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HYADUM II

(Delta-1 Tauri). Many proper star names are famed, while others are mightily obscure. Here is one of the latter, the Latin "Hyadum II" (which bears obvious reference to its membership in the Hyades, star cluster of Taurus) hardly known at all, any more than similarly- named Hyadum I. Instead, they are far better known respectively as Delta and Gamma Tauri. Three stars in a compact collection now carry the name "Delta Tauri," from west to east bright fourth magnitude (3.76) Delta-1, fifth magnitude (4.80) Delta-2, and faint fourth magnitude (4.31) Delta-3. Hyadum II clearly belongs to the brightest, Delta-1. As ordinary hydrogen-fusing stars die, they turn into giants with dead helium cores that quickly light up to fuse their helium to carbon. The Hyades contains four of these helium-fusing class K orange giants, Ain (Epsilon Tauri), Gamma Tau (Hyadum I), Theta-1 Tau, and Hyadum II. At a measured distance of 153 light years, the star would be 3 light years more distant than the cluster center, though more likely the difference is due to small measurement errors. At class K0, the star has a precisely measured temperature of 4965 Kelvin. Allowing for a bit of infrared radiation leads to a luminosity 74 times that of the Sun, which with temperature gives a radius of 11.6 solar and a mass of 2.6 Suns. An interferometer measure of the star's angular diameter gives a radius of 11.8 solar, the agreement between the two evaluations showing that the measured parameters must be correct. The projected rotation speed is a very low 1.2 kilometers per second, which leads to a rotation period that might be as long as 488 days. The Hyades' age is measured (from the kinds of stars it still contains) at 650 million years, which is consistent with the evolution of a class K giant such as Delta-1 Tau. Not quite two minutes of arc away is a dim 13th magnitude star, which if a real companion (which seems unlikely) is a low mass class M0 dwarf at a separation of at least 5200 Astronomical Units (AU) and an orbital period of at least 230,000 years. (Such a companion should long ago have been ripped away by other stars in the cluster.) Far more real is a spectroscopic companion that orbits in 529.8 days (suggesting that the K star's rotation may be synchronized with the orbit). A lunar occultation (whereby the Moon covers the star) shows the neighbor to be a dim 13th magnitude class M dwarf. The two orbit at a mean distance of only 1.76 AU, the orbital eccentricity carrying them from 1.0 AU to 2.5 AU and back again. The occultation observation agrees, giving a distance of 1.9 AU. Like the Hyades in general, Delta-1 is metal-rich, its metal content 25 percent greater than that found in the Sun, which is consistent with an enhancement in the cyanogen (CN) molecule that is noted in the star's spectral class.