Tegmine

TEGMINE (Zeta Cancri). Off to the side of Cancer, the Crab, not part of its classic outline and seven degrees to the west-southwest of Cancer's Beehive Cluster, lies one of the more remarkable multiple stars of the sky, Tegmine, or Zeta Cancri. The Latin name, which refers to the Crab's shell (and was originally meant for the whole constellation), is not quite grammatical, and should as a name be "Tegmen" -- but we are stuck with tradition. And apparently not a long one, as even Smythe and Chambers in 1881 say "there is much shade on the authority which designates it Tegmine." The star was discovered to be triple by William Herschel, and in our own time proved to be quadruple. At a poorly known distance of 84 light years (the multiplicity messing up parallax measures), the combined light of Tegmine's stars appears at just-barely-fifth magnitude (4.64). At first look, the star splits in two, with a fifth magnitude class F8 hydrogen-fusing dwarf (Zeta A) just a second of arc or so away from a sixth magnitude (6.20) F9 dwarf (Zeta-B), the latter not all that dissimilar from the Sun. The pair orbit each other every 59.6 years at a mean distance of 22.1 Astronomical Units, a decent eccentricity taking them between 29 and 15 AU. They were last closest (in orbit) in 1989 and will again pass through "periastron" in 2048. So keep watch.

At left, Zeta Cancri B orbits Zeta Cancri A (at the cross) at an average separation of 22 AU over a period of 59.6 years, while at right, Zeta CaCb (unresolved here) orbits Zeta AB (at the cross) at an average distance of 197 AU over a period of 1115 years, so long that the right-hand orbit is incomplete. The scales here are in seconds of arc; note the size of the right-hand scale and orbit compared with those on the left. Both orbits go in the same direction (as seen at lower right) and both are tilted to the line of sight, though the tilts differ by 21 degrees. In reality, each star is in mutual orbit with the other around a common center of mass. From the Sixth Catalog of Orbits of Visual Binary Stars , W. I. Hartkopf and B. D. Mason, US Naval Observatory Double Star Catalog, 2006.

Kepler's third law then gives a combined mass of 3.0 times that of the Sun. With respective temperatures from their classes of 6200 and 6100 Kelvin, the stars shine with luminosities of 3.5 and 1.8 solar, have radii of 1.6 and 1.2 solar, and masses from stellar structure theory of 1.4 and 1.25 solar, which combine to 2.9 solar, not a bad match at all. Something is wrong, though, as Zeta-B is dim for its class, perhaps the spectral type or distance. Orbiting these 7.7 seconds of arc away is a sixth magnitude (6.0) third component, Zeta C, which is itself double, and which consists of two class G dwarfs, sixth magnitude (6.3) Ca and seventh magnitude (7.1) Cb just a couple tenths of a second of arc apart, giving us a separation of about 5 AU in a nearly circular orbit with a period of 17 years. Respective temperatures for Ca and Cb are about 5900 and 5600 Kelvin, luminosities 1.7 and 0.9 solar, and masses 1.25 and 0.9 solar, Cb even more similar to the Sun. The combined mass of from Kepler's Laws, however, is WAY too small, so again something is clearly wrong with the data. Zeta CaCb then orbits Zeta AB every 1115 years at an average distance of 197 AU, an eccentricity of 0.24 taking the pairs between 244 and 150 AU apart. They were last closest (again, in orbit) in 1970, and it will be a long time before it happens again. Both orbits are tilted to the line of sight. That of Zeta CaCb around the AB pair is tilted to that of A around B by 21 degrees. The total combined mass from orbital theory is then 6.2 as opposed to 4.8 from structure and evolution theory, not bad considering the uncertainties involved. From Zeta CaCb, AB would average 6 degrees apart, while from AB, the outer pair would average about a degree and a half. Off in the distance a few minutes of arc away are much dimmer Zeta D, E, F, and G, whose motions clearly show them to be line-of-sight coincidences. (Thanks to Paolo Colona, who suggested this star.)

Written by Jim Kaler 4/03/09. Return to STARS.