LAMBDA BOO (Lambda Bootis). The main portion of Bootes, the Herdsman, rather resembles a kite with
mighty Arcturus, the luminary, at its
end. Less recognized is a long triangle of stars that lies near
the tail of Ursa Major (the handle of the
Big Dipper) made of Theta and Epsilon Boo (that with much
fainter Iota make the outstretched fingers of the Herdsman) and
modest fourth magnitude (4.18) Lambda. Modest but mysterious, this
class A (A0 peculiar) hydrogen fusing dwarf is still the subject of
study and argument. At a distance of 97 light years, Lambda shines
to us with the luminosity of 16 Suns from an
8900 Kelvin surface, as expected for a two solar mass star.
Spinning with a rapid projected equatorial velocity of 128
kilometers per second and 1.7 times the size of the Sun, the star
rotates with a period of less than two thirds of a day. So far,
all seems relatively normal. The surprise comes in the Lambda's
weird chemical composition. Its outer layers are depleted in
metals (chromium, barium, nickel, titanium) by roughly a factor of
ten, while the other elements come in as more or less normal. Such
stars (the "Lambda Boo stars") are rare: there are only about 50
known. Many class A and B (Elnath and Gamma Corvi for example) stars have odd
abundances as a result of separation (gravitational settling and
radiative lofting) in quiet atmospheres. But Lambda's spin roils
the gasses and hardly makes it quiet! The prevailing idea is that
star was at an early age surrounded by a thick cloud of dusty gas.
Interstellar dust grains absorb metal atoms from the gas onto
themselves, which depletes the gas (a well-recognized phenomenon).
The pressure of starlight keeps the dust pushed away while the
depleted gas settles inward to become part of the star. Or not.
No one really knows. (Lambda Boo was chosen as one of Jim Kaler's
Hundred Greatest Stars.)