SUBRA (Omicron Leonis). The Sickle of Leo is so famed that we tend not to pay much attention to the surrounding stars. Appearing almost as a forefoot of the celestial Lion, just-barely-fourth-magnitude (3.52) Subra extends down and to the right of Regulus. The name "Subra", however, refers to the Lion's mane, which is then decidedly misplaced. The assignment, however, is a mistake, as "Subra" originally referred to Zosma and Chertan (Delta and Theta Leonis), not to the star that is far better known as Omicron Leo, to which the name fell in modern times by error. Best to use the Greek. While seemingly anonymous, Omi Leo is a fascinating double with an oddball primary component. So close together that they cannot be separated directly through the telescope, the duplicity can be examined only through the spectrum and by sophisticated interferometry.
Subra Subra is a close binary star that can be resolved into its components only by using "interferometry," wherein we rely on the interfering properties of light waves. Here, the fainter secondary, Omicron Leonis B, orbits the brighter primary, Omicron Leo A every 14.5 days at a separation of 0.17 AU. The scale is in thousandths of a second of arc. The apparent ellipse is the result of the tilt of the true circular orbit against the plane of the sky. In reality, the two orbit a common center of mass that lies between them, which interferometry cannot locate, so we must rely on the relative orbit in which one star appears to go about the other. (From C. A. Hummel et al. in the Astronomical Journal, vol. 121, p. 1623, 2001.)
At a distance of 135 light years, Omicron Leo consists of a class F (F6) bright giant coupled with a class A (A5) dwarf, the two orbiting each other with a period of 14.498 days. The stars have also been classed as F9 giant plus A7 dwarf, the confusion arising because (1) the combined spectrum is hard to unravel and (2) the chemical compositions are odd. From the combined data, Omi Leo A (the class F star) shines with the light of 37 Suns from a coolish 6100 Kelvin surface, the radius 5.5 times solar, the mass 2.1 solar. Respective parameters for Omi Leo B (the class A dwarf) are 15.5 solar luminosities, 7600 Kelvin, 2.6 solar radii, 1.85 solar masses. Given the masses and orbital period, the two are a mere 0.165 Astronomical Units apart (less than half Mercury's distance from the Sun), the orbit, as expected, nearly a perfect circle. Both are listed as "metallic line stars" with raised abundances of various heavy elements (including the rare earths, those elements from atomic numbers 58 through 71) and deficiencies of calcium and scandium, the effect the result of diffusion, wherein some chemical elements sink under the effect of gravity, while others are raised by radiation pressure. Omi Leo A (the class F giant, or more likely subgiant) is unique in being the coolest known of these chemically odd stars. By all rights, it should not be a metallic line star at all. Once a classic "Am" (class A metallic) star, it has just recently givn up core hydrogen fusion and is now in the process of becoming a true giant. We thus have the good fortune to see a star that has been "caught" as a metallic line star before circulation of stellar gases in the cooler evolving atmosphere have wiped out the chemical peculiarities.
Written by Jim Kaler 6/22/07. Return to STARS.