RSAA Colloquia / Seminars / Feast-of-Facts: Friday, 27 May 2016, 14:00-14:20; Duffield Lecture Theatre

Michael Dopita

"The Discovery of Raman Scattering in HII Regions"

We report here on the discovery of faint extended wings of H observed out to an apparent velocity of 7600 kms−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 OI and Si II resonance lines and stellar continuum UV photons with HI followed by radiative decay to the HI n = 2 level. The broad wings also seen in H and in H result from Raman scattering of the UV continuum in the HI n = 4 and n = 5 levels respectively.The Raman scattering fluorescence is correlated with the intensity of the narrow permitted lines of OI and Si II. In the case of Si II, this is explained by radiative pumping of the same 1023.7°A resonance line involved in the Raman scattering by the Ly radiation field. The subsequent radiative cascade produces enhanced Si II 5978.9, 6347.1 and 6371.4°A permitted transitions. Finally we show that in OI, radiative pumping of the 1025.76°A 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 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 ionisation front.