page_title: Sigma Orionis
section_title: Dust wave
hierarchical_section_title: Sigma Orionis / Dust wave
caption_attribution_description: English: The Flame Nebula and surrounding region. The bright isolated star is Alnilam. Alnitak is difficult to see within the bright nebulosity of the Flame Nebula. The Horsehead Nebula is barely visible at the centre of the image. NGC 2023 is much clearer at these wavelengths, it is the small bright patch of nebulosity just below the Flame Nebula. The red arc surrounds σ Orionis. In this WISE image, blue represents 3.4 microns, cyan is 4.6 microns, green is 12 microns, and red is 22 microns.
context_page_description: Sigma Orionis or Sigma Ori is a multiple star system in the constellation Orion, consisting of the brightest members of a young open cluster. It is found at the eastern end of the belt, south west of Alnitak and west of the Horsehead Nebula which it partially illuminates. The total brightness of the component stars is magnitude 3.80.
context_section_description: In infrared images, a prominent arc is visible centred on σ Ori AB.  It is about 50" away from the class O star, around 0.1 parsecs at its distance.  It is directed towards IC434, the Horesehead Nebula, in line with the space motion of the star.  The appearance is similar to a bowshock, but the type of radiation shows that it is not a bowshock.  The observed infrared emission, peaking at around 45 microns, can be modelled by two approximately black-body components, one at 68K and one at 197 K.  These are thought to be produced by two different sizes of dust grains.
  The material of the arc is theorised to be produced by photoevaporation from the molecular cloud around the Horsehead Nebula.  The dust becomes decoupled from the gas that carried it away from the molecular cloud by radiation pressure from the hot stars at the centre of the σ Ori cluster.  The dust accumulates into a denser region that is heated and forms the visible infrared shape.
  The term "dust wave" is applied when the dust piles up but the gas is largely unaffected, as opposed to a "bow wave" where both dust and gas are stopped.  Dust waves occur when the interstellar medium is sufficiently dense and the stellar wind sufficiently weak that the dust stand-off distance is larger than the stand-off distance of a bow shock.  This would clearly be more likely for slow-moving stars, but slow-moving luminous stars may not have lifetimes long enough to produce a bow wave.  Low luminosity late class O stars should commonly produce bow waves if this model is correct.
caption: The arc in infrared light, with red representing 22 microns.
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