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ALPHA DOR (Alpha Doradus). At mid-third magnitude (3.27), theluminary ofDorado, the Swordfish, is fairly prominent, well outshinig most ofits deep southern surroundings. It does, however, fool the eye, asits modest brightness is the result of it being two stars closetogether that even through a fine optical telescope appear as one. The dominating fourth magnitude (3.64) brighter component is aclass A (A0) giant. Close examination with sophisticatedinstrumentation reveals a fifth magnitude (4.55) companion that islisted as a class B9 subgiant, the pair 175 light years away. Their separation of only a couple tenths of a second of arc renderthe individuals difficult to study. Alpha Dor A has a measuredtemperature of 12,200 Kelvin (and maybe more), anomalously high forclass A0 star, which we adopt for Alpha Dor B as well, as it ismore in line with that expected for a B9 star. Quite possibly, thelight from the primary (A0) star has been contaminated by itscompanion, leading to an erroneous temperature. Adopting thattemperature for both stars (to allow for ultraviolet light) leadsto respective luminosities of 157 and 68 times that of theSun, radii of 2.8 and 1.9 solar, and masses of3.0 and 2.7 solar. The primary (Alpha Dor A) has begun to evolveand has probably given up core hydrogen fusion (hence its giantclassification), while the secondary (Alpha Dor B) is really ahydrogen-fusing dwarf rather and a true subgiant. Close as theyare, the orbit is rather well known and is a bit odd. Averaging9.7 Astronomical Units apart (about the distance of Saturn from theSun), they orbit each other in 12.1 years (far shorter thanSaturn's period as a result of much greater masses andgravitational fields). These orbital parameters give a mass-sum of6.2 times solar, a bit greater than the 5.7 solar derived from theluminosity, temperature, and theory, but given the uncertaintiesthe agreement is rather good. The orbital eccentricity isremarkably high, however, the stars separating by as much as 17.5AU (near Uranus's distance from the Sun) and then screaming in toas close as 1.9 AU (just 25 percent farther than Mars is from theSun), the last close approach taking place in 1998. The measuredrotation speed of the primary of at least 70 kilometers per secondleads to a rotation period of under two days. Of greatestinterest, Alpha Dor A's spectrum is "peculiar" in that it isespecially rich in silicon, the result of separation -- migrationreally -- of chemical elements in a relatively quiet stellaratmosphere, some settling downward because of gravity, otherslofted upward by radiation pressure and effects of magnetic fields. The silicon seems to be concentrated into a magnetic spot, whichtells of a real rotation period of 2.95 days, which shows that theabove rotation speed cannot be correct, probably as a result ofcontaminating light from Alpha Dor B.
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