Orbital proper motions in the protobinary system L1527/IRAS 04368+2557?

Using high angular resolution 7 mm Very Large Array (VLA) observations, we show that the low-mass protostellar object IRAS 04368+2557 in L1527 is composed of two radio sources separated in projection by 25 AU. One of the two components is extended in the direction perpendicular to the outflow powered by IRAS 04368+2557 and is, most likely, a compact accretion disk (R similar to 20 AU), similar to those found in L1551 IRS 5 by Rodriguez et al. As in L1551 IRS 5, the disk found here is small compared to those around T Tauri stars. Tidal interactions with nearby companions provide a natural way of truncating disks, and we argue that the two millimeter-wavelength sources in L1527 trace a compact binary system, where the disk surrounding one of the components has been truncated by the tidal influence of the other. A comparison between observations obtained in 1996 and 2002 reveals large proper motions, which can be only partly attributed to the overall large-scale motion of the region in which IRAS 04368+2557 is located. The remaining residual proper motions might trace the orbital motion of the binary and would suggest a total mass for the system larger than 0.2 M. and likely of the order of 0.5-2 M. This mass (only a small fraction of which is in the disk) is of the same order as that of the extended surrounding envelope of gas and dust traced by far-infrared and submillimeter observations (M-env similar to 0.5 M.), implying that the stars that will eventually form out of IRAS 04368+2557 would have already acquired a significant fraction of their final masses. It is worth noting that multiepoch VLA studies of nearby protobinary systems similar to that presented here could provide direct mass estimates in most nearby star-forming sites. Combined with submillimeter observations of the surrounding envelopes, this would provide a more reliable measure of the evolutionary status of binary protostars.