2016A&A...589A..44G

Context. This paper is the third in a series of NH₃ multilevel imaging studies in well-known, high-mass star-forming regions. The main goal is to characterize kinematics and physical conditions of (hot and dense) circumstellar molecular gas around O-type young stars.
Aims. We want to map at subarcsecond resolution highly excited inversion lines of NH₃ in the high-mass star-forming region W51 Main (distance=5.4kpc), which is an ideal target to constrain theoretical models of high-mass star formation.
Methods. Using the Karl Jansky Very Large Array (JVLA), we 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). These lines arise from energy levels between ∼400K and ∼1700K above the ground state. We also made maps of the (free-free) continuum emission at frequencies between 25 and 36GHz.
Results. We have identified and characterized two main centers of high-mass star formation in W51 Main, which excite hot cores and host one or multiple high-mass young stellar objects (YSOs) at their centers: the W51e2 complex and the W51e8 core (∼6" southward of W51e2). The former breaks down into three further subcores: 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 toward the HC HII region show a clear velocity gradient along the east-west in all lines. The gradient may indicate rotation, although any Keplerian motion must be on smaller scales (<1000AU) 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 (∼5M_☉, assuming [NH₃]/[H₂]=10^–7) with respect to the mass of the central YSO estimated from radio luminosity (>20M_☉), both indicate that the central YSO has already accreted most of its final mass. On the other hand, the nearby W51e2-E, while not showing evidence of rotation, shows signatures of infall in a hot dense core (T∼170K, n_H2∼5x10⁷cm^–3), based on asymmetric spectral profiles (skewed toward the blueshifted component) in optically thick emission lines of NH₃. The relatively large amount of hot molecular gas available for accretion (∼20M_☉ within about half an arcsecond or 2500AU), 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 (n_H2∼2x10⁷cm^–3 and ∼3x10⁶cm^–3), are also hot cores (T_gas∼140 and 200K) and contain a significant amount of molecular gas (M_gas∼30M_☉ and ∼70M_☉, respectively). We speculate that they may host high-mass YSOs either at a previous evolutionary stage or with a mass that is lower than W51e2-E and W51e2-W.
Conclusions. Using high-angular resolution multilevel 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.