56 Pegasi

Characteristics

The star 56 Pegasi was first noted for its changing radial velocity in 1911 by W. W. Campbell. Its inner pair consists of a primary and secondary star locked in a single-lined spectroscopic binary. They follow a nearly circular orbit every 111.15 days, with an average separation of 0.79 astronomical units. The primary is a peculiar bright giant, classified as a K-type giant but with some uncertainty. It shows an excess of barium and a shortage of CN and CH radicals. This star is active, about 229 million years old, and holds 4.3 times the Sun’s mass. It has swelled to 41 times the Sun’s radius and shines with 680 times the Sun’s luminosity. The secondary is much smaller, with just 0.13 times the Sun’s mass. A third component, a white dwarf with 0.85 solar masses, orbits the inner pair. When this white dwarf was an AGB star, it shed mass, transferring s-process elements to the primary and creating its unusual chemical makeup. This outer companion has an orbital period of 15194 days, a moderate eccentricity, and an average separation of 22 AU. The system gives off extra ultraviolet radiation, which must come from the secondary. Simon et al. (1982) classified this object as a subdwarf O star, though it could also be a white dwarf with an accretion disk. Some odd features in the system’s evolution might be explained if the primary is a fast rotator seen nearly pole-on. It may have been spun up during a past mass transfer event with the secondary.