On the Evolutionary State of the Components of the YLW 15 Binary System

Abstract

We report centimeter continuum observations with the Very Large Array (VLA) and the Very Long Baseline Array (VLBA), as well as mid-infrared observations with the Cooled Mid-Infrared Camera and Spectrometer on the Subaru Telescope toward the components of the YLW 15 very young binary system, VLA 1 and VLA 2. The centimeter emission of the two components traces partially thick free-free emission, likely due to collimated, ionized winds. VLA 1 is an embedded protostar, undetected in the near-IR, and possibly in the Class 0 to Class I transition and powering a Herbig-Haro outflow. Its mid-IR emission appears slightly resolved with a diameter of ∼16 AU, possibly tracing circumstellar material from both the envelope and the disk. VLA 2 is a typical Class I object, unresolved in the mid-IR, and is responsible for the strong X-ray emission associated with YLW 15. The expected centimeter "peristellar" emission associated with the X-ray emission is not detected with the VLBA at 6 cm, likely because of the high optical depth of the free-free emission. Strikingly, the near- to mid-IR properties of YLW 15 suggest that VLA 1 is a more embedded young stellar object (YSO), or alternatively, less luminous than VLA 2, whereas orbital proper motions of this binary system indicate that VLA 1 is more massive than VLA 2. This result is apparently against the expected evolutionary scenario, in which one expects that the more massive YSO in a binary system is the more evolved and luminous YSO. Finally, the nearby source YLW 16A is detected with the VLA; its position coincides with reported near-IR and X-ray sources.