Volume 549, January 2013
|Number of page(s)||10|
|Section||Interstellar and circumstellar matter|
|Published online||03 January 2013|
Ammonia in the hot core W 51-IRS2: 11 new maser lines and a maser component with a velocity drift
Max-Planck-Institut für Radioastronomie,
Auf dem Hügel 69,
2 Astronomy Department, Faculty of Science, King Abdulaziz University, PO Box 80203, Jeddah, Saudi Arabia
3 Naval Research Laboratory, Code 7210, Washington, DC 20375, USA
4 Joint ALMA Observatory, Avda. Alonso de Córdova 3107, Vitacura, Santiago de Chile, Chile
Received: 24 July 2012
Accepted: 3 November 2012
With the 100-m telescope at Effelsberg, 19 ammonia (NH3) maser lines have been detected toward the prominent massive star forming region W51-IRS2. Eleven of these inversion lines, the (J, K) = (6, 2), (5, 3), (7, 4), (8, 5), (7, 6), (7, 7), (9, 7), (10, 7), (9, 9), (10, 9), and (12, 12) transitions, are classified as masers for the first time in outer space. All detected masers are related to highly excited inversion doublets. The (5, 4) maser originates from an inversion doublet ~340 K above the ground state, while the (12, 12) transition, at ~1450 K, is the most highly excited NH3 maser line so far known. Strong variability is seen not only in ortho- but also in para-NH3 transitions. Bright narrow emission features are observed, for the first time, in (mostly) ortho-ammonia transitions, at VLSR ~ 45 km s-1, well separated from the quasi-thermal emission near 60 km s-1. These features were absent ~25 years ago and show a velocity drift of about +0.2 km s-1 yr-1. The component is likely related to the SiO maser source in W51-IRS2 and a possible scenario explaining the velocity drift is outlined. The 57 km s-1 component of the (9, 6) maser line is found to be strongly linearly polarised. Maser emission in the (J, K) to (J + 1, K) inversion doublets is strictly forbidden by selection rules for electric dipole transitions in the ground vibrational state. However, such pairs (and even triplets with (J + 2, K)) are common toward W51-IRS2. Similarities in line widths and velocities indicate that such groups of maser lines arise from the same regions, which can be explained by pumping through vibrational excitation. The large number of NH3 maser lines in W51-IRS2 is most likely related to the exceptionally high kinetic temperature and NH3 column density of this young massive star forming region.
Key words: masers / ISM: clouds / ISM: individual objects: W51 / HII regions / ISM: molecules / radio lines: ISM
© ESO, 2013
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.