ALPHA DOR (Alpha Doradus). At mid-third magnitude (3.27), the luminary of Dorado , the Swordfish, is fairly prominent, well outshinig most of its deep southern surroundings. It does, however, fool the eye, as its modest brightness is the result of it being two stars close together that even through a fine optical telescope appear as one. The dominating fourth magnitude (3.64) brighter component is a class A (A0) giant. Close examination with sophisticated instrumentation reveals a fifth magnitude (4.55) companion that is listed as a class B9 subgiant, the pair 175 light years away. Their separation of only a couple tenths of a second of arc render the individuals difficult to study. Alpha Dor A has a measured temperature of 12,200 Kelvin (and maybe more), anomalously high for class A0 star, which we adopt for Alpha Dor B as well, as it is more in line with that expected for a B9 star. Quite possibly, the light from the primary (A0) star has been contaminated by its companion, leading to an erroneous temperature. Adopting that temperature for both stars (to allow for ultraviolet light) leads to respective luminosities of 157 and 68 times that of the Sun, radii of 2.8 and 1.9 solar, and masses of 3.0 and 2.7 solar. The primary (Alpha Dor A) has begun to evolve and has probably given up core hydrogen fusion (hence its giant classification), while the secondary (Alpha Dor B) is really a hydrogen-fusing dwarf rather and a true subgiant. Close as they are, the orbit is rather well known and is a bit odd. Averaging 9.7 Astronomical Units apart (about the distance of Saturn from the Sun), they orbit each other in 12.1 years (far shorter than Saturn's period as a result of much greater masses and gravitational fields). These orbital parameters give a mass-sum of 6.2 times solar, a bit greater than the 5.7 solar derived from the luminosity, temperature, and theory, but given the uncertainties the agreement is rather good. The orbital eccentricity is remarkably high, however, the stars separating by as much as 17.5 AU (near Uranus's distance from the Sun) and then screaming in to as close as 1.9 AU (just 25 percent farther than Mars is from the Sun), the last close approach taking place in 1998. The measured rotation speed of the primary of at least 70 kilometers per second leads to a rotation period of under two days. Of greatest interest, Alpha Dor A's spectrum is "peculiar" in that it is especially rich in silicon, the result of separation -- migration really -- of chemical elements in a relatively quiet stellar atmosphere, some settling downward because of gravity, others lofted upward by radiation pressure and effects of magnetic fields. The silicon seems to be concentrated into a magnetic spot, which tells of a real rotation period of 2.95 days, which shows that the above rotation speed cannot be correct, probably as a result of contaminating light from Alpha Dor B.
Written by Jim Kaler. Return to STARS.