Ammonia in the hot core W51-IRS2: Maser line profiles, variability, and saturation

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
² Astronomy Department, Faculty of Science, King Abdulaziz University, PO Box 80203, Jeddah 21589, Saudi Arabia
³ Xinjiang Astronomical Observatory, Chinese Academy of Sciences, 830011 Urumqi, PR China
⁴ Purple Mountain Observatory (PMO) and Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, 10 Yuanhua Road, Nanjing 210023, PR China
⁵ National Radio Astronomy Obsrevatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA

^★ Corresponding author.

Received: 28 March 2025
Accepted: 19 June 2025

Abstract

W51-IRS2 is known to be one of the most prolific sources of interstellar ammonia (NH₃) maser lines. So far, however, many of these inversion lines have rarely been studied. Here we report spectrally resolved line profiles for the majority of detected features and provide information on the variability of these maser components between 2012 and 2023. This includes the first tentative detection of a (J, K) = (5, 2) maser in the interstellar medium and the first tentative detection of a (6,4) maser in W51-IRS2. Furthermore, we report for the first time NH₃ (9,6) maser emission below local standard of rest velocities of 50 km s^–1 in this source as well as double maser features occasionally seen in other transitions. The detected maser lines strongly indicate vibrational pumping by ≈10 μm photons, which must be abundant due to the high kinetic temperature (≈300 K) of the ammonia emitting gas. The detection of vibrationally excited NH₃, suggesting a vibrational excitation temperature consistent with the kinetic one, and a comparison with measured SiO line profiles, are also presented. For the (10,7) line, we find a tentative correlation between flux density and line width compatible with unsaturated maser emission. The velocity drift of the so-called 45 km s^–1 maser features, reported to be +0.2 km s^–1 yr^–1 between 1996 and 2012, has either slowed down to values <0.1 km s^–1 or has entirely disappeared. In 2023, the component is only seen in ammonia inversion lines that are located at least 800 K above the ground state. The other features have faded. Possible scenarios explaining this phenomenon are discussed.