The First Extrasolar Measurement of Stellar He II and O III Bowen Fluorescence Emission in the EUV: A New Diagnostic of Hot Star Wind Conditions Applied to ɛ Canis Majoris (B 2 II)

Physics – Plasma Physics

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The B bright giant ɛ CMa is one of only two OB stars observable with the Extreme Ultraviolet Explorer (EUVE) spectrometers (between 70 Åand 760ÅOne of the most interesting aspects of this unique spectrum is the presence of strong emission lines at 304 ÅHe II Lyman-α) and 374 ÅO III) due to the Bowen fluorescence mechanism. In this process, the He II 304 Åline pumps a resonance line of O III and the subsequent radiative decay yields several UV emission lines between 3000 Åand 4000 ÅThese lines are observed in nebulae, AGN, and novae, but the final O III transition leads to emission near 400 Åwhich had never before been seen in any astrophysical object outside of the solar system. The Bowen emission lines are formed in the radiation-driven stellar wind of ɛ CMa, as is shown by the Doppler-broadened 304 Åline profile. Our modeling indicates that the He II 304 Åline is sensitive to the X-ray emission in the wind of ɛ CMa, due to the importance of X-rays photoionization in controlling the ionization of helium. We also explore the temperature-sensitivity of the 374 ÅÅand 435 ÅO III lines, and use the non-detection of the latter two lines to place an upper limit on the wind temperature.

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