Zeta Cancri

Zeta Cancri (ζ Cancri, abbreviated Zeta Cnc, ζ Cnc) is a multiple star system in the constellation of Cancer. It is approximately 83.4 light-years from Earth, and has a combined apparent magnitude of +4.67. Since it is near the ecliptic, it can be occulted by the Moon and, very rarely, by planets.

The system is constituted as follows:^[1]

• A binary pair designated Zeta¹ Cancri or alternatively Zeta Cancri AB, the two components of which are themselves designated Zeta¹ Cancri A or simply Zeta Cancri A (formally also named Tegmine /ˈtɛɡmɪniː/, the traditional name of the Zeta Cancri system)^[2] and Zeta¹/Zeta Cancri B.
• A triple star system designated Zeta² Cancri or alternatively Zeta Cancri C, consisting of a single star primary, designated Zeta²/Zeta Cancri Ca, together with a secondary binary pair, designated Zeta²/Zeta Cancri Cb. The binary pair's two components are themselves designated Zeta²/Zeta Cancri Cb1 and Cb2.

Nomenclature[]

ζ Cancri (Latinised to Zeta Cancri) is the system's Bayer designation; ζ¹ Cancri and ζ² Cancri those of its two constituents. The designations of the two constituents as ζ Cancri AB and C, and those of their components - ζ Cancri A, B, Ca, Cb, Cb1 and Cb2 - derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).^[3]

Considerable confusion had developed concerning the catalogue identities of the three bright stars; correct correspondences were worked out by Griffin:^[4]

Zeta Cancri bore the traditional name Tegmine (Tegmen) "the shell (of the crab)".^{[citation needed]} In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)^[5] to catalogue and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.^[6] It approved the name Tegmine for the component Zeta¹ Cancri A on 12 September 2016 and it is now so included in the List of IAU-approved Star Names.^[2]

In Chinese, 水位 (Shuǐ Wèi), meaning Water Level, refers to an asterism consisting of Zeta Cancri, 6 Canis Minoris, 11 Canis Minoris and 8 Cancri.^[7] Consequently, Zeta Cancri itself is known as 水位四 (Shuǐ Wèi sì, English: the Fourth Star of Water Level).^[8]

Properties[]

Zeta Cancri can be resolved as a binary star in small telescopes. Its binary nature was discovered in 1756 by Tobias Mayer. William Herschel resolved the two components that make up Zeta¹ Cancri in 1781. As early as 1831, John Herschel noticed perturbations in Zeta² Cancri's orbit around Zeta¹; this led Otto Wilhelm von Struve, in 1871, to postulate a fourth, unseen, component which orbited closely the visible member of Zeta².^[4] Later observations have resolved this fourth component and have indicated that there may be one or two more unobserved components.^[9][10]

Zeta¹ and Zeta² Cancri are 5.06 arcseconds apart. These two star systems orbit around their common centre of mass once every 1100 years. The radius is about 6.80.^{[clarification needed]}

Zeta¹ Cancri[]

The two components are both yellow-white main sequence dwarfs of spectral class F. The apparent magnitudes of A and B are +5.58 and +5.99, respectively. They are separated, as of 2008, by 1 arcsecond, requiring a large telescope to resolve them, but this separation will increase until the year 2020. They complete one orbit every 59.6 years.^[11] The estimated masses for the pair are 1.28 and 1.18 solar masses, respectively.

Zeta² Cancri[]

Zeta Cancri Ca is the brightest of the three components, having an apparent magnitude of +6.12. It appears to be a yellow G-type star, often reported as G5V, but now thought to be earlier, probably G0V. This star has around 1.15 solar masses.^[12] The tenth magnitude Zeta Cancri Cb is a close pair of red dwarfs. The separation between Ca and Cb is approximately 0.3 arcseconds, and their orbital period is 17 years.

References[]