2016/05/28 by Michael A. Dopita, David C. Nicholls, Ralph S. Sutherland +2
Physics and Astronomy · #Astronomy and Astrophysical Research #Astrophysics and Star Formation Studies #Photon #Radiative transfer #Raman scattering #Raman spectroscopy #Resonance (particle physics) #Scattering #Spectral line #Stellar, planetary, and galactic studies #X-ray Raman scattering #astro-ph.GA
paper · pdf · doi:10.3847/2041-8205/824/1/l13
Accepted for publication in ApJ (Letters). 6pp, 3 figs
arxiv created 2016/05/28 · openalex publication_date 2016/06/08 · arxiv updated 2016/06/15 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05
ABSTRACT We report here on the discovery of faint extended wings of H α 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 H β and in H γ 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 Ly β radiation field. The subsequent radiative cascade produces enhanced Si ii λλ 5978.9, 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 H α 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.