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

Abstract

We present results from a deep imaging search for companions around the young bona-fide and candidate brown dwarfs Cha Hα 1 to 12 in the Cha I dark cloud, performed with HST WFPC2 (R, I, Hα ), VLT FORS1 (VRI), and NTT SofI (JHK_s). We find 16 faint companion candidates around five primaries with separations between 1.5^'' and 7^'' 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^'' SW off the M6-dwarf Cha Hα 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_bol/L_sun) = -3.0 +/- 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. (\cite{Burrows97}) and Chabrier & Baraffe (\cite{Chabrier00}) 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 Hα 2 being a close ( ~ 0.2^'') binary with both components very close to the sub-stellar limit. Our detection limits are such that we should have detected all companions above ~ 1 M_jup with separations >=2^'' (>= 320 AU) and all above ~ 5 M_jup at >= 0.35^'' (>= 50 AU). Based on observations obtained with the NASA/ESA Hubble Space Telescope in program GO 8716, ESO programs 64.I-0493, 66.C-0022, and 66.C-0310 as well as data taken from the public VLT data archive.