Spectral indices of centimeter continuum sources in star-forming regions: Implications on the nature of the outflow exciting sources

We present 6 cm VLA observations of nine regions with molecular or HH outflows that appear to be driven by young stellar objects of low bolometric luminosity. Radio continuum emission at 3.6 cm had been detected previously toward the center of symmetry of 13 outflows in these regions. With the new 6 cm observations we have been able to estimate the spectral index in the 6 to 3.6 cm wavelength range for 11 of these outflow central (Class 0 and Class I) sources. All the spectral indices obtained for these central sources are positive and consistent with partially thick thermal free-free emission. We discuss this result in relation to the current models for the origin of centimeter radio continuum emission in outflow sources. In particular, we find that the observed flux densities of these sources fit very well in the observational correlation between the radio continuum luminosity and the momentum rate of the outflow, and that the results are consistent with the central sources being thermal radio jets. Our data suggest that the ionized part of the jets begins at a distance ≤ 10 AU from the star. In addition, we have derived a positive value for the spectral index of HH 32A, indicative of partially thick thermal emission. For the remaining sources detected in the nine fields observed, our results indicate that the emission is, in general, nonthermal, as expected for background extragalactic sources. However, a few of these sources could be related to the star-forming regions studied.