The Discovery of Raman Scattering in HII Regions

TL;DR

This paper reports the discovery of faint broad wings in H extalpha\, Heta\, and H extgamma\, in HII regions, caused by Raman scattering of UV photons with H I, revealing new insights into the physical processes near ionization fronts.

Contribution

It demonstrates for the first time that Raman scattering of UV photons causes broad wings in Balmer lines in HII regions, linking these features to specific resonance lines and radiative processes.

Findings

Raman scattering causes broad wings in Balmer lines in HII regions.

The wings are linked to resonance lines of O I and Si II and UV photon interactions.

Enhanced permitted lines are explained by radiative pumping and cascades.

Abstract

We report here on the discovery of faint extended wings of H\alpha\ observed out to an apparent velocity of ~ 7600 km/s in the Orion Nebula (M42) and in five HII regions in the Large and the Small Magellanic Clouds. We show that, these wings are caused by Raman scattering of both the O I and Si II resonance lines and stellar continuum UV photons with H I followed by radiative decay to the H I n=2 level. The broad wings also seen in H\beta\ and in H\gamma\ result from Raman scattering of the UV continuum in the H I n=4 and n=5 levels respectively.The Raman scattering fluorescence is correlated with the intensity of the narrow permitted lines of O I and Si II. In the case of Si II, this is explained by radiative pumping of the same 1023.7\AA\ resonance line involved in the Raman scattering by the Ly\beta\ radiation field. The subsequent radiative cascade produces enhanced Si II 5978.9, 6347.1$ and 6371.4\AA\ permitted transitions. Finally we show that in O I, radiative pumping of the 1025.76\AA\ resonance line by the Lyman series radiation field is also the cause of the enhancement in the permitted lines of this species lying near H\alpha\ in wavelength, but here the process is a little more complex. We argue that all these processes are active in the zone of the HII region near the ionisation front