The discovery of Raman scattering in H II regions

Abstract

We report here on the discovery of faint extended wings of Hα observed out to an apparent velocity of ∼7600 km s⁻¹ in the Orion Nebula (M42) and in five H II 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 Hβ and in Hγ 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 Å resonance line involved in the Raman scattering by the Lyβ radiation field. The subsequent radiative cascade produces enhanced Si II λλ5978.9, 6347.1, and 6371.4 Å permitted transitions. Finally, we show that in O I, radiative pumping of the 1025.76 Å resonance line by the Lyman series radiation field is also the cause of the enhancement in the permitted lines of this species lying near Hα in wavelength, but here the process is a little more complex. We argue that all these processes are active in the zone of the H II region near the ionization front.