2013A&A...549A.122M

Many aspects of massive star (>10M_☉) formation are still unclear. In particular, the outflow properties at close distance (100-1000AU) from a massive young stellar object (MYSO) are not yet well established. This work presents a detailed study of the gas kinematics toward the hot molecular core (HMC) G31.41+0.31. To study the HMC 3D kinematics at milli-arcsecond angular resolution, we performed multi-epoch VLBI observations of the H₂O 22GHz and CH₃OH 6.7GHz masers, and single-epoch VLBI of the OH 1.6GHz masers. Water masers present a symmetric spatial distribution with respect to the HMC center, where two nearby (0.2" apart), compact, VLA sources (labeled ``A'' and ``B'') are previously detected. The spatial distribution of a first group of water masers, named ``J1'', is well fit with an elliptical profile, and the maser proper motions mainly diverge from the ellipse center, with average speed of 36km/s. These findings strongly suggest that the ``J1'' water maser group traces the heads of a young (dynamical time of 1.3x10³yr), powerful (momentum rate of ≃0.2M_☉/yr.km/s), collimated (semi-opening angle ≃10°) jet emerging from a MYSO located close (within ≃0.15)" to the VLA source ``B''. Most of the water features not belonging to ``J1'' present an elongated (≃2'' in size), NE-SW oriented (PA≃70°), S-shape distribution, which we denote with the label ``J2''. The elongated distribution of the ``J2'' group and the direction of motion, approximately parallel to the direction of elongation, of most ``J2'' water masers suggests the presence of another collimated outflow, emitted from a MYSO placed near the VLA source ``A''. The proper motions of the CH₃OH 6.7GHz masers, mostly diverging from the HMC center, also witness the expansion of the HMC gas driven by the ``J1'' and ``J2'' jets. The orientation (PA≃70°) of the ``J2'' jet agrees well with that (PA = 68°) of the well-defined V<sub>LSR</sub> gradient across the HMC revealed by previous interferometric, thermal line observations. Furthermore, the ``J2'' jet is powerful enough to sustain the large momentum rate, 0.3M_☉/yr.km/s, estimated from the interferometric, molecular line data in the assumption that the V_LSR gradient represents a collimated outflow. These two facts lead us to favor the interpretation of the V_LSR gradient across the G31.41+0.31 HMC in terms of a compact and collimated outflow.