New extinction and mass estimates from optical photometry of the very low mass brown dwarf companion ct Chamaeleontis b with the Magellan AO system

We used the Magellan adaptive optics system and its VisAO CCD camera to image the young low mass brown dwarf companion CT Chamaeleontis B for the first time at visible wavelengths. We detect it at r′, i′, z′, and Y_S . With our new photometry and T _eff 2500 K derived from the shape of its K-band spectrum, we find that CT Cha B has A_V = 3.4 ± 1.1 mag, and a mass of 14-24 M_J according to the DUSTY evolutionary tracks and its 1-5 Myr age. The overluminosity of our r′ detection indicates that the companion has significant Hα emission and a mass accretion rate 6 × 10^-10 M _yr^-1, similar to some substellar companions. Proper motion analysis shows that another point source within 2″ of CT Cha A is not physical. This paper demonstrates how visible wavelength adaptive optics photometry (r′, i′, z′, Y_S) allows for a better estimate of extinction, luminosity, and mass accretion rate of young substellar companions.