BY DR EMILY BALDWIN
Posted: 23 April, 2009
The clues are in the shadows, say researchers at the Dublin Institute of Advanced Studies studying how giant star-forming structures like the famous Pillars of Creations came into existence.
Massive stars are born in the densest parts of gas clouds, emitting intense ultraviolet radiation that ionises and heats the surrounding gas, blowing out a rapidly expanding bubble known as an HII region. This is a somewhat irregular process, resulting in curious structures forming on the walls of this bubble, such as the so-called pillars of the Eagle Nebula. There are a number of possible formation mechanisms for these pillars, including instabilities in the bubble's expansion or clumpy molecular clouds acting as seeds for the formation of pillars.
Jonathan Mackey, Andrew Lin and colleagues applied the idea of a clumpy gas cloud to computer simulations in order to track the evolution of these pillars and to try to explain why they develop around the edge of ionised gas bubbles surrounding young, very hot stars. "We created a simulation with a random distribution of lots of dense clouds with different sizes and shapes," says Mackey. "We found that in certain cases a number of clouds can merge together in the shadows to form structures that look very like observed pillars."
The simulations revealed that clumps of gas are rapidly added to a shadowed side of a star where it is exposed to less intense ultraviolet radiation. The simulated pillars form in some 150,000 years, however, the age of the Eagle Nebula pillars are estimated to be no older than 100,000 years. Furthermore, the models show that the shadow from a single clump of gas would not attain the required density to form a pillar in that relatively short timescale.
"Many of our models do not produce pillars that are as long and narrow as those in the Eagle Nebula, at least not at the observed gas density. It needs the right configuration of dense clumps of gas to form a long pillar," says Lim. "Unless the shadowed region is already very dense to begin with, it just takes too long to collect and organise the gas into a pillar."
There is still some way to go with recreating the formation of the Eagle Nebula pillars with realistic conditions, especially in terms of the complex chemistry of the interstellar gas and the effects of radiation from diffuse sources, not to mention gravity. "Gravity is relatively unimportant when the pillars are forming, but there comes a point when they get very dense and it cannot be ignored any longer," says Mackey. "We plan to include gravitation in future work so that we can study the next generation of stars which are forming in the pillars."