There is a group of variable pairs of stars known as Algol-type binaries. They are named after the prototype member of this class: Algol, a bright multiple star system in the Perseus constellation. In these stellar couples, the star that was originally brighter, more massive, and more evolved ends up being dimmer and lighter, stripped of material by its companion.
This sordid tale of thievery has many uncertainties, but a new study suggests several exciting phenomena that can explain how the mass transfer between stars takes place. Researchers from the Yunnan Observatories of the Chinese Academy of Science have studied binary system KIC 06852488. As reported in The Astronomical Journal, the peculiar properties of this system could be the blueprint for how Algol variables work.
KIC 06852488 is made of a primary component that is pulsating, and a secondary component with a very strong magnetic field. By using data from NASA's Kepler and Transiting Exoplanet Survey Satellite (TESS), the team was able to track the changes in brightness for the two stars.
It appears that the system experiences the O’Connell effect. As these two stars orbit each other, they periodically pass in front of each other. This is something we can easily track with the brightness profile (the light curve), and they should get equally bright in the two periods in their orbit when they are side-by-side. But one of these side-by-side regions is much brighter than the other; this is the O’Connell effect.
"The variation of the O'Connell effect could be explained by an evolving hot spot on the primary component and an evolving cool spot on the secondary component, and their positions are almost symmetrical with the inner Lagrange L1 point," co-author Professor Sheng-bang Qian said in a statement.
The L1 point is the area where the gravitational pull between the two objects is balanced out, at the point where the two stars are facing each other. Sunspots are related to the magnetic activity of the Sun, and these starspots are likely related to the magnetic activity as well as the stars’ gravitational pull on each other.
The team also reports the detection of six optical flares from the secondary star, all of which were in the energy class of superflares – another indication of major magnetic activity. The secondary star is puffed up, almost completely filling its Roche lobe, the region where its gravity reigns supreme. If material goes beyond the Roche lobe, it would be captured by its companion.
The study raises some intriguing points on the complex lives of binary stars. Algol variables make up 9 percent of all known variable stars, so understanding their formation and evolution is very important.