61 Cygni - Properties

Properties

Although it appears to be a single star to the naked eye, 61 Cygni is in fact a widely separated binary system, composed of two K class (orange) main sequence stars, 61 Cygni A and 61 Cygni B. The brighter star 61 Cygni A is of apparent magnitude 5.2, the fainter 61 Cygni B is 6.1. Both appear to be old-disk stars, with an estimated age that is older than the Sun. The system has a net space velocity of 108 km/s relative to the Sun, which results in the high proper motion across the sky. At a distance of just over 11 light years, it is the 15th nearest known star system to the Earth (not including the Sun). 61 Cygni A is the fourth nearest star that is visible to the naked eye for mid-latitude northern observers, after Sirius, Epsilon Eridani, and Procyon A. Since 1943, 61 Cygni A has served as a stable K5 V "anchor point" of the MK classification system. Starting in 1953, 61 Cygni B has been considered a K7 V standard star (Johnson & Morgan 1953, Keenan & McNeil 1989). This system will make its closest approach at about 20,000 CE, when the separation from the Sun will be about 9 light years.

The two orbit their common barycenter in a period of 659 years, with a mean separation of about 84 A.U.—84 times the separation between the Earth and the Sun. The relatively large orbital eccentricity of 0.48 means that the two stars are separated by about 44 A.U. at periapsis and 124 A.U. at apoapsis. The leisurely orbit of the pair has made it difficult to pin down their respective masses, and the accuracy of these values remain somewhat controversial. In the future this issue may be resolved through the use of asteroseismology.

Component A has about 11% more mass than component B. It has an activity cycle that is much more pronounced than the solar sunspot cycle. This is a complex activity cycle that varies with a period of about 7.5±1.7 years. (An earlier estimate gave a period of 7.3 years.) The combination of starspot activity combined with rotation and chromospheric activity is characteristic of a BY Draconis variable. Because of differential rotation, this star's surface rotation period varies by latitude from 27 to 45 days, with an average period of 35 days.

The outflow of the stellar wind from component A produces a bubble within the local interstellar cloud. Along the direction of the star's motion within the Milky Way, this extends out to a distance of only 30 AU, or roughly the orbital distance of Neptune from the Sun. This is lower than the separation between the two components of 61 Cygni, and so the two most likely do not share a common atmosphere. The compactness of the astrosphere is likely due to the low mass outflow and the relatively high velocity through the local medium.

Component B displays a more chaotic pattern of variability than A, with significant short-term flares. There is an 11.7-year periodicity to the overall activity cycle of B. Both stars exhibit stellar flare activity, but the chromosphere of component B is 25% more active than for component A. As a result of differential rotation, the period of rotation varies by latitude from 32 to 47 days, with an average period of 38 days.

There is some disagreement over the evolutionary age of this system. Kinematic data gives an age estimate of about 10 Gyr. Gyrochronology, or the age determination of a star based on its rotation and color, results in an average age of 2.0 ± 0.2 Gyr. The ages based on chromospheric activity for A and B are 2.36 Gyr and 3.75 Gyr, respectively. Finally the age estimates using the isochrone method, which involve fitting the stars to evolutionary models, yield upper limits of 0.44 Gyr and 0.68 Gyr. However, a 2008 evolutionary model using the CESAM2k code from the Côte d'Azur Observatory gives an age estimate of 6.0 ± 1.0 Gyr for the pair.