The immediate environment of the Class 0 protostar VLA 1623, on scales of similar to 50-100 au, observed at millimetre and centimetre wavelengths

We present high angular resolution observations, taken with the Very Large Array (VLA) and Multiple Element Radio Linked Interferometer Network radio telescopes, at 7 mm and 4.4 cm, respectively, of the prototype Class 0 protostar VLA 1623. At 7 mm we detect two sources (VLA 1623A and VLA 1623B) coincident with the two previously detected components at the centre of this system. The separation between the two is 1.2 arcsec, or similar to 170 au at an assumed distance of 139 pc. The upper limit to the size of the source coincident with each component of VLA 1623 is similar to 0.7 arcsec, in agreement with previous findings. This corresponds to a diameter of similar to 100 au at an assumed distance of 139 pc. Both components show the same general trend in their broad-band continuum spectra, of a steeper dust continuum spectrum shortward of 7 mm and a flatter spectrum longward of this.

We estimate an upper limit to the VLA 1623A disc mass of <= 0.13 M-circle dot and an upper limit to its radius of similar to 50 au. The longer wavelength data have a spectral index of a similar to 0.6 +/- 0.3. This is too steep to be explained by optically thin free-free emission. It is most likely due to optically thick free-free emission. Alternatively, we speculate that it might be due to the formation of larger grains or planetesimals in the circumstellar disc. We estimate the mass of VLA 1623B to be <= 0.15 M-circle dot. We can place a lower limit to its size of similar to 30 x 7 au, and an upper limit to its diameter of similar to 100 au. The longer wavelength data of VLA 1623B also have a spectral index of alpha similar to 0.6 +/- 0.3. The nature of VLA 1623B remains a matter of debate. It could be a binary companion to the protostar, or a knot in the radio jet from VLA 1623A.