Waves in Orion, a surfers paradise for ghosts

Surfer on a beach in Ensenada, Baja California, Mexico.

Surfer on the San Miguel beach in Ensenada, Baja California, Mexico.

Can you imagine a bunch of ghosts surfing in the Orion Nebula? In Ensenada there are a few places where one can always see surfers in their black wetsuits waiting for a wave to catch. Recently, I started to imagine rather more transparent, whitish ghosts riding whispy waves on a glowing reddish ocean of gas and dust illuminated by four bright stars arranged in a Trapezium. Don´t worry, I am not on any weird drugs…it´s just the discovery of 3D Kelvin-Helmholtz instabilities in the Orion Nebula that triggered my imagination …

When it was published, I missed the original discovery paper by Berné et al. in Nature, but recently came across the paper by Hendrix et al. (2015) on 3D simulations of Kelvin-Helmholtz instabilities to explain the wave patterns that Berné et al. discovered in Orion. The rippled structures can be seen in 3.6 micron observations of the Spitzer infrared space telescope.

The simulations done by Hendrix et al. include the dust grains that produce the infrared radiation in the 3D hydrodynamical simulations and even explore how different grain sizes move and accumulate differently in the wave pattern. They show how the geometric position of the illuminating star and the orientation with respect to us is important for the observed structure. Since Berné et al. actually found three different sites of wave patterns and since unfavorable illumination and orientation can make the detection much more difficult, who knows how many wave patterns are actually present on the shores of Orion.

From the phenomenological point of view, the Kelvin-Helmholtz instability occurs when a more tenuous fluid flows over a denser one in the presence of a transverse velocity gradient. Given the right conditions, seed ripples can then be enhanced and evolve into full-fletched waves. Ever since I applied these concept to explain some patterns in helical radio jets back during my PhD-Thesis, I find these instabilities fascinating.

Coming back to the views in Ensenada, there is an auto-dealer on the coastal boulevard that has one of those air dancers, wiggly tubes made of fabric with air flowing through them. They are a nice model for light astrophysics jets, such as those coming out of active galactic nuclei. Most of them show the socalled kink-instability, making them bend and stand up again and again. But my favorite one, at the auto-dealer, actually produces a very nice helical Kelvin-Helmholtz instability, which can appear in quasar jets such as S5 1803+784. So, next time you see surfers on the ocean or one of those air dancers, they might transport your thoughts right back to outer space.


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