2018A&A...619A.107S

Context. Weak and compact radio continuum and H₂O masers are preferred tracers of the outflow activity nearby very young stars.
Aims. We want to image the centimeter free-free continuum emission in the range 1-7 cm (26-4GHz), which arises in the inner few 1000 au from those young stars also associated with bright H₂O masers. We seek to study the radio continuum properties in combination with the H₂O maser kinematics to quantify the outflow energetics powered by single young stars.
Methods. We made use of the Karl G. Jansky Very Large Array (VLA) in the B configuration at K band and the A configuration at both Ku and C bands in order to image the radio continuum emission toward 25 H₂O maser sites with an angular resolution and thermal rms on the order of 0.1" and 10µJy/beam, respectively. These targets add to our pilot study of 11 maser sites previously presented. The sample of H₂O maser sites was selected among those regions that have accurate distance measurements, obtained through maser trigonometric parallaxes, and H₂O maser luminosities in excess of 10^–6L_☉.
Results. We present high-resolution radio continuum images of 33 sources belonging to 25 star-forming regions. In each region, we detect radio continuum emission within a few 1000 au of the H₂O masers' position; 50% of the radio continuum sources are associated with bolometric luminosities exceeding 5x10³L_☉, including W33A and G240.32+0.07. We provide a detailed spectral index analysis for each radio continuum source, based on the integrated fluxes at each frequency, and produce spectral index maps with the multifrequency synthesis deconvolution algorithm of CASA. The radio continuum emission traces thermal bremsstrahlung in (proto)stellar winds and jets that have flux densities at 22GHz below 3mJy and spectral index values between -0.1 and 1.3. We prove a strong correlation (r>0.8) between the radio continuum luminosity (L_rad) and the H₂O maser luminosity (L_H2_O) of (L_8GHz/mJy.kpc²)=10^3.8x(L_H2_OL_☉^)0.74. This power-law relation is similar to that between the radio continuum and bolometric luminosities, which confirms earlier studies. Since H₂O masers are excited through shocks driven by (proto)stellar winds and jets, these results provide support to the idea that the radio continuum emission around young stars is dominated by shock ionization, and this holds over several orders of magnitude of stellar luminosites (1-10⁵L_☉).