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.