Not all stars are lucky enough to have their own brood of planets. Conditions may arise in the Universe under which protoplanetary disks scatter faster than planets can form. This is well illustrated by the stellar association Cygnus OB2, which was observed by a group of astronomers. Depending on the environment, protoplanetary disks are preserved in 1–40% of young stars, although they are all of the same age.
The Cygnus OB2 region, where the violet glow is shown in X-rays, and the rest in infrared light. Image source: NASA
The stellar association Cygnus OB2 is located approximately 4600 light years from Earth. It is not a star cluster because most of the stars are not bound by gravity and will eventually scatter throughout the galaxy (in clusters, stars are held together by gravity). The population of the association is represented mainly by young and hot stars, around each of which there should be a protoplanetary disk. The stars themselves were formed from such a disk, and something, and often a lot, always remains superfluous.
At the same time, the saturation of space in the Cygnus OB2 region with young stars creates uncomfortable conditions for planet-forming disks. Intense radiation in the ultraviolet and X-ray range causes the material of the disks to evaporate and be carried away. This phenomenon is called photoevaporation: gas in the protoplanetary disk heats up and ionizes, and internal radiation pressure from the star pushes it out of the disk.
Under normal conditions, stars like our Sun can disperse a protoplanetary disk in 5–10 million years. Hot, bright O and B stars do this in a shorter period of time, leaving no chance for planet formation, a new study shows.
Scientists created a composite image of Cygnus OB2 from images from the Chandra X-ray Observatory and the Spitzer Infrared Observatory. Chandra shows areas of intense X-ray emission, while Spitzer reveals dust (disks) and stars.
Analysis of the image showed that less massive stars in the association, located in a less dense environment of neighbors, have protoplanetary disks, which were detected in 40% of the stars. They are not so hot that they disperse dust and gas around them. In denser stellar groups, only 18% of stars had protoplanetary disks. In the most extreme and dense environments, only 1% of stars retain protoplanetary disks. When there are many stars, they are bright, hot and located close enough to each other, this does not contribute to the formation of planets and the origin of life. Another coin in the treasury of knowledge about where to look for life in the Universe. There are so many stars in it that observational resources should be used only after a very thoughtful choice.