PI BOO (Pi Bootis). From the heart of the nineteenth century, Smythe and Chambers tell us that Pi Bootis (in Bootes, the Herdsman, not quite eight degrees east-southeast of Arcturus) is "A neat double star...both white" and that "This is a fine object." Together the two make a barely fifth magnitude (4.53) seemingly-single star that breaks into a mid-fifth magnitude (4.94) cool-side class B (B9) hydrogen-fusing dwarf set just 5.5 seconds of arc from a fainter sixth magnitude (5.79) cooler class A (A6) dwarf that together make a striking pair, the western and brighter called Pi-1, the fainter Pi-2. First observed by Herschel, the duo has drawn together by some two seconds of arc over the past two centuries, though the motion has not been enough to enable calculation of an orbit. Just over two minutes of arc away is an 11th magnitude third member, Pi Boo C (Pi-1 and Pi-2 also called Pi A and B) that seemingly belongs to the system and that, given a distance of 309 light years (give or take 25), would be a borderline G-K dwarf. Observations that go back to the 1920s suggest that Pi-1 may have a spectroscopic companion (about which nothing is known, not even the orbital period), making the whole affair quadruple. Pi-1's temperature (averaged from wildly scattered measures) comes in at 12,400 Kelvin, making it more like a B8 dwarf, not surprising given an odd chemical composition (see below), while Pi-2's is estimated at 8100 K. From these and distance we respectively find luminosities of 151 and 34 times that of the Sun and radii of 2.7 and 3.0 solar. With a projected equatorial rotation speed of just 24 kilometers per second, Pi-1 takes under 5.6 days to make a full rotation, while speedier Pi-2's numbers are 140 km/s and under 1.1 days, which is consistent with Pi-1 having a small companion that through tidal interactions has slowed it down. Theory gives masses of 3.4 and 2.3 Suns for Pi-1 and Pi-2, but seem to show that the latter is older than the former, which suggests that the measured basic parameters are off (supported by Pi-2 being found larger than Pi-1). The rapid rotation of P-2 may play a role by distorting the star. From the current separation, Pi-1 and 2 must be at least 500 Astronomical units apart, which from the masses and Kepler's Laws give an orbital period of at least 5000 years. Much more distant Pi C would be at least 12,000 AU from the inner pair and take at least half a million years to make a full circuit. From Pi-1, Pi-2 would glow with the light of perhaps 60 full Moons, while from Pi-2, Pi-1 would twice as bright, whereas from these, Pi Boo C would appear little brighter than our Venus. From C's perspective, each of the A-B pair would glow as bright as our gibbous Moon, the two separated by at most a couple degrees.

What really makes the system special is that Pi-1 is a fine example of a "mercury-manganese star," in which, while iron is somewhat depleted (two-thirds solar compared with hydrogen), the element mercury is enhanced by a factor of 350,000! Such weirdness is caused by diffusion, wherein in quiet, unstirred stellar atmospheres (the result of slow rotation) some elements settle downward under the force of gravity, while others are lofted upward by the intense radiation, the odd chemistry making them hard to classify. Consistently, faster-spinning Pi-2 Boo seems normal, allowing us to speculate that the difference is due to Pi-1 having a (very uncertain) companion. (Thanks to Tim Benedictis, who suggested this star.)
Written by Jim Kaler 4/22/11. Return to STARS.