A stellar four-beat waltz

A stellar four-beat waltz
A system composed of four gravitationally bound stars has been followed by spectroscopy for several years by a team of researchers. This quadruple system is unstable and simulations of its future evolution show that, eventually, the stars may collide, which is likely to produce a thermonuclear supernova. The evolution of such stellar quadruple systems thus represents a new way to explain the observations of supernovae.

Stars like company, and unlike our Sun, most stars in the Milky Way have one or more stellar companions. It is now recognized that binary stars play a major role in a wide range of observed astrophysical eventsIt is now recognized that binaries play a major role in a wide range of observed astrophysical events, and mergers of binaries are notably at the origin of the recent detection of gravitational wave emissions. In addition, binary stars allow us to derive fundamental stellar parameters such as masses, radii, and luminosities with better accuracy than stars alone. They represent the jewels on which various astrophysical subjects rest.

While binaries have received a lot of attention so far, higher-order stellar systems have remained in the background until recently, despite the fact that they exhibit a wide variety of interactions, especially in tight systems.cart until recently, despite the fact that they exhibit a wide variety of interactions, especially in tight systems. Stellar quadruples represent only a marginal fraction (a few percent) of all multiple systems. The complex evolution of these high-order multiple systems involves mass transfers and collisions, leading to mergers that are also possible progenitors of thermonuclear supernovae. These supernovae represent standard candles for setting the distance scale of the Universe, despite the fact that the evolutionary channel(s) leading to the progenitors of such supernova explosions are still highly debated.

In 2017, Thibault MERLE - Research Unit in Astronomy and Astrophysics, Faculty of Sciences of the Free University of Brussels - discovered a spectroscopic quadruple (HD 74438), i.e. a system of four stars, in the Gaia-ESO survey. The Gaia-ESO survey is a public spectroscopic survey that provides a detailed overview of the stellar content of the Milky Way by characterizing more than 100 000 stars. He was then able to track this system with high-resolution spectrographs at the University of Canterbury’s Mount John Observatory in New Zealand and the Southern African Large Telescope in South Africa. These observations have determined that this system is composed of 4 gravitationally bound stars: a short-period binary orbiting another short-period binary on a longer orbital period (2+2 configuration). Its belonging to the open cluster IC 2391 makes it the youngest spectroscopic quadruple system discovered so far (43 Ma million years) and among the quadruple systems with the shortest external orbital period (6 years).

Thanks to the spectroscopic analysis, it was possible to show that this quadruple system undergoes dynamical effects on long time scales compared to the orbital periods. Indeed, one of the inner binaries should evolve on a circular orbit while it has an eccentric orbit. This is explained by the gravitational effect of the distant binary companion which can increase the eccentricity.

State-of-the-art simulations of the future evolution of this system show that these gravitational dynamics can lead to one or more collisions and mergers producing white dwarfs whose mass is just below the Chandrasekhar limit. A star like our Sun will end its life as a white dwarf, and the mass of white dwarfs cannot exceed the so-called Chandrasekhar limit. If it does, as a result of mass transfer or fusion, it collapses and produces a thermonuclear supernova. It is interesting to note that it is now suspected that 70-85% of all thermonuclear supernovae result from the explosion of white dwarfs with masses below the Chandrasekhar limit. The evolution of stellar quadruples such as HD 74438 represents a promising new path for their formation.