Most stars form in clusters called clusters or associations that include very massive stars. These giant stars emit large amounts of high-energy radiation that can destroy relatively fragile disks of dust and gas that are in the process of merging to form new planets.
The team of astronomers used NASA ‘s Chandra X-ray Observatory in combination with ultraviolet, optical, and infrared data to show where the most treacherous places in a star cluster might be, where the chances of planets forming are reduced.
The object of the observations was the Cygnus OB2 cluster, the closest large cluster of stars to our Sun, at a distance of about 4600 light-years. The cluster contains hundreds of massive stars, as well as thousands of lower-mass stars. The team used long observations of Chandra aimed at different regions of Cygnus OB2, and the resulting set of images was then stitched together into one large image.
The deep Chandra observations revealed diffuse X-ray luminescence between the stars, and also created a list of young stars in the cluster. This list was combined with other optical and infrared data to create the best census of young stars in the cluster.
In this new combined image, Chandra data (purple) shows diffuse X-ray emission and young stars in Cygnus OB2, while infrared data from NASA’s Spitzer Space Telescope (red, green, blue, and cyan) shows young stars and cooler dust and gas throughout the region.
In this crowded stellar environment, there is a large amount of high-energy radiation produced by stars and planets. X-rays and intense ultraviolet light together can have a devastating effect on planetary disks and systems in the process of formation.
Planet-forming disks around stars naturally disappear over time. Part of the disk falls into the star, and part is heated by the star’s X-rays and ultraviolet radiation and evaporates under the influence of wind. The latter process, known as “photoevaporation,” typically takes 5 to 10 million years for medium-sized stars before the disk disappears. If there are massive stars nearby that produce the most X-rays and ultraviolet radiation, this process can be accelerated.
Using this data, the researchers found clear evidence that planet-forming disks around stars do indeed disappear much faster when they are close to massive stars that produce a lot of high-energy radiation. Disks also disappear faster in regions where stars are more closely packed together.
For regions of the OB2 Swan with lower high-energy emission and fewer stars, the fraction of young stars with disks is about 40%. For regions with more high-energy emission and more stars, this fraction is about 18%. The strongest effect – that is, the worst place for a potential planetary system – is at a distance of about 1.6 light-years from the most massive stars in the cluster.
A separate study by the same team examined the properties of diffuse X-rays in the cluster. They found that the higher-energy diffuse radiation comes from areas where the winds of gas flying away from massive stars collide with each other. This causes the gas to become hotter and emit X-rays. The less energetic radiation likely comes from the collision of the gas in the cluster with the gas surrounding the cluster.
Two separate articles describing Chandra’s data on Cygnus OB2 are available. The article on planetary hazard zones, led by Mario Giuseppe Guarcello (National Institute of Astrophysics in Palermo, Italy), appeared in the November 2023 issue of the Astrophysical Journal Supplement Series and is available here. An article on diffuse radiation led by Juan Facundo Albacete-Colombo (University of Rio Negro, Argentina) is published in the same issue of Astrophysical Journal Supplement and is available here.
The Chandra program is operated by NASA’s Marshall Space Flight Center in Huntsville, Alabama. The Chandra X-ray Center at the Smithsonian Astrophysical Observatory oversees science operations from Cambridge, Massachusetts, and flights from Burlington, Massachusetts.
JPL operated the Spitzer Space Telescope mission for NASA’s Science Mission Directorate in Washington, DC, until the mission was decommissioned in January 2020. Science operations were conducted at the Spitzer Science Center at the California Institute of Technology. The spacecraft were based at Lockheed Martin Space in Littleton, Colorado. The data are archived at the Infrared Science Archive, managed by IPAC at the California Institute of Technology. Caltech operates JPL for NASA.
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