EDP Sciences
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Volume 384, Number 3, March IV 2002
Page(s) 999 - 1011
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/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)

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 ( AI = 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 $ $M_{\rm jup}$ 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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