MU LEP (Mu Leporis). The stars of Orion and its surroundings are so luminous in the night sky that we tend to ignore a figure that in any other setting might be rather prominent, Lepus, the Hare, which lies directly south of the Hunter. After all, its brightest star (Arneb, Alpha) is close to second magnitude, and even Mu Lep, well down in the Greek alphabet, hits third, albeit at the fainter end (3.31). Though at first sight appearing as a common class B, hydrogen-fusing dwarf (at B9 on the very cool side of the class and seemingly not much different from Vega or Sirius), it has some unusual properties to recommend it, as seen following the usual physical rundown. Mu Lep lies at a well-determined distance of 186 light years, give or take only 4. Its temperature, from which we can find the amount of ultraviolet light, is also well-determined at 12,600 Kelvin, which is more in line with that of a B8 dwarf (and indeed the star has been called a B7 subgiant). From these we find a luminosity of 256 times that of the Sun, a radius of 3.4 solar, and a rotation period (from a projected equatorial speed of 15 kilometers per second) of under 11.3 days. For its class, the spin is slow, suggesting that the star's rotation axis may be at least somewhat pointed at Earth. Theory gives a hefty mass of 3.75 times that of the Sun, and indeed shows the star to be a dwarf, one around halfway through its allotted 220 million-year hydrogen- fusing lifetime. The dichotomy in the spectral class is consistent with an odd chemistry, and sure enough, Mu Lep is a fine example of a "mercury-manganese" star, one of the brightest, in which these and a variety of other elements (notably rare earths like europium) are greatly enhanced, while others like calcium are depleted, as a result of a combination of gravitational settling and radiative lofting in quiet unstirred atmospheres in slowly rotating stars (fast rotation causing circulation that mixes the gases), and Mu Lep certainly falls into that category. Actual measures show that manganese is raised over the solar value (relative to hydrogen) by a factor of 180, while mercury is up by around 70,000 times. The latter element has in fact never even been identified in the Sun, its solar abundance inferred from the chemistry of primitive meteorites. Mu Lep is also very oddly a notable source of X-rays, indeed is the brightest cool-class-B mercury-manganese star to have them. But unlike the Sun, such stars should NOT radiate X-rays. An offset in the X-ray position relative to the star plus evidence in the high-energy spectrum strongly suggest that the X-rays are coming from an active companion, perhaps a low-mass star still undergoing its formation. If so, the offset suggests a separation of 53 Astronomical Units, which if accurate (a big "if") would indicate from Kepler's Laws an orbital period of about 200 years. A bit of controversy also hangs about Mu Leporis proper. It is noted as being a variable (magnitude 2.97-3.36) magnetic Alpha-2 Canum Venaticorum (Cor Caroli) star. But the variation is unconfirmed, and direct measures indicate no magnetic field, the disagreement a bit strange for such a bright member of a prominent constellation.
Written by Jim Kaler 1/28/11. Return to STARS.