The shape of the California and Orion A Clouds from two different perspectives at a spatial resolution of 15 light-years. The colours indicate density, with red colours representing higher values. The images are based on the 3D reconstruction by Sara Rezaei Khoshbakht and Jouni Kainulainen. Credit: Rezaei Khoshbakht & Kainulainen (2022) / MPIA
Astronomers from MPIA and Chalmers used the Gaia space probe’s observations of tens of thousands of stars to figure out the 3D shapes of the California Cloud and the Orion A Cloud, which are large molecular clouds that form stars. In traditional 2D images, they look like they have the same shape and that they have similar-looking dust and gas filaments. In 3D, though, they look very different. In reality, their densities are much more different than what their pictures on the sky would make you think. This answer answers a question that has been around for a long time: why do these two clouds make stars at different rates?
Stars are made in big clouds of gas and dust in space. More specifically, stars form where there is the most of this kind of matter. The temperatures drop to almost absolute zero, and the tightly packed gas collapses under its own weight, making a star. Sara Rezaei Khoshbakht says, “Density, which is the amount of matter packed into a given volume, is one of the most important properties that determine how well stars form.” She is an astronomer at the Max Planck Institute for Astronomy in Heidelberg, Germany, and the main author of a new article that came out today in The Astrophysical Journal Letters.
In the pilot study described in this article, Sara Rezaei Khoshbakht and co-author Jouni Kainulainen used a method that lets them figure out the 3D shapes of molecular clouds to look at two huge clouds that are making stars. Kainulainen is a scientist who used to work at MPIA and now works at the Chalmers University of Technology in Gothenburg, Sweden. The Orion A Cloud and the California Cloud were their main goals.
Usually, it’s hard to figure out how dense clouds are. Jouni Kainulainen says, “Everything we see when we look at things in space is their two-dimensional projection on an imaginary celestial sphere.” He is an expert at figuring out how cosmic matter affects the light from stars and using that information to figure out how dense things are. Kainulainen adds, “Normal observations don’t have enough depth. So, the only density we can usually figure out from these kinds of data is what is called the “column density.””
The mass added along a line of sight divided by the projected cross-section is the column density. So, those column densities don’t always match up with the real densities of molecular clouds, which makes it hard to connect cloud properties to the activity of making stars. In fact, the images of the two clouds that show the thermal dust emission from this work seem to have similar shapes and densities. But their very different rates of making stars have puzzled astronomers for a long time.
Instead, the new 3D reconstruction shows that these two clouds are not that similar after all. Even though the 2D images make the California Cloud look like a string, it is actually a flat sheet of material that is almost 500 light-years long and has a large bubble that extends below it. So, you can’t give the California Cloud a single distance, which has big effects on how you understand its properties. From where we are on Earth, it looks like it is almost edge-on, which makes it look like a filament. Because of this, the sheet’s real density is much lower than what the column density suggests. This explains the difference between the previous estimates of the sheet’s density and the rate at which stars are being made in the cloud.
How does the Orion A Cloud look in three dimensions? The team confirmed the dense filamentary structure seen in the 2D images. But its real shape is also different from what we see in two dimensions. Orion A is pretty complicated, with more dust and gas condensing along the ridge that stands out. On average, Orion A is a lot denser than the California Cloud, which explains why it is making more stars.
During her Ph.D. at MPIA, Sara Rezaei Khoshbakht, who also works at Chalmers University of Technology, came up with the 3D reconstruction method. It involves looking at how the light from stars changes when it goes through clouds of gas and dust, which Gaia and other telescopes can measure. Gaia is a project of the European Space Agency (ESA). Its main goal is to figure out how far away more than a billion stars in the Milky Way are. For the 3D reconstruction method, these distances are very important.
Sara Rezaei Khoshbakht says, “We looked at and compared the light from 160,000 stars in the California Cloud and 60,000 stars in the Orion A Cloud.” With a resolution of only 15 light-years, the two astronomers were able to figure out the shapes and densities of the clouds. Rezaei Khosbakht says, “This is not the only way astronomers figure out the shapes of clouds in space.” “But ours gives solid and reliable results with no strange numbers.”
By adding a third dimension, this study shows that it could help researchers learn more about how stars form in the Milky Way. Sara Rezaei Khoshbakht says, “I think one important result of this work is that it challenges studies that only use column density thresholds to figure out how stars form and compare them to each other.”
But this work is just the beginning of what astronomers want to do. Sara Rezaei Khoshbakht is working on a project that will eventually show where dust is in the Milky Way and how it is related to the birth of stars.
Further information: Sara Rezaei Kh. et al, Three-Dimensional Shape Explains Star Formation Mystery of California and Orion A, The Astrophysical Journal Letters (2022). DOI: 10.3847/2041-8213/ac67db
Sara Rezaei Kh. et al, Detailed 3D structure of Orion A in dust with Gaia DR2, Astronomy & Astrophysics (2020). DOI: 10.1051/0004-6361/202038708
Journal information: Astrophysical Journal Letters , Astronomy & Astrophysics
Source: Max Planck Society