A Twist on the “Trunk” – IC1396 and Van den Berg 142 by Takayuki Yoshida


Out in the reaches of the constellation of Cepheus some 2400 light years from Earth, a cloud of hydrogen gas and dust harbors young star cluster IC 1396. These newborn stars emit their light upon the scene… shedding infrared radiation through a 20 light year wide corridor known as the “Elephant’s Trunk”…

Cataloged by Dreyer as far back as 1888, galactic cluster IC 1396 has long been known to have an air of nebulosity around it and perhaps a shroud of mystery as well. As telescopes improved, so did the view and observers began to notice dark patches and a bright, sinuous rim. The dark interstellar clouds took a very special observer in the late 1800s to discover them – E.E. Barnard – and he labeled his discovery B163. Nothing more than a cold area in space – obscuring dust waiting to gel into stars. Just another dark hole obscuring a mystery inside IC 1396… and tiny patch of nebula that would one day be known as Van den Berg 142.

In 1975 Robert B. Loren (et al) was the first to report on the molecular cloud structure in IC 1396. His observations were made using the Kitt Peak scope, doing their best to confirm the hypothesis that cometary like structure was the result of an ionization front as it progressed into neutral hydrogen territory. High density gases, a dark rimmed nebula… But, they still didn’t quite grasp what lay inside – a concentration of interstellar gas and dust that is being illuminated and ionized by a very bright, massive star.

And the tiny dense globules hiding from the intense ultraviolet rays…

In 1996, G. H. Moriarty Schieven was the first to announce H I “Tails” from cometary globules in IC 1396. In his reports he writes: “IC 1396 is a relatively nearby, large, H ii region ionized by a single O6.5 V star and containing bright rimmed cometary globules. We have made the first arcminute resolution images of atomic hydrogen toward IC 1396, and have found remarkable “tail” like structures associated with some of the globules and extending up to 6.5 pc radially away from the central ionizing star. These H i “tails” may be material which has been ablated from the globule through ionization and/or photodissociation and then accelerated away from the globule by the stellar wind, but which has since drifted into the “shadow” of the globules.” This report was the first results of the Galactic Plane Survey Project began by the Dominion Radio Astrophysical Observatory and opened the gateway into the twisted tale of the “Trunk”.

The Elephant’s Trunk nebula is an intense concentration of interstellar gas which contains embedded globule IC 1396A and is now believed to be the site of star formation. Located inside the opening where the stellar winds have cleared a cavity are two very young stars – their pressure driving the material outwards and revealing the presence of protostars.

In 2003, Alaina Henry picked up the ball once again. “Since emission line stars are
relatively rare, the discovery of a cluster of emission line stars is adequate proof that star formation is taking place in a cluster. In addition, young stars often display variable luminosity. It is thought that non-constant mass accretion rates cause variations in the luminosity of young stellar objects. BRC 37 is a small globule in the extended, HII region, IC 1396. It is about I’ wide and 5′ long in the optical, and has a bright rim of Ho emission in the north, due to recombination of ionized hydrogen. The source of the ionization is thought to be the 06 star, HO 206267, which lies several degrees away on the sky. The infrared source, IRAS 21388+5622 is located at the head of the globule and showed another signature of star formation in BRC 37 by discovering a bipolar molecular outflow associated with the IRAS source. We identify eight likely young stellar objects in BRC 37, based on the presence of an infrared excess. We also identify four of our observed sources with Ho emission line stars. Of these 11 sources, five are sub-stellar objects, below the hydrogen burning limit. While the eleven objects in table 1 are apparently young stellar objects, it is likely that there are many more young stellar objects in BRC 37… ”

As recently as mid-2005 even more discovery was made by Astrofisico di Arcetri at the end of a 16 year study. “In spite of the relatively high far-infrared luminosities of the embedded sources H2O maser emission was detected towards three globules only. Since the occurrence of water masers is higher towards bright IRAS sources, the lack of frequent H2O maser emission is somewhat surprising if the suggestion of induced intermediate- and high-mass star formation within these globules is correct. The maser properties of two BRCs are characteristic of exciting sources of low-mass, while the last one (BRC 38) is consistent with an intermediate-mass object.”

Around 18 months later at the beginning of 2007, Konstantin V. Getman (et al) used the Chandra X-Ray Observatory to draw conclusions on this same strange area as well: “The IC 1396N cometary globule (CG) within the large nearby H II region IC 1396 has been observed with the ACIS detector on board the Chandra X-Ray Observatory. We detect 117 X-ray sources, of which ~50-60 are likely members of the young open cluster Trumpler 37 dispersed throughout the H II region, and 25 are associated with young stars formed within the globule…. We find that the Chandra source associated with the luminous Class 0/I protostar IRAS 21391+5802 is one of the youngest stars ever detected in the X-ray band.”

Is there even more things yet to be discovered inside the twisted “Trunk”? Astronomers haven’t stopped looking. Just as recently as November 2008 yet another study was released Zoltan Bolag (et al) searching for protoplanetary discs. “Overall, our observations support theoretical predictions in which photoevaporation removes the gas relatively quickly (<=105 yr) from the outer region of a protoplanetary disk, but leaves an inner, more robust, and possibly gas-rich disk component of radius 5-10 AU. With the gas gone, larger solid bodies in the outer disk can experience a high rate of collisions and produce elevated amounts of dust. This dust is being stripped from the system by the photon pressure of the O star to form a gas-free dusty tail." What will the future hold? My many thanks to Takayuki Yoshida of Northern Galactic for turning me on to this incredible image which sparked my desire to learn and share what I’d learned about this region. Arigato!

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