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 Ha 1 to 12 in the Cha I dark cloud, performed with HST WFPC2 (R, I, Ha), VLT FORS1 (VRI), and NTT SofI (JHK). 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 Ha 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 (0.47 mag in I) as towards the primary, the companion candidate has log (L(bol)/L(odot) = -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. (1997) and Chabrier & Baraffe (2000) models. The probability for this companion candidate to be an unrelated fore- or background object is smaller than 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 Ha 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 Jup with separations above 2" (320 AU) and all above 5 Jup at 0.35" (50 AU).