Largest touching stars ever observed will eventually smash into each other as black holes

The stars, which are located in a neighboring dwarf galaxy known as the Small Magellanic Cloud, are in partial contact with each other. In fact, one of the stars is currently "feeding" off of the other. They orbit each other once every three days and they are the most massive touching stars observed to date.

"This binary star is the most massive contact binary observed so far," study co-author Daniel Pauli, a Ph.D. student at the University of Potsdam, explained in a statement. "The smaller, brighter, hotter star, 32 times the mass of the Sun, is currently losing mass to its bigger companion, which has 55 times our Sun’s mass."

The scientists used computer models and compared them to their real observations. They found that one of the touching stars, also referred to as contact binaries, will likely become a black hole and will feed on the other star. Shortly afterward, the other star will likely also become a black hole.

Touching stars to merge as black holes in 18 billion years

Based on their models and observations, the scientists believe the stars will become black holes in roughly a couple million years' time. They will then orbit each other for billions of years before smashing into each other with massive force. That event will generate ripples in the fabric of space-time — also known as gravitational waves — that would, in theory, be detectable by instruments on Earth.

“Thanks to gravitational wave detectors Virgo and LIGO, dozens of black hole mergers have been detected in the last few years," said lead author Matthew Rickard, a Ph.D. student at UCL. "But so far we have yet to observe stars that are predicted to collapse into black holes of this size and merge in a time scale shorter than or even broadly comparable to the age of the universe."

“Our best-fit model suggests these stars will merge as black holes in 18 billion years," Rickard continued. "Finding stars on this evolutionary pathway so close to our Milky Way galaxy presents us with an excellent opportunity [to] learn even more about how these black hole binaries form.” 

Study Abstract:

Context: Most massive stars are believed to be born in close binary systems where they can exchange mass, which impacts the evolution of both binary components. Their evolution is of great interest in the search for the progenitors of gravitational waves. However, there are unknowns in the physics of mass transfer as observational examples are rare, especially at low metallicity. Nearby low-metallicity environments are particularly interesting hunting grounds for interacting systems as they act as the closest proxy for the early universe where we can resolve individual stars.

Aims: Using multi-epoch spectroscopic data, we complete a consistent spectral and orbital analysis of the early-type massive binary SSN 7 hosting a ON3 If∗+O5.5 V((f)) star. Using these detailed results, we constrain an evolutionary scenario that can help us to understand binary evolution in low metallicity.

Methods: We were able to derive reliable radial velocities of the two components from the multi-epoch data, which were used to constrain the orbital parameters. The spectroscopic data covers the UV, optical, and near-IR, allowing a consistent analysis with the stellar atmosphere code, PoWR. Given the stellar and orbital parameters, we interpreted the results using binary evolutionary models.

Results: The two stars in the system have comparable luminosities of log(L1/L ) = 5.75 and log(L2/L ) = 5.78 for the primary and secondary, respectively, but have different temperatures (T1 = 43.6 kK and T2 = 38.7 kK). The primary (32M ) is less massive than the secondary (55 M ), suggesting mass exchange. The mass estimates are confirmed by the orbital analysis. The revisited orbital period is 3 d. Our evolutionary models also predict mass exchange. Currently, the system is a contact binary undergoing a slow Case A phase, making it the most massive Algol-like system yet discovered.

Conclusions: Following the initial mass function, massive stars are rare, and to find them in an Algol-like configuration is even more unlikely. To date, no comparable system to SSN 7 has been found, making it a unique object to study the efficiency of mass transfer in massive star binaries. This example increases our understanding of low-mass binary evolution and the formation of gravitationalwave progenitors.