Researchers and laymen found huge common envelope around double star

Following a tip-off from hobby researchers, a team of astronomers led by Stefan Kimeswenger from the Institute for Astro- and Particle Physics at the University of Innsbruck has succeeded in providing the first evidence of a huge, fully developed common envelope around a binary star system.

The scientists report on their discovery of a new class of galactic nebulae in the scientific journal "Astronomy & Astrophysics", according to a statement from the University of Innsbruck on Tuesday.

The structure came to the attention of a group of German-French amateur astronomers. They painstakingly searched through digitised archives of historical celestial images in search of unknown structures. In the process, they found a fragment of the nebula on photographic plates from the 1980s.

120 individual images combined into one big whole

They then called in scientific experts, including the Innsbruck researcher Kimeswenger, who is now the first author in the publication. The growing team then sifted through and combined a wide variety of images of the region from several telescopes and satellites over the past 20 years. In addition, the scientists pointed telescopes at the double star system from South America. The discovery photo was then produced from 120 individual images taken over several months.

This shows the fully developed envelope of a "common envelope system" (CE), according to the release. This is a structure that surrounds a binary star system. Such a structure can form at the end of stars' lives when they inflate. "Since a very large fraction of stars are in binary stars, this affects how they evolve at the end of their lives. This is because in tighter binary systems, the inflating outer part of a star coalesces as a common envelope around both stars. Inside this envelope, however, the core regions of the two stars behave practically undisturbed and follow their evolution as if they were independent of each other," Kimeswenger explained.

Diameter of the main cloud is 15.6 light years

In the case studied here, a common main cloud with a diameter of 15.6 light years formed in the meantime. That is "almost a million times larger than the distance of the Earth to the Sun and much larger than the distance of our Sun to its nearest neighbouring star. But fragments with an extension of even 39 light years were also found," said the astronomer: "Since the object lies slightly above the Milky Way, the nebula could develop largely undisturbed by other clouds in the surrounding gas."

Based on the comprehensive information, the researchers were also able to create a model of the binary star system. According to this, it houses a white dwarf star with a temperature of around 66,500 degrees Celsius. In addition, there is a normal star with a temperature of "only" about 4,700 degrees Celsius and a slightly lower mass than our Sun. These two orbit each other every eight hours and two minutes at a cosmically small distance of only 2.2 solar radii. The side of the normal star facing the white dwarf is therefore strongly heated by the hot partner star, which leads to regular fluctuations in the brightness of the system.

The envelope now discovered consists of the outer material of the white dwarf. The complete, widely distributed envelope weighs more than one solar mass in total. The structure was formed about 500,000 years ago, the researchers report.

According to Kimeswenger, such nebulae are "of great importance for understanding the development of stars in their final phase. They also help us to understand how they enrich space with heavy elements, which in turn are important for the development of planetary systems, including our own.