}$ ( $\ge$ 50 AU).

Key words: stars: binaries: visual -- individual: Cha H $\alpha$ 1 to 12 -- low-mass, brown dwarfs -- pre-main sequence

Offprint request: R. Neuhäuser, rne@mpe.mpg.de

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