CU VIR (CU Virginis). How can we turn down a star with the name "CU"
Virginis? Sadly, "CU" does not honor Champaign-Urbana, home of the
University of Illinois (and of
HAL in the seminal movie "2001: A Space Odyssey"), but is a variable star name, the fifth
magnitude (nominally 5.01) star in eastern Virgo changing between 4.98 and 5.05 every
0.52 days. It is, however, aside from catalogue names, all we have:
not only does CU Vir not have a Greek
letter (understandable), but even Flamsteed ignored it in spite
of nearby 92 and 106 Vir at magnitudes 5.91 and 5.42 being
considerably fainter. CU Virginis has been described as "one of the
most enigmatic stars on the upper main (dwarf)
sequence." A class A (A0p) hydrogen-fusing dwarf (the "p" standing for
"peculiar") 258 light years away (give or take 5), CU Vir is a magnetic
variable with greatly enhanced atmospheric silicon, and falls into
the general class of "Alpha-2 Canum
Venaticorum stars" (which occur among the F, A, and B dwarfs).
They are similar to the "metallic-line stars," in which some elements
are lofted to the surface by radiation, while others sink under the
force of gravity. In stars like Alpha-2 CVn and CU Vir, there is
also a strong magnetic field that concentrates enhanced elements into
large, more or less polar starspots. The magnetic field axis is
inclined to the rotation axis (in CU's case a field more than 6000
times Earth's is tilted by 74 degrees), so as the star rotates, the
spots swing in and out of view and the star varies in brightness.
CU Vir is too blue and, with a temperature of 12,800 Kelvin, too hot and blue
for its A0 class, and B9p is probably more like it. After accounting
for ultraviolet radiation,
it shines with the light of 108 Suns, which
with temperature gives it a radius of 2.1 times that of the Sun.
Theory then shows the star to have a hefty mass of 3.2 times solar
and to fall on or close to the "zero age main sequence," that is it
has just begun its hydrogen-fusing career and over the course of its
280 million year lifetime will slowly brighten, cool, and
expand.
Now things get peculiar indeed. Of this class of magnetic dwarfs,
CU Vir has the shortest known period, just 0.5207 days (a bit under
12.5 hours) The projected equatorial rotation speed of 135
kilometers per second gives a rotation period under 0.79 days, which
with the true rotation period gives an axial tilt against the line
of sight of 41 degrees. Moreover, CU Vir is the only magnetic dwarf
of its kind to emit pulses of radio waves, two per rotation period,
that (like a neutron-star pulsar) are
related not to actual stellar pulsations but to beamed radiation
hitting the Earth as the star spins. The radio emission seems to
be coming from well above the star's surface. Better yet, CU Vir's
rotation period suddenly (though slightly) increased in the 1980s,
the spin rate slowing a fraction of a percent. Nobody knows why.
There is no companion with which to interact and there seems to be
no significant wind that might pull on the magnetic field to slow
the star down. It's not even yet known if the slowdown is continuous
or is a singular "glitch" (such things seen regularly in neutron
stars). For that matter, there is no agreement on where the magnetic
fields of such stars come from in the first place. They may be
primordial, their origins within the clouds of dusty gas from which the
stars were birthed. If any
star deserves better popular recognition, indeed at least a Flamsteed
number, it's this one. (Summaries of properties are given by:
D.M.Pyper et al., Monthly Notices of the Royal Astronomical Society,
vol. 431, p. 2106, 2013; J.Krucka et al., Astronomy and Astrophysics,
537, A14, 2012, K.K.Lo et al., MNRAS, 421, 3346, 2012; quote from
Z.Mikulasek et al., A&A, 534.)
Written byJim Kaler 5/23/14. Return to STARS.