29 Cygni

Characteristics

29 Cygni is a main-sequence star of spectral type A2 V, making it an A-type star. It is a Delta Scuti variable, pulsing at a rate of 0.0267 cycles per day, and it also belongs to the chemically peculiar Lambda Boötis class. It was the first star of this type found to be a pulsating variable. Its brightness varies by up to about 0.02 magnitudes across multiple periods, with the strongest period lasting 39 minutes. A magnetic field has been measured, with an average quadratic field strength. The star’s mass falls between 1.67 and 1.75 times that of the Sun, and it spins with a certain projected rotational velocity. Interferometric data show it is seen nearly pole-on, with an axial tilt that suggests its equatorial rotation speed is close to the breakup limit. This rapid spin gives the star an oblate shape: its equatorial radius is larger than its polar radius. Because of gravity darkening, the surface temperature changes with latitude, reaching 6,200 K at the poles and 8,800 K at the equator. Its photosphere emits about 10 times the Sun’s total light.

The star’s age is not settled. Its motion through space points to a 99 to 99.7 percent chance it belongs to the Argus Association, which would make it between 30 and 50 million years old. However, its position on the Hertzsprung–Russell diagram suggests an age between that of the 115-million-year-old Pleiades and the 414-million-year-old Ursa Major moving group. A model where the star is a rapid rotator seen nearly pole-on, which would lower its inferred luminosity and age, was first doubted based on asteroseismic data but later confirmed by CHARA array interferometry. The age of its companion, HIP 99770 b, has been estimated by matching its mass and brightness to substellar evolution models. Several stars appear close to 29 Cygni in catalogs, such as the yellow, 7th-magnitude HD 192661, but all are unrelated background objects. The naked-eye stars b1 Cygni and b2 Cygni, located about one and two degrees away, also lie at different distances.

Planetary system and substellar companion

The 29 Cyg system exhibits far-infrared excess emission about 13 times brighter than the stellar photosphere, indicating a cold debris disk directly detected in Herschel images. The disk likely lies beyond 150 au from the star, at a luminosity-scaled distance similar to the Kuiper belt. The star is orbited by the superjovian planet HIP 99770 b, discovered by direct imaging and astrometry. It is a late L dwarf (L6 to L9.5) with an estimated temperature of about 1300 K, at a luminosity-scaled distance similar to Jupiter's. The planet's orbital plane is aligned with the star's spin axis to within 2 sigma, comparable to other imaged planetary systems. This spin-orbit alignment and evidence for enhanced metallicity relative to the host star suggest HIP 99770 b formed in a protoplanetary disk via a bottom-up process.