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(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.