A low-metallicity massive contact binary undergoing slow Case A mass transfer: A detailed spectroscopic and orbital analysis of SSN 7 in NGC 346 in the SMC

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 (32 M⊙) 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 massive star binary evolution and the formation of gravitational wave progenitors.