Turbulence a Key to Birth of Massive Stars
Scientists have long known that stars are formed from swirling clouds of gas and dust that coalesce. But why some of these stellar nurseries give rise to ordinary stars like our sun and others can pop out stars 15 to 30 times as massive is something of a conundrum.
Astronomers working with the Submillimeter Array in Mauna Kea, Hawaii think they may have found the key difference: turbulence.
The Submillimeter Array (SMA) is so named because it probes the universe in wavelengths of light from 0.3 to 1.7 millimeters (0.01 to 0.07 inches). Most radiation in this range comes from the cold interstellar gas and dust from which stars and planets form.
The ability to monitor this radiation comes in handy when astronomers are trying to peer into stellar nurseries, whose cocoons ? all that dust and gas ?block visible light.
A team of astronomers at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., used the SMA to probe two such cocoons located 15,000 light years away in the constellation Serpens Cauda.
"The SMA enables us to see the dust and gas in the cocoon with amazing details, and to probe the initial stages of massive star formation," said team member Qizhou Zhang.
The team's findings will be detailed in an upcoming issue of The Astrophysical Journal.
A baby star is born
The basics of star formation are pretty clear: a cloud of cosmic gas tucked away in some part of a galaxy spins and coalesces under the pull of gravity. As it does so, the gas grows denser and hotter until nuclear fusion ignites and a baby star is born.
The gravitational pull that condenses the gas also tends to fragment it, fracturing the condensing cloud into smaller and smaller pieces. Astronomers think this fragmentation may inhibit the formation of massive stars because the resulting pieces are too small ? they eventually become mundane-sized stars like our sun.
But young, massive stars are clearly seen in some clouds; for example, the Orion Nebula (located in the Hunter's sword of the Orion constellation) is host to a cluster of newborn stars called the Trapezium that are many times more massive than our sun and 100,000 times as bright.
So some mechanism must allow the behemoth babies to form in these birthing clouds.
Some astronomers posit that such young, massive stars are the result of collisions between smaller nascent stars. But this method requires an "extreme environment," Zhang told SPACE.com, and the stellar nurseries they have examined don't have a high enough density of protostars for collisions to occur.
Another proposal that some force must be counteracting gravity and suppressing fragmentation in the clouds, allowing the massive stars to form outright, gathering gas as smaller stars do.
Two such forces are known to be at work in gas clouds: thermal pressure and turbulence. The thermal pressure is the result of the intense heat radiating from the protostars. The turbulence is likely the result of spiral waves in galaxies, supernovas interacting with the clouds, or outflows of material from newborn stars, said Zhang's coauthor, Thushara Pillai.
Previous work had suggested that thermal pressure was the strongest influence opposing fragmentation, but the new SMA study finds that turbulence is more important, at least at certain spatial scales.
"What's unique about these SMA observations is that we can check some of the hypotheses for massive star formation against the observations for the first time," Zhang said. "Unlike what has been assumed in theoretical models, we found that fragmentation is suppressed in these clouds, not by stellar heating but rather by turbulence."
This was contrary to previous theories because it was thought that "when you trigger those turbulence [events], they die down very quickly," Zhang said. But that doesn't seem to be the case, and some feedback that perpetuates the turbulence must be in play.
While this study has shed some light on massive star formation, it's a first step that will be followed-up by a more comprehensive survey of the regions where massive stars form that will look for signatures of stellar outflow and potential feedbacks of turbulence
"We have just started to understand the initial conditions in distant, massive star-forming regions," Thushara Pillai said. "A large survey that we have launched with the SMA will, in the near future, reveal the nature of more of such objects."
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