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.