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G. Duchene, A. M. Ghez, C. McCabe (UCLA)
We present new near-infrared (1.6 and 2.2 micron) adaptive optics images with the 10m-Keck II telescope of the low-mass pre-main sequence multiple system V773 Tau. In addition to the already known unresolved double-lined spectroscopic binary and its 83 milliarcsec companion, our images reveal a fourth star located only 0.21'' (projected distance: 30 AU) away from the brightest component. This object appears to be much redder than the other stars in the system. We also obtained a medium-resolution (R=3500) long-slit spectrum of this object which covers the 2.0-2.4 micron wavelength range. The spectrum of this dim fourth component in the system shows no photospheric feature but has a small Br\gamma emission line (equivalent width of about 0.5Å). If this object is not an extincted background giant, which the hydrogen emission line seems to exclude, it is very reminiscent of a small class of objects known as "infrared companions" to T Tauri stars. The tight visual binary has been followed over the last years through speckle interferometry technique, and these data indicate a strange behaviour with a large "jump" in position angle. This may reveal some strong photometric variability in the candidate IRC, which would reinforce its status. We also emphasize that such a quadruple system, with four stars located within 30 AU or so, is extremely rare among main sequence solar-type objects while many T Tauri binaries in Taurus turned out to have additionalcompanions when observed with higher resolution and/or sensitivity. This may indicate that this region is all but typical of star formation in the Galaxy.