An international team of astronomers, led by scientists from the Physics Department of the University of Warwick (Great Britain), has published the results of a study of an unusual star in the Milky Way - a supermassive white dwarf. According to researchers, it was formed in the process of fusion of stars - two medium-sized white dwarfs. This was reported in the journal Nature Astronomy.
The object was discovered 150 light-years from Earth by the telescope of the European Space Agency Gaia. The star was identified as a supermassive white dwarf and was named WDJ0551 + 4135.
Recall that white dwarfs represent the final stage in the evolution of small stars (up to 10 solar masses). After burning all the hydrogen in such stars, thermonuclear reactions cease, and they continue to glow due to the residual thermal energy. Most white dwarfs have a huge density and a small diameter comparable to the diameter of the Earth. As a rule, these stars on average have about 0.6 solar masses. Object WDJ0551 + 4135 is almost twice as heavy - 1.14 solar masses.
Scientists continued their further study of the white dwarf, having studied the data obtained by the spectrometer of the European observatory Herschel Space Observatory. An analysis of the atmosphere of the object showed that it has an unusual mixed composition, which cannot be explained by the natural course of evolution of one star.
- Herschel Space Observatory Space Telescope
- © ESA / Herschel / NASA / JPL-Caltech; acknowledgement T. Pyle & R. Hurt
“This star is a case we have not seen before. The outer hydrogen layer - sometimes with the presence of helium, and sometimes just a mixture of helium and carbon - is still the expected option. But what you do not expect to see is the simultaneous combination of hydrogen and carbon. Despite the fact that between them there should be a thick layer of helium, excluding such a phenomenon. Seeing this, we could not understand what was the matter, ”said the main author of the work, an employee of the physics department of the University of Warwick, Dr. Mark Hollands.
Having obtained the results of the analysis of the atmosphere of the star, scientists suggested that they are dealing with an object formed as a result of the merger of two white dwarfs of medium size and different composition.
Another key to understanding the unusual nature of the white dwarf, scientists call it a "solid" age. According to astronomers, the older the star, the faster its orbital speed around the center of the galaxy. Scientists also learned to determine the age of white dwarfs by the rate of cooling of such objects. The researchers found a mismatch between the cooling parameters of WDJ0551 + 4135 and its orbital speed. It was found that inside the Milky Way, this supermassive white dwarf moves faster than 99% of its nearby "brothers", having the same cooling parameters.
“The concept of normal stellar evolution does not allow us to give any explanation for this combination: the mass is twice the average value for white dwarfs, and the kinematic age is more than could be expected from the cooling parameters. You can only explain what happened by the merger of two white dwarfs, ”added Dr. Hollands.
Today it is one of the few well-known united white dwarfs, the researchers note. Its uniqueness is that most of the mergers in the Galaxy occur between objects with different masses, whereas in this case it occurred between stars of approximately the same size. According to scientists, the merger occurred about 1.3 billion years ago, while the history of the two source stars before the merger can be several billion years old.