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.