The formation of massive stars.: I.: High-resolution millimeter and radio studies of high-mass protostellar candidates

We used the Owens Valley Millimeter Array and the Very Large Array to obtain interferometric maps at millimeter and centimeter wavelengths in both the continuum and various lines [HCO+ (1-0), (HCO+)-C-13 (1-0), SiO (v = 0, J = 2-1) and (HCN)-C-13(1-0)] toward a sample of 11 high-mass protostellar candidates. These sources are known from a previous study to be associated with dense gas and dust and to not be associated with H II regions. All 11 sources were detected in HCO+ (1-0), nine in the millimeter continuum and five (of eight observed) in the centimeter continuum. The derived physical parameters confirm the high-mass nature of these molecular clumps and suggest they are gravitationally bound. Molecular outflows were detected toward six sources, with flow masses and momenta much higher than in low-mass young stellar objects. In many of the sources the molecular emission is organized in substructures, resolved both spatially and in velocity. We find that the sources may be characterized by their degree of fragmentation, turbulence, and outflow activity, with the sample dividing into two groups: group 1 cores have multiple peaks but with a clearly dominant component and larger line widths and are systematically associated with outflows, while group 2 cores have several comparable subentities, smaller line widths, and no association with outflows. We speculate that more massive cores may form from smaller cores via coalescence or competitive accretion. Even conservative estimates of outflow mass-loss rates, however, indicate that accretion is the dominant process in the later formation of massive protostars from such cores. We find a flattening of the outflow mass spectra with increasing flow velocities, at variance with previous studies that suggest a steepening with increasing flow velocities. In the light of this result we suggest a reevaluation of the wide-angle wind momentum-driven flow models to describe the acceleration of outflows in the earliest stages of massive star formation.