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.