To understand how the universe was formed and how we got here, ancient stars are some of the best objects to study — yet astronomers know surprisingly little about the conditions in which they were formed. Dominant cosmological theories posit that stars are formed in molecular clouds that are sufficiently large and dense that it has the conditions necessary to create new stars. These same experts have been uncertain, however, about some of the specifics of those conditions. What is it really like in those interstellar nurseries known as molecular clouds?
According to a recent study in The Astrophysical Journal, some of the molecular clouds out of which stars are formed can be captured by an adjective one does not often associate with stars: fluffy.
Scientists at Japan’s Kyushu University learned this, in collaboration with researchers from Osaka Metropolitan University, by studying the Small Magellanic Cloud (SMC), a dwarf galaxy near the Milky Way located roughly 20,000 lightyears from Earth. Because the SMC contains approximately twenty percent of the heavy elements of the Milky Way, it is believed to closely resemble the cosmic environment of the early universe from 10 billion years ago. That is why astronomers used the ALMA (Atacama Large Millimeter Array) radio telescope in Chile, which is powerful enough to capture higher-resolution images of the SMC.
“Our study was motivated by a fundamental question: how did star formation occur in the early universe?” Dr. Kazuki Tokuda, an Earth and planetary sciences professor at Kyushu University in Fukuoka, Japan, told Salon.
Molecular clouds in the Small Magellanic Cloud (ALMA (ESO/NAOJ/NRAO), Tokuda et al., ESA/Herschel)“We sought to understand the formation and evolution of stellar nurseries — molecular clouds where stars are born — under conditions similar to those billions of years ago,” Tokuda added. “Typically, studying ancient star-forming regions requires observing galaxies that are tens of billions of light years away.”
Next the scientists assembled a dataset covering 17 distinct molecular clouds associated with massive young stellar objects. While coordinating this data from multiple programs can be challenging, Tokuda explained that it also presented “a unique opportunity.”
"Recent observations of our own Milky Way have increasingly highlighted the importance of filamentary molecular clouds as the primary sites of star formation."
“We focused on aspects of these datasets that had not yet been fully explored, and this approach not only tested our ability to integrate diverse observations but also revealed intriguing details about the evolution of star-forming regions in the SMC,” Tokuda said.
This is where the “fluffiness” factor comes into play. Tokuda and the other scientists wanted to understand if filaments, or threadlike fibers, form during star formation, as this reveals key details about their density and overall composition. In the paper the researchers concluded that “even if filaments form during star formation, their steep structures may become less prominent and transit to a lower-temperature state.” Even though prior observation of the Milky Way showed these filaments were present in molecular clouds which became sites for star formation, Alma’s SMC studies demonstrate that stars can also form in fluffier conditions.
“Recent observations of our own Milky Way have increasingly highlighted the importance of filamentary molecular clouds as the primary sites of star formation,” Tokuda said. “However, more diffuse, fluffy molecular clouds have not received as much attention over the past decade.”
Although these structures are lighter than the molecular clouds with filamentary structures, they are still quite similar in other crucial aspects.
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“The environment, such as an adequate supply of heavy elements, is crucial for maintaining a filamentary structure and may play an important role in the formation of planetary systems,” Tokuda said in the study’s accompanying press statement. “In the future, it will be important to compare our results with observations of molecular clouds in heavy-element-rich environments, including the Milky Way galaxy. Such studies should provide new insights into the formation and temporal evolution of molecular clouds and the universe.”
The researchers determined that stars can be formed in a diverse range of structures, but that there are “systematic differences in the physical properties of filamentary and non-filamentary clouds. The former tend to have smaller velocity dispersions relative to their column densities and exhibit higher temperatures.” Additionally filamentary clouds tend to have faster velocities and at an increasing width relative to their columns' density, “consistent with the relationship observed in the [Large Magellanic Cloud],” a dwarf galaxy near the Milky Way.
Example of a filamentary (left) and fluffy (right) molecular cloud in the Small Magellanic Cloud captured by the ALMA telescope (ALMA (ESO/NAOJ/NRAO), Tokuda et al.)
Finally, they added that “the high temperatures observed in the filaments suggest that they likely preserve the heated conditions related to their cloud formation. In addition, [young stellar objects] with proto-stellar outflows have been found in some filamentary clouds.”
When their research into the SMC is synthesized with the growing body of knowledge about other galaxies, Tokuda told Salon that he hopes one day astronomers will be able “to deepen our understanding of how molecular clouds form and evolve under different conditions.”
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Dr. Avi Loeb, a Harvard University astronomer, told Salon that the paper illuminates a previously mysterious story involving the history of the universe.
“The data indicates that the youngest stars form in filaments of gas,” Loeb said. “Subsequently the gas cools and fragments into later generations of stars in less filamentary structures. This behavior is not shared in star forming environments that are more enriched in heavy elements.”
He added, “The new behavior sheds new light on what star formation in the early universe, before the primordial gas was enriched with heavy elements.”
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