17 COM (17 Comae Berenices). Separated by 145 seconds of arc, fifth
magnitude (5.29) 17 Com A (in Coma
Berenices, Berenices Hair, south of the Big Dipper's handle) and seventh magnitude
(6.65, almost sixth) 17 Com B are usually considered to be an easy
"binocular" double. Merged
together, they shine at magnitude 5.01. Flamsteed 17 sometimes refers
to "A" alone, leaving "B" as HR 4751 from the Bright Star Catalogue
("A" being HR 4752). Both are given as class A (A0p) "peculiar" dwarfs, though 17 Com B is also
listed as A2 to F2. The duplicity rests on their common motions both
across and along the line of sight. The Hipparcos parallaxes, however, call
out a rather resounding "no," as the distance of 17-A is 238 light
years, while that of 17-B is 223 light years. The likely respective
errors of 4 and 6 light years cannot bring them any closer together
than Alpha Centauri is to us, making
it highly questionable that they constitute a binary system. The
common motion is more likely to be the result of their both being
members of the Coma Berenices open cluster (which is the major
part of the larger modern constellation),
whose stars are moving through space together while slowly orbiting
some vague common center of mass and also interacting with one
another. With a distance to the cluster of around 280-300 light
years, 17 A and B are on the near side of it. We'll therefore consider
them as separate stars that share a common name. Even if the two
were at the same distance, they would be separated by more than 10,000
Astronomical Units which, in the environment of a cluster, seems a
bit much for the pair to be any kind of stable orbiting binary. One
interesting possibility is that they WERE a pair that has been
separated. About 400 million years old, the cluster as a whole is
falling apart, its stars gradually escaping its weak gravitational
grip.
It's no surprise that brighter 17 A is by far the more studied. The
"p" suffix to the class makes it a "magnetic" double of the "Alpha-2
Canum Venaticorum" class (Cor
Caroli, not far away), the star's field measured at around 1000
times that of Earth. In the similar metallic stars, some chemical
elements in their outermost layers sink under the force of gravity,
while others are elevated by radiation. The result is a wacky, very
non-solar chemical mix. The magnetic stars add tilted magnetic
fields that concentrate some of the elements into "starspots" that,
as the stars rotate, swing in and out of view. As a result, 17-A
varies between magnitudes 5.27 and 5.40 over a 5.08 day rotation
period, with chromium, europium, and strontium strongly enhanced.
With a poorly-known temperature of 10,200 Kelvin, the star shines
with the light of 42 Suns, which in turn gives
a radius of 2.1 times solar. A projected equatorial rotation
velocity of 18 kilometers per second gives a rotation period of less
than 5.82 days, which with the true value shows the rotation axis
to be inclined to the plane of the sky by 60 degrees. Application
of theory yields a mass of 2.4 Suns and solidly confirms "dwarfhood."
The temperature of lesser 17 Comae B is around 7700 Kelvin, consistent
with a class of A3, which we'll adopt: classes of these peculiar stars
are compromised by the odd abundances (that of 17-A compromised too).
The star radiates at a rate of 8 Suns, its radius 1.6 times solar.
A rotation speed of at least 26 km/s gives a rotation period of under
3.1 days, theory giving a mass of 1.7 Suns. It too is a peculiar
magnetic dwarf, with a field around 750 times the strength of Earth's.
While "A" and "B" are unlikely to be a binary, "B" itself is triple! It has a spectroscopic companion in a 68.3
day eccentric orbit, which if of low mass would orbit at an average
distance of 0.4 AU. Then a 13th magnitude companion hovers at just
under two seconds of arc that, if really bound to "B," from its
brightness is red M2 dwarf at least 130 AU away and that takes more
than 1000 years to go all the way around
Written byJim Kaler 7/25/14. Return to STARS.