Cepheus A HW2: A powerful thermal radio jet

At angular resolution of ∼1″ the Cepheus A East radio source is known to consist of 16 compact components clustered within a 25″ radius region, most of which are aligned in stringlike structures. We present multifrequency VLA radio continuum observations of Cep A HW2, the elongated radio object believed to be associated with the most luminous (∼ 104 L⊙) source in the region. In the frequency range from 1.5 to 43 GHz, we find that its flux density increases with frequency as ν0.69, while the angular size of its major axis decreases with frequency as ν-0.57. The above frequency dependences are very close to the theoretical values of ν0.6 and ν-0.7 expected for a biconical thermal jet and make Cep A HW2 the best example known of this type of object. We suggest that Cep A HW2 is responsible for at least part of the complex outflow and excitation phenomena observed in the region. The estimated ionized mass-loss rate in this source, ∼8 × 10-7 M⊙ yr-1, is about 100 times larger than the value expected for a star of the same luminosity in the main sequence.