Huge disks of gas and dust have been found near the center of our galaxy. Radiation in the region is so intense that astronomers expected it to rip such disk formations apart, creating an intriguing puzzle as to how they survived.
Dark clouds 100 light-years from the heart of the Milky Way are in the process of forming stars 2-15 times the mass of the sun, making them an exciting target for astronomers from the University of Bonn. But around giant stars just a few million years old, the team found disks that may eventually form in planetary systems. Such large stars emit enormous amounts of high-energy light, particularly in the ultraviolet. The pressure applied by this sort of radiation disperses clouds.
"We expected that the enormous radiative energy of these giant beasts [would] evaporate the material around their smaller neighbours in less than one million years," says Dr. Andrea Stolte, first author of the paper in Astronomy and Astrophysics. Clearly this has not occurred, but Stolte and her colleagues are at a loss to know why.
The disks were not rare aberrations. One cluster, known as the Quintuplet, hosts 26 of these disks, which surround 4% of its stars. The smaller Arches cluster has at least 20. However, their survival in the more developed Quintuplet cluster makes it even harder to imagine that these disks are just short-lived objects we happen to be seeing before they vanish in a puff of dust.
Disks in clusters dominated by stars with between two and ten solar masses are known to last for 3-10 million years. With the stars in these two clusters growing larger still, and intense UV radiation from the surrounding environment, Stolte and her colleagues did not expect the disks to have survived the estimated 2.5 and 4 million years that the clusters have existed.
The authors propose that the objects they are detecting could be big brothers to the disks seen around T Tauri stars, which are thought to be the predecessors of planets. Planetary formation in disk environments such as these have been dismissed before, but Stolte says, “If there is enough material – who knows?”
The main puzzle, however, is how these disks exist at all given their high-energy environment. The authors propose the possibility that the disks are somehow more resilient to radiation than those in less exotic environments. They also suggest that, rather than surviving from the parent stars' formation, the disks may be constantly replenished as larger stars strip material from smaller companions.
While admitting the question remains unsolved, Stolte favors the latter theory, saying, "Many unknown processes take place in these rich, young star clusters. The tight interaction and mass flow between numerous close twins observed in other star-forming environments might also be the explanation for the dusty discs we found in these massive clusters."