Threads or flats? It all comes back to perspective

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, Sarah Rezaei Khoshbacht and Johnny Kainolin determined the three-dimensional shape of two star-forming large molecular clouds, the California cloud and the Orion cloud. They appear similarly on conventional 2D images, containing filaments of dust and gas with an apparently similar density. But in 3D it looks completely different. In fact, their densities are much more varied than their celestial-projected images would suggest. This finding solves the longstanding mystery of why these two clouds form stars of different intensities.

Cosmic clouds of gas and dust are the home of stars. More specifically, stars form in the densest pockets of this material. Temperatures drop to near absolute zero, and the densely packed gas collapses under its own weight, eventually forming a star. “Density, that is, the amount of matter compressed into a given volume, is one of the critical properties that determine the efficiency of star formation,” says Sarah Rezai Khoshbacht. She is an astronomer at the Max Planck Institute for Astronomy in Heidelberg and lead author of a new article published today in the astrophysical journal Letters.

In an experimental study described in this article, Sarah Rezaei Khoshbacht and co-author Johnny Quinolin applied a method that allowed them to reconstruct the 3-D morphology of molecular clouds in two giant star-forming clouds. Kinulainen is a researcher at Chalmers University of Technology in Gothenburg, Sweden. He also worked at MPIA. Their targets were the Orion-A cloud and the California cloud.

It is usually difficult to measure the density in clouds. “All we see when observing objects in space is their two-dimensional projection onto what is supposed to be a celestial sphere,” explains Johnny Kainolin. He is an expert in interpreting the effect of cosmic matter on starlight and calculating densities from this data. Kainulainen adds: “Traditional notes lack the necessary depth. Therefore, the only density we can usually derive from this data is what is known as the vertical density.”

Column density is the particles of matter grouped along the line of sight divided by the projected cross section. Therefore, these vertical densities do not necessarily reflect the actual densities of molecular clouds, which is a problem when linking cloud properties to star-forming activity. The two cloud images examined in this work, which show the emission of thermal dust, show similar structures and densities. However, the very different rates of star formation have puzzled astronomers for many years.

New 3D reconstructions now show that these two clouds are not the same. Despite its threadlike appearance in the 2D images, the California cloud is a flat sheet of material about 500 light-years across with a large bubble extending beneath it. Therefore, one space cannot be assigned to the California cloud, which has important implications for the interpretation of its properties. From our viewpoint from Earth, the California cloud is almost perfectly aligned to the edge, giving the illusion of a stringy structure. As a result, the actual cloud density is much lower than the plume density would suggest, which explains the discrepancy between previous density estimates and the cloud’s star formation rate.

And what does the Orion-A 3D cloud look like? The team confirmed the dense filamentous structure seen in the 2D images. However, the actual shape of the cloud is also different from what we see in 2D. Orion A is rather complex, with additional condensations along the ridge notable from gas and dust. On average, Orion A is denser than the California cloud, which explains its more pronounced star-forming activity.

Sarah Rezaei Khoshbacht, also at Chalmers, developed the 3D reconstruction method while earning her PhD at MPIA. It analyzes the change in starlight as it passes through these clouds of gas and dust, as measured by the Gaia spacecraft and other telescopes. Gaia is a project of the European Space Agency (ESA) whose primary purpose is to precisely measure the distances of more than a billion stars in the Milky Way. These distances are critical 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 Sarah Rezaei Khoshbacht. The researchers reconstructed the cloud structures and density at a resolution of only 15 light-years. “This is not the only approach that astronomers use to identify spatial cloud structures,” adds Rezaei Khosbakht. “But our method provides robust and reliable results without numerical implications.”

This study proves that it has the potential to improve the study of star formation in the Milky Way by adding a third dimension. Sarah Rezaei Khoshbacht concluded: “I think an important outcome of this work is that it challenges studies that rely solely on plume density values ​​to infer and compare star formation properties.”

However, this work is only the first step towards what the astronomers hope to achieve. Sarah Rezaei Khoshbachht is pursuing a project that will eventually determine the spatial distribution of dust throughout the Milky Way and clarify its connection to star formation.

additional information

The team consists of Sarah Rezaei Khoshbacht (Max Planck Institute for Astronomy, Heidelberg, Germany and Chalmers University of Technology, Department of Space, Earth and Environment, Gothenburg, Sweden). [Chalmers]) and Johnny Quinoline (Chalmers).

This work used data from the European Space Agency (ESA) Gaia mission, which was processed by the Gaia Data Processing and Analysis Consortium (DPAC). DPAC has been funded by national institutions, specifically those institutions participating in the Gaia multilateral agreement.


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