2015A&A...573A.109G

This paper is the second in a series of ammonia (NH₃) multilevel imaging studies in high-mass star forming regions. We want to identify the location of the maser emission from highly excited levels of ammonia within the W51 IRS2 high-mass star forming complex that was previously discovered in a single dish monitoring program. We have used the Karl Jansky Very Large Array (JVLA) at the 1 cm band to map five highly excited metastable inversion transitions of NH₃, (J,K)=(6,6), (7,7), (9,9), (10,10), and (13,13), in W51 IRS2 with ∼0.2" angular resolution. We present detections of both thermal (extended) ammonia emission in the five inversion lines, with rotational states ranging in energy from about 400K to 1700K, and point-like ammonia maser emission in the (6,6), (7,7), and (9,9) lines. For the point-like emission, we estimate lower limits to the peak brightness temperatures of 1.7x10⁵K, 6x10³K, and 1x10⁴K for the (6,6), (7,7), and (9,9) transitions, respectively, confirming their maser nature. The thermal ammonia emits around a local standard of rest velocity of V_LSR=60km/s, near the cloud's systemic velocity, appears elongated in the east-west direction across 4" and is confined by the HII regions W51d (to the north), W51d1 (to the east), and W51d2 (to the west). The NH₃ masers are observed in the eastern tip of the dense clump traced by thermal NH₃, offset by 0.65" to the east from its emission peak, and have a peak velocity at ∼47.5km/s. No maser components are detected near the systemic velocity. The NH₃ masers arise close to but separated from (0.65" or 3500AU) the rare vibrationally excited SiO masers, which are excited in a powerful bipolar outflow driven by the deeply embedded high-mass young stellar object (YSO) W51-North. This means that the two maser species cannot be excited by the same object. Interestingly, the NH₃ masers originate at the same sky position as a peak in a submm line of SO₂ imaged with the Submillimeter Array, tracing a face-on circumstellar disk or ring around W51-North. In addition, the thermal emission from the most highly excited NH₃ lines, (10,10) and (13,13), shows two main condensations, the dominant one towards W51-North with the SiO and H₂O masers, and a weaker peak at the NH₃ maser position. We propose a scenario where the ring seen in SO₂ emission is a circumbinary disk surrounding (at least) two high-mass YSOs, W51-North (exciting the SiO masers) and a nearby companion (exciting the NH₃ masers), separated by 3500 AU. This finding indicates a physical connection (in a binary) between the two rare SiO and NH₃ maser species.