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

摘要

Context. This paper is the third in a series of NH _(3) ?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 NH _(3) in the high-mass star-forming region W51 Main (distance?=?5.4 kpc), 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 hbox{$arcs$} 2 ? 0 hbox{$arcs$} 3 angular resolution in five metastable ( J = K ) inversion transitions of ammonia (NH _(3) ): ( J,K ) = (6, 6) , (7,?7), (9,?9), (10,?10), and (13,?13). These lines arise from energy levels between ~ 400 K and ~ 1700 K above the ground state. We also made maps of the (free-free) continuum emission at frequencies between 25 and 36 GHz. 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 NH _(3) ?is observed in absorption, and two additional dusty cores, W51e2-E ( ~ 0 hbox{$arcs$} 8 to the East) and W51e2-NW ( ~ 1 ′′ ?to the North), where hot NH _(3) ?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 ( & 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 [NH _(3) ]/[H _(2)] = 10~(-7) ) 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 showing evidence of rotation, shows signatures of infall in a hot dense core ( T ~ 170 K, n _(H_(2)) ~ 5 × 10~(7) cm ~(-3) ), based on asymmetric spectral profiles (skewed toward the blueshifted component) in optically thick emission lines of NH _(3) . 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 ( n _(H_(2)) ~ 2 × 10~(7) cm ~(-3) ?and ~ 3 × 10~(6) cm ~(-3) ), are also hot cores ( T _(gas) ~ 140 and 200 K) and contain a significant amount of molecular gas ( M _(gas) ~ 30 M _(⊙) ?and ~ 70 M _(⊙) , 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 NH _(3) ?metastable lines, we characterized the physical and dynamical properties of four individual high-mass young stars forming in the W51 Main clump.