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ACUBENS

(Alpha Cancri). Though Bayer's Alpha star, Acubens, at faint fourth magnitude (4.25), ranks only fourth in the constellation Cancer, after Beta (Al Tarf), Delta (Asellus Australis), even Iota, probably because of its position as a southern claw of the celestial crab. The star's name, which it actually shares with a northern claw (Iota), is derived from an Arabic word that means just that, "the claw," and is the same root from which is derived the name Zubenelgenubi (Alpha Librae), the star that represents the southern claw of Scorpius. Acubens, 175 light years away, is something of a mystery. Seemingly a simple white class A (A5) star, its spectrum displays ultra-strong absorptions of particular metals that make it a "metallic line" or "Am" star. Metallic line stars are typically strongly enhanced in elements like zinc, strontium, zirconium, barium and others. The phenomenon is a surface effect in which some elements sink to lower layers under the action of gravity, while others rise, pushed upward by radiation. To be such chemically peculiar stars, they must rotate slowly, such that the surface gases are not stirred up. Class A stars tend to rotate quickly however, so there should be slowing mechanism, which for the Am stars is duplicity (each acting tidally to slow the other). Acubens, though, rotates more quickly than usual (at least 68 kilometers per second at the equator). It is, however, a double star. It is close enough to the ecliptic plane that the Moon occasionally passes over and occults it. Stars normally wink out very quickly when the Moon covers them; Acubens winked out twice, showing that it consists of two identical stars only 0.1 seconds of arc (5.3 astronomical units) apart. Each is presumably a dwarf Am star with double the mass of the Sun, each shining with 23 solar luminosities. Given the masses and separation, they should orbit each other every 6.1 years. Neither can be discriminated from the other, however. Moreover, Acubens is not just double, but quadruple. Eleven seconds of arc away is a 12th magnitude companion that is ITSELF double -- about all that is known of it. If the two are identical, both would be dim class M dwarfs. Separated by at least 600 astronomical units, the faint pair must take at least 6300 years to orbit the bright pair (which from the faint pair would appear about as far apart as the angular diameter of the full Moon).