January 13, 2013
Dense Cloud Causing Astronomers To Re-Examine Processes Of Star Formation
redOrbit Staff & Wire Reports — Your Universe Online
An unusually dense cloud located near the center of the galaxy does not appear to be forming any massive stars, astronomers at the California Institute of Technology (Caltech) have discovered, and that behavior has them puzzled as it appears to defy the rules of star formation.
The darkness of the cloud means that it is dense enough to block light, and according to common scientific belief, gas clouds with that kind of density should start forming clumps of yet denser material, which ultimately collapse due to their own gravity and go on to form stars, Caltech senior postdoctoral scholar Jens Kauffmann and colleagues from the Harvard-Smithsonian Center for Astrophysics said.
Such a phenomenon occurred in the Orion Nebula, they note, but despite the fact G0.253+0.016 is a full 25-times denser than Orion, only a handful of stars are being born there. The Caltech team says the cloud´s star formation rate is 45-times lower than what they might have expected out of such a dense celestial body, and even those stars being formed are on the small side.
Based on a series of observations, the astronomers believe they have discovered the reason for the lack of star production in this dense cloud — it has not formed the clumps of denser gas necessary for star formation, and the cloud itself appears to be swirling at such high speeds that it cannot slow down long enough to collapse into stars.
Their findings, which are scheduled for publication in the Astrophysical Journal Letters and were presented this week during the 221st meeting of the American Astronomical Society in Long Beach, California, suggest there may be more involved in the process of star formation than scientists had originally believed, and that dense gas alone may not be enough of an indicator that the process occurs in a particular region.
“To determine whether the cloud contained clumps of denser gas, called dense cores, the team used the Submillimeter Array, a collection of eight radio telescopes on top of Mauna Kea in Hawaii,” Caltech officials explained. “In one possible scenario, the cloud does contain these dense cores, which are roughly 10 times denser than the rest of the cloud, but strong magnetic fields or turbulence in the cloud disturbs them, thus preventing them from turning into full-fledged stars.”
“However, by observing the dust mixed into the cloud's gas and measuring N2H+–an ion that can only exist in regions of high density and is therefore a marker of very dense gas–the astronomers found hardly any dense cores,” they added. “Next, the astronomers wanted to see if the cloud is being held together by its own gravity–or if it is swirling so fast that it is on the verge of flying apart. If it is churning too fast, it can't form stars.”
For this step, they used the 23 radio telescopes that comprise the Combined Array for Research in Millimeter-wave Astronomy (CARMA) in eastern California. Using CARMA, the team measured the velocities of the cloud´s gas, discovering it moves as much as 10 times faster than normal for these types of clouds. In fact, they said the structure itself was just barely held together by its own gravity.
“The CARMA data revealed yet another surprise: the cloud is full of silicon monoxide (SiO), which is only present in clouds where streaming gas collides with and smashes apart dust grains, releasing the molecule,” the institute said. “Typically, clouds only contain a smattering of the compound. It is usually observed when gas flowing out from young stars plows back into the cloud from which the stars were born. But the extensive amount of SiO in the galactic-center cloud suggests that it may consist of two colliding clouds, whose impact sends shockwaves throughout the galactic-center cloud.”
“G0.253+0.016 may eventually be able to make stars, but to do so, the researchers say, it will need to settle down so that it can build dense cores, a process that could take several hundred thousand years. But during that time, the cloud will have traveled a great distance around the galactic center, and it may crash into other clouds or be yanked apart by the gravitational pull of the galactic center. In such a disruptive environment, the cloud may never give birth to stars,” they added.