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 H α 1 to 12 in the Cha I dark cloud, performed with HST WFPC2 ( R , I , H α ), VLT FORS1 ( VRI ), and NTT SofI ( JHK 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 α 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. ([CITE]) and Chabrier & Baraffe ([CITE]) models. The probability for this companion candidate to be an unrelated fore- or background object is ≤ $ 0.7\%$, its colors are marginally consistent with a strongly reddened background K giant. One other companion candidate has infrared colors consistent with an early T-dwarf. In addition, we present indications for Cha H α 2 being a close (~$ 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 ∼ 1 M jup with separations $\ge $ $2^{\prime \prime}$ ( ≥ $ 320$ AU) and all above ~$ 5$ M jup at ≥ $ 0.35^{\prime \prime}$ ( ≥ $ 50$ AU).