45 LEO (45 Leonis = CX Leonis). Rho Leonis, 6.5 degrees southeast of Regulus (the line between the two paralleling the ecliptic), seems to be a veritable magnet for variable stars. Surrounding it within a circle of two degrees radius are sixth magnitude 44 (or DE) Leonis, 49 (TX) Leonis, and 45 (CX) Leonis, the numbers from the Flamsteed catalogue, the double letters indicating variability. TX Leo is a rather subtle class A eclipsing binary (magnitude 5.66-5.75), while DE is a class M (M3) giant that was touted as a semi- regular variable, but which seems to change by only a few hundredths of a magnitude. While at nominal magnitude 6.04 the faintest of them, CX (45) is probably also the most interesting. At a distance of 421 light years (give or take 16), 45=CX is an enigmatic white class A (A0p) "dwarf" that varies between magnitudes 5.97 and 6.15 over a 1.445 day interval. The "p" indicates a "peculiar" spectrum and thus an odd chemical composition. Serious chemical weirdness is caused in metallic-line "Am" stars (like Alpha Cancri and Alpha Piscium) by radiative lofting of some elements and gravitational settling of others. The effects are hardly subtle, some elements enhanced beyond their solar values by hundreds of thousands of times. In Ap stars like this one, the oddities are especially concentrated into large magnetic patches usually near the rotation poles. Since the rotation axes are tilted to the line of sight, the stars vary both in brightness and in "chemistry" as the starspots rotate in and out of view. The prototype is Alpha-2 Canum Venaticorum (the principal member of Cor Caroli). CX=45 is noted to be enhanced in silicon, chromium, copper, zirconium, the "rare earths" (europium a common example), and deficient in calcium and scandium, while the magnetic field is measured to be as great as 250 or so times that of Earth. On the face of it, with a temperature of 10,040 Kelvin, 45 Leo radiates at a rate of 64 times that of the Sun, which yields a radius of 2.7 times solar. While the projected equatorial rotation speed is controversial, the best value seems to be 11 kilometers per second, which gives a rotation period of under 12.1 days. Presuming the variation period of 45 Leo to be produced by rotation gives an axial tilt of 7 degrees from the line of sight and a true velocity of 92 km/s, near the upper edge for such stars (higher rotation speeds stirring the stellar atmospheres and reducing the anomalies). Theory then suggests a mass of 2.7 Suns and clearly shows the star to be a hydrogen fusing dwarf.

But there is a problem: duplicity, even multiplicity. We can dismiss 11th magnitude 45 Leo B, which is a line of sight coincidence. There seems, though, to be a real spectroscopic companion eccentrically orbiting 45 Leo with a period of 34.7 years. At the same time, a lunar occultation, in which the Moon passed over the star, revealed that 45 Leo was made of TWO identical components just 0.02 seconds of arc apart, each of which would then radiate 32 solar luminosities and carry 2.3 solar masses, and bring the rotation speed of the magnetic star down to 65 km/s. But both interferometry and spectroscopy did not reveal any such duplicity. On the other hand given the proper orbit, one of the close pair could be the spectroscopic companion, so there might be one star, two, or possibly even three. We may have to wait out the stars' movements until around the year 2030. (Thanks especially to J. Zverko et al., Astrophysical Bulletin, vol. 67, p. 57, 2012.)

Written by Jim Kaler 6/07/13. Return to STARS.