A Westerbork Synthesis Radio Telescope 1400 and 350 MHz Continuum Survey of the Cygnus OB2 Association, in Search of Hot Massive Stars

Abstract

We present a radio continuum survey at 1400 and 350 MHz of a region of 2°×2° centered on the Cygnus OB2 association (d = 1.7 kpc), using the Westerbork Synthesis Radio Telescope (WSRT) with angular resolutions of, respectively, 13" and 55". The resulting 5 sigma flux-density limits of, respectively, ~2 mJy and ~10-15 mJy are a significant improvement over previous surveys. We detected 210 discrete sources with sizes less than 1.9θ beam (beam size), 98 of which at both frequencies. We also detected 28 resolved sources (sizes>1.9θ beam) still having well-defined peak intensities. The observed spectral index α 1400 350 distribution and source count strongly suggest an excess of sources of Galactic origin in the direction of Cyg OB2. We have searched for positional coincidences of the detected sources in our list with other radio, infrared, and optical objects from various surveys by using the likelihood ratio (LR) method. Furthermore, we looked for objects that show characteristics of either optically thick stellar winds (α >~ +0.6), or nonthermal emission (-1