An important advance in understanding how clusters of stars like our sun are formed has been made by a team that includes seven astronomers at Penn State Univ. and two at other universities. Using data from NASA's Chandra X-ray Observatory and infrared telescopes, the astronomers have shown that earlier theories about the process that creates star clusters in giant clouds of gas and dust cannot be correct.
"Our findings mean we need to develop new ideas, based on these new data, about how stars like our sun form within large clusters of stars," said Konstantin Getman, the senior research scientist at Penn State who led the study.
The simplest of the earlier ideas is that stars form into clusters when the center of a giant cloud of gas and dust condenses, pulling in material from its surroundings until it becomes dense enough to trigger star formation. Because star formation occurs in the center of the cloud first, according to this idea, the stars in the middle of the cluster should be the oldest. However, when Getman and his coauthors studied two clusters where Sun-like stars currently are forming in the center of the Flame Nebula and the Orion Nebula Cluster, they discovered that the stars on the outskirts of these clusters actually are the oldest.
"These latest data from the Chandra X-ray Observatory suggest that some other process must be forming these Sun-like stars," said Eric Feigelson, a prof. of astronomy, astrophysics and statistics at Penn State, and a member of the discovery team. The team's results will be published in two separate papers in The Astrophysical Journal.
Getman, Feigelson and their colleagues developed a new two-step approach that led to this discovery. First, they used Chandra data on the brightness of the stars in x-ray wavelengths of light to determine the stars' masses. Then they determined how bright these stars are in infrared light using ground-based telescopes and data from NASA's Spitzer Space Telescope.
The astronomers then combined this information with existing theoretical models in order to estimate the ages of the stars throughout the two clusters—but they discovered that the results were contrary to what the standard basic model at that time predicted.
At the center of the Flame Nebula (NGC 2024), the stars were about 200,000 years old, while those on the outskirts were about 1.5 million years old. In the Orion Nebula (NGC 1976), star ages ranged from 1.2 million years in the middle of the cluster to almost 2 million years near the edges.
"A key conclusion from our study is that we can reject the basic model that assumes clusters form from the inside out," Feigelson said. "We now need to consider more complex models emerging from theoretical models of clustered star formation."
Explanations for the new findings can be grouped into three broad notions. The first is that star formation continues to occur in the inner regions of a star-forming cloud because the gas there is denser than in the more diffuse outer regions—the center contains more material for building stars. Over time, if this gas density in the outer regions of the cloud falls below a threshold where it can no longer collapse to form stars, star formation will cease in the outer regions but stars will continue to form in the inner regions, leading to a concentration of younger stars there.
Another idea is that old stars have had more time to drift away from the center of the cluster or to be kicked outward by interactions with other stars. One final notion is that the new observations could be explained if young stars were formed in massive filaments of gas that fall toward the center of the cluster.
Previous studies of the Orion Nebula Cluster revealed hints of this reversed age spread but these earlier data were inconclusive. This new research provides the first data-based evidence of such age differences in the Flame Nebula. "The next steps will be to see if we find this same age range in other young clusters," said Penn State graduate student Michael Kuhn, who also worked on the study.
Source: Penn State Univ.