3D cosmic dust clouds | urania

Astronomers solve the mystery of the different star-forming activity of two similar-looking molecular clouds.

Using tens of thousands of stars observed by the Gaia spacecraft, MPIA astronomers and Chalmers revealed the three-dimensional shapes of two star-forming large molecular clouds – the California cloud and the Orion A cloud. density. However, they look completely different in 3D. In fact, its density is very different from what its sky-level projected images might suggest. This finding solves an age-old mystery why these two clouds form stars at different rates.

Cosmic clouds of gas and dust are the birthplace of stars. More specifically, stars form in the densest clumps of this material. The temperature there drops to nearly absolute zero, and the densely packed gas collapses under its own weight, eventually forming a star. Density, that is, the amount of material compressed into a given volume, is one of the main properties that determine the efficiency of star formation processes Says Sarah Rezaei Khoshbacht, an astronomer at the Max Planck Institute for Astronomy in Heidelberg, Germany and lead author of a new article published May 16, 20222 in the astrophysical journal Letters.

In the experimental study presented in this article, Sarah Rezaei Khoshbacht and co-author Johnny Quinolin used a method that allows 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, and previously also worked at MPIA. The subjects of these studies were Orion A Cloud and California Cloud.

Measuring the density in clouds is usually difficult. All we see when observing objects in outer space is their two-dimensional projection on an imaginary celestial sphere Johnny Kainolainen explains it. It is an experiment in interpreting the effects of space matter on starlight and calculating density from this data. Kainulainen adds: Conventional notes lack the necessary depth. Therefore, the only density that we can usually infer from this data is the so-called column density.

Column density is the mass added along the line of sight divided by the projected cross section. Therefore, the plume densities do not necessarily reflect the actual densities of molecular clouds, which is a problem when it comes to correlating cloud properties with star-forming activity. In fact, the thermal dust emission images of the two clouds examined in this paper have a seemingly similar structure and density. However, the dramatically different rates of star formation have puzzled astronomers for many years.

The new 3D reconstruction shows, however, that these two clouds are not the same to each other. Despite the fibrous appearance of the 2D images, the California cloud is a flat lobe of matter about 500 light-years across with a large bubble stretching beneath it. Therefore, it is not possible to allocate a single space to it, which has important consequences for the interpretation of its properties. From our point of view on Earth, they are oriented roughly from the edge up, which only mimics a fibrous structure. As a result, the actual cloud density is much lower than indicated by the plume density, which explains the discrepancy between previous density estimates and the cloud’s star formation rate.

And what does Orion A Cloud look like in 3D? The team confirmed its dense fibrous structure seen in the 2D images. However, its actual shape is also different from what we see in 2D. Orion A is very complex with additional condensation along the protruding edge of the gas and dust. On average, Orion A is denser than the California cloud, which explains its greater star-forming activity.

Sara Rezai Khoshbacht developed a 3D reconstruction method during her PhD thesis. It analyzes changes in stellar light as it passes through clouds of gas and dust as measured by the Gaia probe and other telescopes. Gaia’s primary goal is to accurately 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 correlated the light from 160,000 and 60,000 stars for the California Cloud and Orion Cloud A. Sarah Rezaei Khoshbacht says: Two astronomers reconstructed the cloud’s shape and density with an accuracy of only 15 light-years. This is not the only method astronomers use to determine the spatial structures of clouds Rezaie Khoshbachht adds. But our results provide robust and reliable results without any numerical implications.

The study shows that by adding a third dimension, it can improve research into star formation in the Milky Way. I think an important outcome of this work is that it challenges research that relies solely on plume density thresholds to identify and compare star formation properties with each other. Sarah Rezaei Khoshbacht concludes.

However, this work is only the first step in the direction that astronomers want to achieve. Sara Rezai Khoshbacht is implementing a project that will eventually reveal the spatial distribution of dust across the Milky Way and its relationship to star formation.

Agnieszka Nowak

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Source: MPG

In the illustration: the California cloud and the Orion cloud from two different perspectives with a spatial resolution of 15 light-years. Colors indicate intensity, while red indicates higher values. The images are based on a 3D reconstruction by Sarah Rezaei Khoshbacht and Johnny Kainolin. Source: Rezaei Khoshbakht & Kainulainen (2022) / MPIA.

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