FR Scuti (FR Sct) is a hierarchical triple star system located in the constellation of Scutum. It is classified as a VV Cephei-type system, which typically consists of a cool red supergiant and a hot companion, but in this case the hot component is itself a close binary, making the overall system a triple. It was once thought to be a symbiotic binary, exhibiting unique spectral emission lines and photometric variability, but is now though to be a detached eclipsing binary with additional irregular pulsations of the cool supergiant. Another star system similar to FR Scuti is TIC 290061484.
Characteristics
The system consists of a cool red supergiant primary and a smaller hotter companion. The hot companion is itself a close binary, likely consisting of two massive B-type stars (inferred from the VV Cephei classification and emission lines). The two hot stars occupy an inner orbit, while the M supergiant has an wider orbit. This hierarchical structure makes it unique among VV Cephei systems, which are rare massive binaries related to, but distinct from, symbiotic stars. The system is also a radio source, likely due to thermal emission from plasma ionized by the hot components' ultraviolet radiation interacting with the M supergiant's wind.
Spectral features The spectrum is composite, showing emission lines typical of VV Cephei systems (hydrogen, [Fe II]), but uniquely includes stronger [Fe III] and [O III] lines, which may indicate hotter components or effects from the hot binary's duplicity. These features were first noted in 1956.
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