An international team of astronomers has recently identified a new class of galactic nebulae. Their discovery offers evidence for a fully developed shell of a common envelope system (CE). The CE is the common envelope phase of a binary star system.
Astrophysicist Stefan Kimeswenger said, “Toward the end of their lives, normal stars inflate into red giant stars. Since a very large fraction of stars is in binary stars, this affects the evolution at the end of their lives. In close binary systems, the inflating outer part of a star merges as a common envelope around both stars. However, inside this gas envelope, the cores of the two stars are practically undisturbed and follow their evolution like single independent stars.”
Many stellar systems that are about to develop a common envelope had already been discovered due to their specific and high brightness. However, the fully developed envelope of a CE and its ejection into interstellar space had not been observed in this form so far.
Kimeswenger said, “These envelopes are of great importance for our understanding of the evolution of stars in their final phase. Moreover, they help us to understand how they enrich the interstellar space with heavy elements, which are then in turn important for the evolution of planetary systems, such as our own.”
“The discovery shows the importance of the newly discovered galactic nebulae and adds an explanation for why the probability of their discovery is low: They are too large for the field of view of modern telescopes, and at the same time, they are very faint. Moreover, their lifetime is short, at least when considered in cosmic time scales. It is only a few hundred thousand years.”
Scientists made this discovery while searching historical images for unknown objects in the now digitized archives and finally found a fragment of a nebula on photographic plates from the 1980s.
The team then compiled and combined observations from the past 20 years from public archives of various telescopes and data from four different space satellites. They rule out their first assumption, namely the discovery of a planetary nebula caused by the remnants of dying stars.
Kimeswenger said, “The diameter of the main cloud is 15.6 light-years across, almost 1 million times larger than the distance of the earth to the Sun and much larger than the distance of our Sun to its nearest neighboring star. Moreover, fragments as large as 39 light-years apart have also been found. Since the object lies slightly above the Milky Way, the nebula was able to develop largely undisturbed by other clouds in the surrounding gas.”
By combining all the information, scientists successfully created an object model. The object consists of a close binary system of a 66,500-degree white dwarf star and a normal star with a mass slightly below the Sun. Both stars orbit each other in only 8 hours and 2 minutes and at a distance of only 2.2 solar radii.
Because of the small distance, the companion star with a temperature of around 4,700 degrees is strongly heated at the side facing the white dwarf. This leads to extreme phenomena in the star spectrum and very regular variations in brightness.
There is a gigantic envelope around both stars that consists of the outer material of the white dwarf. At just over one solar mass, this material is heavier than the white dwarf and its companion star and was ejected into space some 500,000 years ago.
Kimeswenger said, “Another part of the puzzle related to the discovery of the new class of galactic nebulae has not yet been solved. It is even possible that this system is related to a nova observation made by Korean and Chinese astronomers in 1086. In any case, the positions of the historical observations match very well with those of our object described here.”
- Stefan Kimeswenger et al. YY Hya and its interstellar environment. DOI: 10.1051/0004-6361/202039787