A luminous infrared companion in the young triple system WL 20

We present spatially resolved near- and mid-infrared (1-25 mum) imaging of the WL 20 triple system in the nearby (d = 125 pc) rho Ophiuchi star-formin g cloud core. We find WL 20 to be a new addition to the rare class of "infr ared companion systems," with WL 20 E and WL 20 W displaying Class II (T Ta uri star) spectral energy distributions (SEDs) and total luminosities of 0. 61 and 0.39 L., respectively, and WL 20 S, the infrared companion, having a Class I (embedded protostellar) SED and a luminosity of 1.0-1.8 L.. WL 20 S is found to be highly variable over timescales of years, to be extended ( 40 AU diameter) at mid-IR wavelengths, and to be the source of the centimet er emission in the system. The photospheric luminosities of 0.53 L. for WL 20 E and 0.35 L. for WL 20 W estimated from our data, combined with existin g, spatially resolved near-IR spectroscopy, allow us to compare and test cu rrent pre-main-sequence evolutionary tracks. The most plausible, nonaccreti ng tracks describing this system are those of D'Antona & Mazzitelli publish ed in 1998. These tracks give an age of (2-2.5) x 10(6) yr and masses of 0. 62-0.68 M. for WL 20 E and 0.51-0.55 for WL 20 W. The age and mass of WL 20 S cannot be well determined from the currently available data. WL 20 E and W fall into the region of the H-R diagram in which sources may appear up t o twice as old as they actually are using nonaccreting tracks, a fact that may reconcile the coexistence of two T Tauri stars with an embedded protost ar in a triple system. The derived masses and observed projected separation s of the components of the WL 20 triple system indicate that it is in an un stable dynamical configuration, and it may therefore provide an example of dynamical evolution during the pre-main-sequence phase.