Hot ammonia around young O-type stars III. High-mass star formation and hot core activity in W51 Main

Context. This paper is the third in a series of NH3 multilevel imaging studies in well-known high-mass star forming regions. Aims. We want to map at sub-arcsecond resolution highly-excited inversion lines of NH3 in the high-mass star forming region W51 Main (distance=5.4 kpc). Methods. Using the Karl Jansky Very Large Array (JVLA), we have mapped the hot and dense molecular gas in W51 Main, with ∼0. ′′2–0.′′3 angular resolution, in five metastable (J=K) inversion transitions of ammonia (NH3): (J,K)=(6,6), (7,7), (9,9), (10,10), and (13,13). Results. We have identified and characterized two main centers of high-mass star formation in W51-Main: the W51e2 complex and the W51e8 core (∼6′′ southward of W51e2). The former breaks down into three further sub-cores: W51e2-W, which surrounds the well known hypercompact (HC) HII region, where hot NH3 is observed in absorption, and two additional dusty cores, W51e2-E (∼0. ′′8 to the East) and W51e2-NW (∼1′′ to the North), where hot NH3 is observed in emission. The velocity maps towards the HC HII region show a clear velocity gradient along east-west in all lines. The gradient may indicate rotation, though any Keplerian motion must be on smaller scales (<1000 AU) as we do not directly observe a Keplerian velocity profile. The absence of outflow and/or maser activity and the low amount of molecular gas available for accretion (∼5 M , assuming [NH3]/[H2]=10) with respect to the mass of the central YSO estimated from radio luminosity (>20 M ), both indicate that the central YSO has already accreted most of its final mass. On the other hand, the nearby W51e2-E, while not displaying evidence for rotation, shows signatures of infall in a hot dense core (T ∼ 170 K, nH2 ∼ 5 × 107 cm−3), based on asymmetric spectral profiles (skewed towards the blueshifted component) in optically thick emission lines of NH3. The relatively large amount of hot molecular gas available for accretion (∼20 M within about half an arcsecond or 2500 AU), along with strong outflow and maser activity, indicates that the main accretion center in the W51e2 complex is W51e2-E rather than W51e2-W. Finally, W51e2-NW and W51e8, although less dense (nH2 ∼ 2 × 107 cm−3 and ∼ 3 × 106 cm−3), are also hot cores (Tgas ∼140 and 200 K) and contain a significant amount of molecular gas (Mgas ∼30 M and ∼70 M , respectively). We speculate that they may host high-mass YSOs either at a previous evolutionary stage to or with lower mass than W51e2-E and W51e2-W. Conclusions. Using high-angular resolution multi-level imaging of highly-excited NH3 metastable lines, we characterized the physical and dynamical properties of four individual high-mass young stars forming in the W51 Main clump.