VLTI/AMBER detection of a K=9.5 very low mass star

The precise determination of both the dynamical mass and infrared photometry of the close companion to AB Dor A, AB Dor C (0.090 solar masses), has provided an important benchmark for calibration of theoretical evolutionary models of low-mass young stars. However, comparison of the observed magnitudes of AB Dor C with these models suggests that they could overpredict the flux of this object, a trend also found in other young systems. One of the ambiguities remaining in AB Dor C is the possible binary nature of this star; in fact, should AB Dor C be close binary (about 10 mas separation), it could reconcile observations and models. We will report on VLTI/AMBER observations of AB Dor C addressed to discriminate between both scenarios: AB Dor C as a single object or a binary brown dwarf. We used a non-standard "off-axis" fringe tracking that allowed the detection of AB Dor C. This is, to our knowledge, one of the weakest objects detected by an infrared interferometer (Ks=9.5).