A Twinkle in the Wolf’s Eye – IC 4406: A Hubble Visualization by Jukka Metsavainio

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During the month of May, the “Wolf” rises and prowls the skies after midnight. Lupus was one of the 48 original constellations listed by the first century astronomer Ptolemy and on its western border is a Wolf-Rayet planetary nebula – IC 4406 – which contains some of the hottest stars known to be in existence. What exactly lay inside this 1900 light year distant torus-shaped cloud of dust? Then let’s really step inside this Hubble dimensional visualization by Jukka Metsavanio and take a closer look…

Whenever we present a dimensional visualization it is done in two fashions. The first is called “Parallel Vision” and it is much like a magic eye puzzle. When you open the full size image and your eyes are the correct distance from the screen, the images will seem to merge and create a 3D effect. However, for some folks, this doesn’t work well – so Jukka has also created the “Cross Version”, where you simply cross your eyes and the images will merge, creating a central image which appears 3D. As we learned some time ago, it might not always work for all people, but there are a few other tricks you can try. Now sit back and prepare to be blown away…

The rectangluar appearance of planetary nebula, IC 4406, isn’t such a great mystery. We know from looking at a great number of objects that our point of view affects how we see things and we realize we’re seeing this incredible structure almost in the plane of its equator. Astronomers believe the entirety of the nebula is shaped like a prolate spheroid – where the polar diameter is greater than the equatorial diameter. Why such an unusual shape? Quite probably because IC 4406 is believed to be bipolar. No. It’s not going to freak out on you… It simply means this planetary nebula has an axially symmetric bi-lobed appearance. This may be the beginnings or the endings of the evolutionary stages of all planetary nebulae – but it does have its quirks.

While the function that shapes this structure isn’t exactly clear to astronomers, many believe it may belong to the physical process known as bipolar outflow – continuous highly energetic streams of gas emanating from the poles of a star. What types of stars? Again, it isn’t always clear. Bipolar outflow can occur with protostars where a dense, concentrated jet produces a supersonic shock fronts. More evolved young stars, such as T-Tauri types, also produce bow shocks visible at optical wavelengths that we refer to as Herbig-Haro objects. Evolved stars produce spherically-symmetric winds (called post-AGB winds) that are focused into cones and eventually become classic planetary nebula structures. There is even speculation that these outflows may be impacting with interstellar dust surrounding the star or supernova remnants. But… what exactly causes these beautiful structures we see inside?

According to C.R. O’Dell: “This progression begins with dark tangential structures showing no alignment with the central star and location near the main ionization front. At the end of the progression in the largest nebulae, the knots are located throughout much of the ionized zone, where they are photoionized on the side facing the central star and accompanied by long tails well aligned radially. This modification of characteristics is what would be expected if the knots were formed near or outside the main ionization front, obtaining densities high enough to lead to their being only partially ionized as they are fully illuminated by the Lyman continuum (Lyc) radiation field. Their expansion velocities must be lower than that of the main body of the nebular shell. Their forms are altered by exposure to the radiation field from the star, although it is not clear as to the relative role of radiation pressure acting on the dust component vis-à-vis ionization shadowing.”

However, there is something a bit unusual about IC 4406, isn’t there? That’s right. It contains a Wolf-Rayet star. Descended from O-types, these massive, extremely luminous beauties have strong stellar winds and are well-known for spouting off their unprocessed outer H-rich layers. The dense, high-velocity winds then rip at the superheated stellar photosphere, unleashing ultra-violet radiation which in turn causes fluorescence in the line-forming wind region. Most continue on to become Ib or Ic type supernovae, and just a very few (only 10%) become the central stars of planetary nebulae. So is the beautiful patterns we see in IC 4406 the beginning or the end? Says C.R. O’Dell:

“We find knots in all of the objects, arguing that knots are common, simply not always observed because of distance. The knots appear to form early in the life cycle of the nebula, probably being formed by an instability mechanism operating at the nebula’s ionization front. As the front passes through the knots they are exposed to the photoionizing radiation field of the central star, causing them to be modified in their appearance. This would then explain as evolution the difference of appearance like the lacy filaments seen only in extinction in IC 4406… Theoretical models have considered only symmetric instabilities, but there seems to be nothing that precludes the formation of elongated concentrations like one sees in IC 4406.”

In the meantime, many of you will recognize these filaments in this planetary by its more common name – the “Retina Nebula” – the third to have its spatial distribution of H2 and CO emissions mapped to prove that the equatorial density is caused by the high-velocity outflow of the progenitor AGB star – and perhaps the twinkle in its eye could have either the beginnings or the end of what may have been planetary systems. Says R. Sahai: “It is suggested that the equatorial tori observed or deduced in IC 4406 results from ‘born again’ disks formed through the destruction of planetary systems at the end of the AGB evolutionary phase.”

Are these filaments shaped by magnetic fields? The work of Hanna Dahlgren opens some very interesting ideas: “We propose a theory where the magnetic fields control the sculpting and evolution of small-scale filaments. This theory demonstrates how the substructures may form magnetized flux ropes that are twisted around each other, in the shape of double helices. Similar structures, and with similar origin, are found in many other astrophysical environments.” And will they survive? Says C.R. O’Dell:

“What the future holds in store for the knots in PN is quite important because whichever mechanism is producing them is locking a substantial fraction of the mass into molecular knots and these knots are escaping from the gravitational field of the central star (Meaburn et al. 1998). The process of photoionization means that there will be photoevaporation of material from the knots. The situation will be very much like the proplyds in the Orion Nebula, where the inner molecular core is heated by photons of less than 13.6 eV, causing a slow flow of gas away from the core. When this gas reaches the knots’ ionization front it is photoionized and heated, then it is rapidly accelerated to a velocity of about 10 km s. The estimated evaporation timescale for the outward moving knots is several thousand years. Many or most of them will therefore survive the hot-luminous phase close to the star and will be ejected into the surrounding interstellar medium.”

As just another twinkle in the Wolf’s eyes…

Many thanks to JP Metsavainio of Northern Galactic for his magic with Hubble Space Telescope images and allowing us this incredible look inside another mystery of space.