2003ApJ...592..288D

We present new multiepoch near-infrared (NIR) and optical high angular resolution images of the V773 Tau pre-main-sequence triple system, a weak-line T Tauri star (WTTS) system in which the presence of an evolved, ``fossil'' protoplanetary disk has been inferred on the basis of a significant IR excess. Our images reveal a fourth object bound to the system, V773 Tau D. While it is much fainter than all other components at 2 µm, it is the brightest source in the system at 4.7 µm. We also present medium-resolution K-band adaptive optics spectroscopy of this object, which is featureless with the exception of a weak Brγ emission line. Based on this spectrum and on the spectral energy distribution (SED) of the system, we show that V773 Tau D is another member of the small class of ``infrared companions'' (IRCs) to T Tauri stars (TTSs). It is the least luminous, and probably the least massive, component of the system, as opposed to most other IRCs, which suggests that numerous low-luminosity IRCs such as V773 Tau D may still remain to be discovered. Furthermore, it is the source of the strong IR excess in the system. We therefore reject the interpretation of this excess as the signature of a fossil (or ``passive'') disk and further suggest that these systems may be much less frequent than previously thought. We further show that V773 Tau C is a variable classical TTS (CTTS) and that its motion provides a well-constrained orbital model. We show that V773 Tau D can be dynamically stable within this quadruple system if its orbit is highly inclined. Finally, V773 Tau is the first multiple system to display such a variety of evolutionary states (WTTS, CTTS, IRC), which may be the consequence of the strong star-star interactions in this compact quadruple system.