The Formation of Massive Stars. High Resolution Millimeter and Radio Studies of High-Mass Protostellar Candidates

by

S. Molinari, L. Testi, L.F. Rodriguez, Q. Zhang

Abstract:

We used the Owens Valley Millimeter Array and the Very Large Array to obtain interferometric maps at millimeter and centimeter wavelengths, both in the continuum and in various lines (HCO+, H13CO+, SiO and H13CN), 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 not associated with HII regions. All 11 sources were detected in HCO+, 9 in mm-continuum and 5 (out of 8 observed) in cm-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 6 sources, with flow masses and momenta much higher than in low-mass Young Stellar Objects (YSOs). In many of the sources the molecular emission is organised in sub-structures, resolved both spatially and in velocity. We find that the sources may be characterised 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, larger linewidths, and are systematically associated with outflows, while group 2 cores have several, comparable sub-entities, smaller linewidths, 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 re-evaluation of the wide-angle wind momentum-driven flow models to describe the acceleration of outflows in the earliest stages of massive star formation.


Mantained by: Leonardo Testi