Astronomers solve the mystery of the different star-forming activities of two similar-looking molecular clouds
Using tens of thousands of stars observed by the Gaia spacecraft, Sara Rezaei Khoshbakht and Jouni Kainulainen have determined the 3D shape of two large star-forming molecular clouds, the California Cloud and the Orion A Cloud. They appear similarly structured on conventional 2D images, containing filaments of dust and gas with apparently comparable densities. In 3D, however, they look very different. In fact, their densities are far more diverse than their images projected onto the celestial plane suggest. This result solves the long-standing mystery of why these two clouds form stars with different intensities.
Cosmic clouds of gas and dust are the birthplaces of stars. More specifically, stars form in the densest pockets of such material. Temperatures plummet to near absolute zero, and the densely packed gas collapses under its own weight, eventually forming a star. “Density, i.e. the amount of matter compressed in a certain volume, is one of the crucial properties that determines the efficiency of star formation,” says Sara Rezaei Khoshbakht. She is an astronomer at the Max Planck Institute for Astronomy in Heidelberg and the lead author of a new article published today in The Astrophysical Journal Letters.
In a pilot study described in this article, Sara Rezaei Khoshbakht and her co-author Jouni Kainulainen applied a method that allowed them to reconstruct the 3D morphology of molecular clouds in two giant star-forming clouds. Kainulainen is a researcher at Chalmers University of Technology in Gothenburg, Sweden. He also used to work at the MPIA. Their targets were the Orion-A cloud and the California cloud.
It is usually difficult to measure density in clouds. “All we see when observing objects in space is their two-dimensional projection on what is supposed to be a celestial sphere,” explains Jouni Kainulainen. He is an expert in interpreting the influence of cosmic matter on starlight and calculating densities from such data. Kainulainen adds: “Traditional observations lack the necessary depth. Therefore, the only density we can normally derive from such data is what is known as the columnar density.”
Column density is the matter particles summed along a line of sight divided by the projected cross-section. Therefore, these columnar densities do not necessarily reflect actual densities of molecular clouds, which is problematic when relating cloud properties to star-forming activity. The images of the two clouds examined in this work, which show thermal dust emission, show similar structures and densities. However, their very different rates of star formation have puzzled astronomers for many years.
The new 3D reconstruction now shows that these two clouds are not that similar. Despite the filamentous appearance in the 2D images, the California cloud is a flat sheet of material nearly 500 light-years across with a large bubble extending beneath it. Therefore, one cannot assign a single distance to the California cloud, which has significant implications for the interpretation of its properties. From our perspective from Earth, the California cloud is almost exactly edge-aligned, giving the illusion of a filamentary structure. As a result, the actual density of the cloud is much lower than the column density suggests, explaining the discrepancy between the previous density estimates and the cloud’s star-forming rate.
And what does the Orion-A cloud look like in 3D? The team confirmed the dense filamentous structure seen in the 2D images. However, the actual morphology of the cloud is also different from what we see in 2D. Orion A is fairly complex, with additional densifications along the prominent gas and dust ridge. On average, Orion A is much denser than the California cloud, which explains its more pronounced star-forming activity.
Sara Rezaei Khoshbakht, who also works in Chalmers, developed the 3D reconstruction method during her PhD at MPIA. It analyzes the change in starlight as it traverses these clouds of gas and dust, as measured by the Gaia spacecraft and other telescopes. Gaia is a European Space Agency (ESA) project whose primary purpose is to accurately measure the distances to over a billion stars in the Milky Way. These distances are crucial for the 3D reconstruction method.
“We analyzed and combined the light from 160,000 and 60,000 stars, respectively, for the California cloud and the Orion A cloud,” says Sara Rezaei Khoshbakht. The two researchers reconstructed the cloud structures and densities with a resolution of just 15 light years. “This is not the only approach astronomers use to determine spatial cloud structures,” adds Rezaei Khosbakht. “But our method provides robust and reliable results without numerical artifacts.”
This study proves it has the potential to improve the study of star formation in the Milky Way by adding a third dimension. “I think an important finding of this work is that it challenges studies that rely solely on column density values to infer and compare star formation properties,” concludes Sara Rezaei Khoshbakht.
However, this work is only the first step towards what the two astronomers hope to achieve. Sara Rezaei Khoshbakht is pursuing a project that will ultimately determine the spatial distribution of dust throughout the Milky Way and elucidate its connection to star formation.
The team consists of Sara Rezaei Khoshbakht (Max Planck Institute for Astronomy, Heidelberg, Germany and Chalmers University of Technology, Department of Space, Earth and Environment, Gothenburg, Sweden [Chalmers]) and Jouni Kainulainen (Chalmers).
This work used data from the European Space Agency (ESA) mission Gaia, processed by the Gaia Data Processing and Analysis Consortium (DPAC). The DPAC was funded by national institutions, specifically those institutions participating in the Gaia Multilateral Agreement.
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