Hot ammonia around young O-type stars

II. JVLA imaging of highly excited metastable NH₃ masers in W51-North

¹ Joint Institute for VLBI in Europe, Postbox 2, 7990 AA Dwingeloo, The Netherlands
e-mail: goddi@jive.nl
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Astronomy Department, Abdulaziz University, PO Box 80203, 21589 Jeddah, Saudi Arabia
⁴ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁵ Centro de Radioastronomía y Astrofisíca, UNAM, Morelia, CP 58089 Michoacán, México, Mexico
⁶ US Naval Research Laboratory, Code 7213, Washington, DC 20375, USA

Received: 28 July 2014
Accepted: 1 November 2014

Abstract

Context. This paper is the second in a series of ammonia (NH₃) multilevel imaging studies in high-mass star forming regions.

Aims. 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.

Methods. 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.

Results. We present detections of both thermal (extended) ammonia emission in the five inversion lines, with rotational states ranging in energy from about 400 K to 1700 K, 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 1.7 × 10⁵ K, 6 × 10³ K, and 1 × 10⁴ K 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 = 60 km s^-1, 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 ~47.5 km s^-1. No maser components are detected near the systemic velocity. The NH₃ masers arise close to but separated from (0.̋65 or 3500 AU) 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.

Conclusions. 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.