A Luminous Infrared Companion in the Young Triple System WL 20

We present spatially resolved near-infrared and mid-infrared (1-25 microns)imaging of the WL 20 triple system in the nearby (d=125 pc) rho Ophiuchistar-forming cloud core. We find WL 20 to be a new addition to the rare classof "infrared companion systems", with WL 20:E and WL 20:W displaying Class II(T-Tauri star) spectral energy distributions (SEDs) and total luminosities of0.61 and 0.39 L_sun, respectively, and WL 20:S, the infrared companion, with aClass I (embedded protostellar) SED and a luminosity of 1.0-1.8 L_sun. WL 20:Sis found to be highly variable over timescales of years, to be extended (40 AUdiameter) at mid-infrared wavelengths, and to be the source of the centimeteremission in the system. The photospheric luminosities, estimated from our data,allow us to compare and test current pre-main-sequence evolutionary tracks. WL20:E and WL 20:W fall into the region of the H-R diagram in which sources mayappear up to twice as old as they actually are using non-accreting tracks, afact which may reconcile the co-existence of two T-Tauri stars with an embeddedprotostar in a triple system. The derived masses and observed projectedseparations of the components of the WL 20 triple system indicate that it is inan unstable dynamical configuration, and may therefore provide an example ofdynamical evolution during the pre-main-sequence phase.Comment: AASTeX 5.0, 17 pages, 4 tables, 9 figures, accepted by AJ, to appear Feb. 200