The Nature of the Radio Sources within the Cepheus A Star-forming Region

Abstract

We present multifrequency, matching-beam, VLA radio continuum observations of the Cep A East radio source, known to consist of 16 compact ( 1") components clustered within a 25" radius region, most of which are aligned in stringlike structures. We find that the spectral indices of the emission from these compact objects, in the frequency interval from 1.5 to 15 GHz, cover a wide range, from -0.6 to 0.7. Positive spectral indices are exhibited by sources 2, 3b, 3c, and 3d. The first and last of these objects, the brightest sources within Cep A East, exhibit in addition elongated morphologies and angular-size and flux-density dependences with frequency that suggest they correspond to confined jets of ionized gas. Most of the objects that appear in string structures exhibit a mixture of flat and negative spectral indices across their faces, which indicates the presence of both thermal and nonthermal emission. The spectral indices of the integrated emission from sources 1b, 4, 6, and 7a are in the range between - 0.3 and -0.1 while those of sources 1 a, 5, 7b, and 7c are even more negative (-0.6 < alpha < -0.4). We suggest that the radio emission from the string sources arises in shocks resulting from the interaction of confined stellar winds with the surrounding medium. The duality in emission mechanisms is expected in shock waves where a small fraction of the electrons are accelerated to relativistic velocities, giving rise to nonthermal emission, while most of the electrons produce thermal free-free emission. We find that the nonthermal emission dominates the thermal emission when the density of the thermal electrons is below a critical density of 5 x 103 cm -3. We also observed the Cep A West radio source, which consists of two compact components and an elongated, diffuse, champagne-like structure. We find that the spectral index between 1.5 and 5 GHz of the peak and of the integrated emission from the latter component are -0.1 and -0.4, respectively. We suggest that the radio emission from the diffuse source arises from shocked gas at the edge of a cavity driven by a wind that originates from the brightest compact radio object within this region, which shows a spectral index at the peak of the emission of 0.6.