Protostellar Outflows at the EarliesT Stages (POETS). I. Radio thermal jets at high resolution nearby H${}_2$O maser sources

Abstract

Abridged. 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 $0.1^{\prime \prime }$ and 10 $\mu$Jy beam${}^{-1}$, respectively. These targets add to our pilot study of 11 maser sites presented in Moscadelli et al. (2016). 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>$0.8) between the radio continuum luminosity (L${}_{\mathrm{rad}}$) and the H${}_2$O maser luminosity (L${}_{{\mathrm{H}}_2\mathrm{O}}$) of (${\mathrm{L}}_{8GHz}/mJy\times \mathrm{kp}{\mathrm{c}}^2)=10^{3.8}\times ({\mathrm{L}}_{{\mathrm{H}}_2\mathrm{O}}/{\mathrm{L}}_{\odot }{)}^{0.74}$. Since H${}_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.

arXiv comment25 pages, 7 figures, 4 tables, accepted by Astronomy & Astrophysics

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