A) Two color composite in the mid-IR of the region around σ Ori AB (white asterisk). Green is WISE 12 μm, whereas red is WISE 22 μm. The large IR counterpart of the GS206-17+13 HI shell surrounds σ Ori AB. The east side of the bubble is marked by the large molecular cloud L1630, which includes the Horsehead Nebula (HH). The projected distance between the Horsehead and σ Ori, as shown by the white line along which the Spitzer/IRS observations are positioned (Fig. 4), is about 0.5 degrees or 3.7 pc (at 385 pc; Caballero 2008). The white cross reveals the offset position used for the Spitzer/IRS background subtraction. B) Smoothed Hα image from SHASSA (4′ resolution). The gas emission around σ Ori AB (IC 434) originates from a photo-evaporation flow of the L1630 molecular cloud. The bright part appears roughly circular with a radius ~0.75°. C) Schematic view of the same region, showing the IR bubble (solid outline) and the Hα emission (dashed circle), created by the photo-evaporation flow (arrows) of the L1630 molecular cloud (filled black rectangle).