OMI AND (Omicron Andromedae). Andromeda, the central character in the ancient Andromeda myth, has a strong representation in the sky in long strings of stars that come off the northeastern corner of the Great Square of Pegasus (Alpha Andromedae and Delta Pegasi being the same star.) The constellation boundaries spread far from the classic figure. At the far western boundary, 14 degrees north of the Great Square (and Beta Pegasi) lies rather bright fourth (3.62, nearly third) Omicron Andromedae, which, since it is just east of the border with Lynx, has the distinction of being Flamsteed's number 1 as well. At first look, it's a rather odd close orbiting double consisting of a fourth magnitude (3.73) class B (B6) emission line giant (a "Be" star) coupled with a sixth magnitude (6.03) class A (A2) "peculiar" (magnetic) star only a few hundredths of a second of arc apart, which at a distance of 687 light years (give or take 77) corresponds to an average separation of 12.8 AU (with a modest eccentricity) and an orbital period of 5.6 years. Adopting a small degree of interstellar dimming (0.19 magnitudes) and a temperature of 14,490 Kelvin for the Be star (from which we evaluate the degree of ultraviolet radiation), we find a hefty luminosity of 3945 times that of the Sun, a radius of 9.9 times solar, and a mass of 6.5 Suns, theory showing the star to be a subgiant that will shortly begin to fuse its core helium to carbon and oxygen. Interferometric measures of radius give a satisfyingly close 11.5 solar radii. A projected equatorial rotation velocity of 286 kilometers per second gives a rotation period under 1.7 days. A Be star is enclosed in a rotating disk that gives off hydrogen (and other) emission lines. The disks are variable, Omi And typically varying between around magnitude 3.5 and 3.65, in outbursts rather like Gamma Cas. In some stars the disks disappear and re-appear at will. The Be phenomenon is somehow related to high rotation velocity, but nobody knows quite how. Winds may well be involved. The class A companion should have a temperature around 9000 Kelvin (there is no measurement), which gives a much lower luminosity of 170 Suns, a radius of 1.3 solar, and a mass of 3 Suns, all of these numbers rather uncertain. The bright Be star (confusingly called Omi And A, rendering the A star Omi B) has a companion as well with a measured period of 117 days and an apparent mean separation of 62 AU. Both orbits (AbAa and AB) give highly unreasonable masses, so it's doubtful that that the orbits are accurate (not surprising for such a complex star). There is also a suggestion of a fourth member that was originally thought to orbit the A star (Omi B), but that might orbit the other. It's all surprisingly non-definitive for a star this observable and this bright.
Written byJim Kaler 12/02/16. Return to STARS.