Astronomers map ‘danger zone’ of planet-forming disks in star clusters

Most stars form collections called clusters or federations that contain very massive stars. These massive 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.

A team of astronomers uses NASA’s Chandra X-ray Observatory, combined with ultraviolet, optical, and infrared data, to determine where the most dangerous spots in star clusters are, reducing the chance of planets forming. I showed you the location.

The observation target is Cygnus OB2, the closest large star cluster to the Sun, approximately 4,600 light-years away. This cluster contains hundreds of heavy stars and thousands of low-mass stars. The research team used long-term Chandra observations pointing to different regions of Cygnus OB2 and stitched the resulting series of images into one large image.

Chandra’s deep observations mapped the diffuse X-ray glow between stars and also provided an inventory of young stars within the cluster. We combined this inventory with others using optical and infrared data to create the best population survey of young stars in this cluster.

In this new composite image, Chandra data (purple) shows the diffuse X-ray emission and young stars of Cygnus OB2, along with infrared data (red, green, blue, cyan) from NASA’s now-retired Spitzer Space Telescope reveals young stars. And it creates cold dust and gas throughout the region.

In these crowded stellar environments, there is a large amount of high-energy radiation produced by stars and planets. X-rays and intense ultraviolet radiation can have devastating effects on planetary disks and systems that are in the process of forming.

The disks that form planets around stars naturally disappear over time. Part of the disk falls onto the star, and some is heated by X-rays and ultraviolet light from the star and evaporates in the wind. The latter process, known as “photoevaporation,” typically takes 5 million to 10 million years for an average-sized star to destroy its disk. This process could be accelerated if there is a nearby massive star that produces the most X-rays and ultraviolet light.

Using this data, the researchers found clear evidence that planet-forming disks around stars actually die out much faster when they approach massive stars that produce large amounts of high-energy radiation. Also, in regions where stars are more densely packed, the disk dies out faster.

In the region of Cygnus OB2, which has less high-energy radiation and fewer stars, the proportion of young stars with disks is about 40%. In regions with higher-energy radiation and more stars, the proportion is about 18%. The strongest influence, and therefore the worst location for a star to become a potential planetary system, is within about 1.6 light-years of the most massive star in the cluster.

Another study by the same team investigated the properties of diffuse X-ray emission within the cluster. They discovered that the high-energy, diffuse radiation originates from regions where winds of gas blown from massive stars collide with each other. This heats the gas and produces X-rays. The lower energy emission probably occurs when the gas within the cluster collides with the gas surrounding the cluster.

Two separate papers are available describing Cygnus OB2’s Chandra data. The paper on planetary danger zones, led by Mario Giuseppe Guarcello (National Institute of Astrophysics, Palermo, Italy), was published in the November 2023 issue of the Astrophysics Magazine Supplement series.

A paper on diffuse radiation led by Juan Facundo Albacete-Colombo (University of Rio Negro, Argentina) was published in the same issue of The Astrophysical Journal Supplement Series.