New Extinction and Mass Estimates from Optical Photometry of the Very Low Mass Brown Dwarf Companion CT Chamaeleontis B with the Magellan AO System

Abstract

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<SUB>S</SUB> . With our new photometry and T <SUB>eff</SUB> ~ 2500 K derived from the shape of its K-band spectrum, we find that CT Cha B has A<SUB>V</SUB> = 3.4 ± 1.1 mag, and a mass of 14-24 M<SUB>J</SUB> 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<SUP>-10</SUP> M <SUB>☉</SUB> yr<SUP>-1</SUP>, 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<SUB>S</SUB> ) allows for a better estimate of extinction, luminosity, and mass accretion rate of young substellar companions. <P />This paper includes data gathered with the 6.5 m Magellan Clay Telescope at Las Campanas Observatory, Chile.