Context : Weak and compact radio continuum and H _ { 2 } O 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 ( 26–4 GHz ) , which arises in the inner few 1000 au from those young stars also associated with bright H _ { 2 } O masers .
We want to study the radio continuum properties in combination with the H _ { 2 } O maser kinematics , in order to eventually 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 in the A configuration at both Ku and C bands , in order to image the radio continuum emission towards 25 H _ { 2 } O maser sites with an angular resolution and thermal rms of the order of 0 \aas@@fstack { \prime \prime } 1 and 10 \mu Jy beam ^ { -1 } , respectively .
These targets add to our pilot study of 11 maser sites presented in .
The sample of H _ { 2 } O maser sites was selected among those regions having an accurate distance measurement , obtained through maser trigonometric parallaxes , and H _ { 2 } O maser luminosities in excess of 10 ^ { -6 } L _ { \odot } .

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 H _ { 2 } O masers ’ position ; 50 % of the radio continuum sources are associated with bolometric luminosities exceeding 5 \times 10 ^ { 3 } L _ { \odot } , 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 multi-frequency-synthesis deconvolution algorithm of CASA .
The radio continuum emission traces thermal bremsstrahlung in ( proto ) stellar winds and jets , with flux densities at 22 GHz below 3 mJy , 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 H _ { 2 } O maser luminosity ( L _ { H _ { 2 } O } ) of ( L _ { 8 GHz } / mJy kpc ^ { 2 } ) = 10 ^ { 3.8 } \times ( L _ { H _ { 2 } O } / L _ { \odot } ) ^ { 0.74 } .
This power-law relation is similar to that between the radio continuum and bolometric luminosities , which confirms earlier studies .
Since H _ { 2 } O 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 - 10 ^ { 5 } L _ { \odot } ) .

Conclusions :