September 1, 2014
Chemical Fingerprint Of Sibling Stars Due To Mixing Of Gas During Formation
Chuck Bednar for redOrbit.com - Your Universe Online
Astrophysicists and computational astronomers from the University of California, Santa Cruz have discovered why sibling stars look alike – those formed from a single cloud share the same chemical fingerprint due to early, fast and turbulent mixing of gas in the giant molecular clouds where star formation occurs.
Stars are made primarily of hydrogen and helium, but they also contain trace amounts of elements such as carbon, oxygen and iron, study authors Mark Krumholz and Yi Feng explained. Scientists can determine how abundant each of those trace elements is by carefully measuring the wavelength of light coming from those stars.
When two stars are selected at random, the abundance of their trace elements will differ slightly, with one possessing more iron or carbon than another, they noted. However, two stars selected from the same gravitationally-bound star cluster always share the same abundances, much like how family members share the same basic genes.
“The pattern of abundances is like a DNA fingerprint, where all the members of a family share a common set of genes,” Krumholz, an associate professor of astronomy and astrophysics at UCSC, said in a statement Sunday. He added that it was important to measure this so-called fingerprint because most of the time, stellar families drift apart and migrate to different parts of the galaxy.
Since those abundances are set at birth, astronomers have often wondered if it would be possible to determine if two stars that are now located on opposite ends of the galaxy actually came from the same giant molecular cloud when they formed billions of years ago – and, if so, could they even be able to track down our Sun’s long-lost siblings?
As explained in the latest edition of the journal Nature, Krumholz and Feng, a graduate student at the university, developed supercomputer simulations of interstellar gas coming together to form a cloud which eventually collapses under its own gravity to form a star cluster. Since studies of interstellar gas show far greater variation in chemical abundances than typically observed in stars within the same open star cluster, the researchers added tracer dyes to the simulation’s two gas streams.
They placed red dye in one stream and blue in another, and their results showed extreme turbulence as the two streams came together. This turbulence effectively mixed the tracer dyes together, and by the time the cloud began collapsing and forming stars, the material that formed the stars had turned purple in color. As a result, the stars that formed were also of that hue, which Krumholtz said explains why stellar siblings have the same abundances.
“The simulation revealed exactly why stars that are born together end up having the same trace element abundances: as the cloud that forms them is assembled, it gets thoroughly mixed very fast,” he said. “This was actually a surprise: I didn’t expect the turbulence to be as violent as it was, and so I didn’t expect the mixing to be as rapid or efficient. I thought we’d get some blue stars and some red stars, instead of getting all purple stars.”
Their research, which was supported by NASA and the National Science Foundation (NSF), also demonstrated that the mixing occurs very quickly, before much of the gas becomes a star. This is good news when it comes to the search for the sun’s siblings, since it indicates that the chemical uniformity of star clusters is commonplace, and that even those stars resulting from clouds that produce few of them have extremely similar chemical signatures.
“The idea of finding the siblings of the sun through chemical tagging is not new, but no one had any idea if it would work. The underlying problem was that we didn't really know why stars in clusters are chemically homogeneous, and so we couldn't make any sensible predictions about what would happen in the environment where the Sun formed,” Krumholz said. “This study puts the idea on much firmer footing and will hopefully spur greater efforts toward making use of this technique.”
Image 2 (below): This is an image from a computer simulation shows a collision of two streams of interstellar gas, leading to gravitational collapse of the gas and the formation of a star cluster at the center. In this image, the gas streams were labeled with blue and red "tracer dyes," and the purple color indicates thorough mixing of the two gas streams during the collapse. Credit: Y. Feng and M. Krumholz
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