... nebula

This article is dedicated to the memory of Wolfgang Wiedenhöver, who lead the efforts on the developments of the correlator array backend MACS for CHAMP.

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...$\zeta$ Ori

The spectral type of $\zeta$ Ori is O 9.5 Ib, as compared to O 9.5 V for $\sigma$ Ori. The two stars are at roughly the same projected distance from the Horsehead. Depending on the location along the line of sight and the amount of shielding by the protruding material between the star and the nebula (see Fig. 1 of Pound et al.), the illumination from $\zeta$ Ori may be important.

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