December 26, 2013
Infrared Dark Cloud - artist's impression based on ALMA data. Credit: Bill Saxton & Alexandra Angelich (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO)
We are fairly certain we know how stars come to be, but our current understanding only applies to stars that are similar to Sun in mass, or only a couple of times heavier. Why there are massive stars, having to 10 to over 100 times more mass than Sun, remains a mystery.
Using Atacama Large Millimeter/submillimeter Array (ALMA), astronomers observed so called Infrared Dark Clouds. These clouds are dark, cold and dense objects that should be full of stars being born, but something is postponing the birth of average stars and giving a chance for massive stars to be born.
Simply put, stars are born in vast clouds of molecules, mostly hydrogen and helium. At first molecules are bound together to create small lumps, but over the time, they grow larger. After a certain point, the gravitational pull of this body becomes strong enough to attract even more material, causing a cloud to begin its rotation finally creating a accretion disk. When enough mass is accumulated for nuclear fusion to begin, the star is born.
One would think that for massive star all you need is more material, a bigger cloud of molecules, but it's not that simple. Without some mechanism postponing the star formation by supporting the cloud and preventing it from collapsing on itself, bigger cloud would only lead to more average stars. If the star forms too quickly, its mass remains average no matter how big the cloud is.
Using ALMA, an international team of astronomers led by Jonathan Tan of the University of Florida, Gainesville, USA, looked for deuterium isotope in Infrared Dark Clouds. Deuterium bonds with other molecules in cold conditions, but after the star is formed, surrounding disk is heated and deuterium dissipates. By observing one such cloud 10,000 light-years away, his team found it to be rich with deuterium and thus cold and without stars.
"A starless core would indicate that some force was balancing out the pull of gravity, regulating star formation, and allowing vast amounts of material to accumulate in a scaled-up version of the way our own Sun formed," Tan said. "Alternatively, there has been speculation that two separate models of star formation are needed: one for Sun-like stars and one for these massive stars."
By observing these Infrared Dark Clouds, we hope to gain an insight into the birth of stars that are even 150 times heavier than our Sun.