First detection of a THz water maser in NGC 7538-IRS1 with SOFIA and new 22 GHz e-MERLIN maps

Context. The formation of massive stars (M> 10M⊙, L > 103L⊙) is still not well understood. Accumulating a large amount of mass infalling within a single entity in spite of radiation pressure is possible if, in addition to several other conditions, enough thermal energy is released. Despite numerous water line observations over a broad range of energies obtained with the Herschel Space Observatory, observations were not able to trace the emission from the hot core around the newly forming protostellar object in most of the sources. Aims. We wish to probe the physical conditions and water abundance in the inner layers of the host protostellar object NGC 7538-IRS1 using a highly excited H2O line. Water maser models predict that several THz water masers should be detectable in these objects. We therefore aim to detect the o-H2O 82,7−73,4 line in a star forming region for the first time. Model calculations have predicted this line to show maser action. Methods. We present SOFIA observations of the o-H2O 82,7−73,4 line at 1296.41106 GHz and a 616−523 22 GHz e-MERLIN map of the region (the very first 22 GHz images made after the e-MERLIN upgrade). In order to be able to constrain the nature of the emission – thermal or maser – we used near-simultaneous observations of the 22 GHz water maser performed with the Effelsberg radiotelescope and e-MERLIN. A thermal water model using the RATRAN radiative transfer code is presented based on HIFI pointed observations. Molecular water abundances are derived for the hot core. Results. The o-H2O 82,7−73,4 line is detected toward NGC 7538-IRS1 with one feature at the source velocity (–57.7 kms-1) and another one at –48.4 kms-1. We propose that the emission at the source velocity is consistent with thermal excitation and is excited in the innermost part of the IRS1a, in the closest circumstellar environment of the massive protostellar object. The other emission is very likely the first detection of a water THz maser line, pumped by shocks due to the IRS1b outflow, in a star-forming region. Assuming thermal excitation of the THz line, the water abundance in the hot core of NGC 7538-IRS1 is estimated to be 5.2 × 10-5 with respect to H2.

Effect on Doppler Resonance From a Near-Surface Shear Layer

For waves generated by a wave source which is simultaneously moving and oscillating at a constant frequency ω, a resonance is well known to occur at a particular value τres of the nondimensional frequency τ = ωV/g (V: source velocity relative to the surface, g: gravitational acceleration). For quiescent, deep water, it is well known that τres = 1/4. We study the effect on τres from the presence of a shear flow in a layer near the surface, such as may be generated by wind or tidal currents. Assuming the vorticity is constant within the shear layer, we find that the effects on the resonant frequency can be significant even for sources corresponding to moderate shear and relatively long waves, while for stronger shear and shorter waves the effect is stronger. Even for a situation where the resonant waves have wavelengths about 20 times the width of the shear layer, the resonance frequency can change by ∼ 25% for even a moderately strong shear VS/g = 0.3 (S: vorticity in surface shear layer). Intuition for the problem is built by first considering two simpler geometries: uniform current with finite depth, and Couette flow of finite depth.

Analysis on the Characteristic of Cross-Correlated Field and Its Potential Application on Source Localization in Deep Water

The interference characteristics of cross-correlated broadband fields received by a single hydrophone deployed near the bottom at two different ranges are analyzed in this paper. The ray theory is used to derive the interference pattern, which is a combination of two kinds of interference phenomena. One kind of interference period contains the information of radial source velocity, and the other one is related to source depth. The source motion parameters, including the range and time of closet point of approach (CPA), constant source velocity and source depth, can be estimated by computing the Fourier transform of cross-correlated broadband fields when the existence of a CPA is apparent in the data. If the CPA is not evident, only the radial source velocity and the discrimination of surface versus submerged source can be provided. Note that the proposed method is a broadband technique. Experimental results confirm this single hydrophone estimation method of radial source velocity and source depth.