Download KOI-54 Claude Plymate There is a star system about 45 light years

KOI-54
Claude Plymate
There is a star system about 45 light years away in the constellation Cygnus. The system we
know as HD 187091 (also known as KOI-54 for Kepler Object of Interest 54) is an
undistinguished 8th magnitude A star or was before the Kepler telescope took a close look. As it
turns out, the system is anything but typical or uninteresting. KOI-54 has been found to be a
close binary system, consisting of nearly twin A stars in highly eccentric (e = 0.83) 41.8 day
orbits about their mutual center of mass. These main sequence stars reside right at the bottom
of the instability strip on the H-R diagram.
The instability strip is a nearly vertical region in the H-R diagram. Stars that fall within the
instability strip tend to vary in brightness by physically pulsating. The best known example are
the Cepheid variables. Cepheids are giant stars that pulsate with periods of days to months and
can vary in brightness by more than a magnitude. In 1908, it was discovered that the brightness
variation and period of cepheids are tightly correlated. By measuring the period and luminosity
variation, the distance to a cepheid can be easily calculated. Hubble (Edwin, not the telescope)
used this period-luminosity relationship to determine the distance to galaxies in which he could
resolve individual cepheids. Those measurements were central to Hubble establishing the
expansion of our universe.
The instability strip crosses the main sequence in the A to F stellar class (roughly 1 - 2 solar
mass) range. It has been discovered that main sequence stars within the instability strip are
able to pulsate as well. Main sequence pulsations tend to be much weaker - typically tenths of a
magnitude - and quicker - a few hours to a day - compared to cephieds. Such pulsators are now
known as Scuti or Doradus stars.
Kepler found that BOTH components in KOI-54 are pulsators. And at least some of their
pulsation modes are synced to their orbital period! The strongest pulsation modes turn out to
be 90 and 91 times their orbital period. Apparently, these modes are natural resonance periods
for the two stars. The stars are nearly but not exactly identical with slightly different resonance
periods. Every time the stars swing by each other (pariastron) they get a kick that excites their
natural oscillation modes. One star happens to be right at 90 and the other 91 times the orbital
period! It appears likely that this pulsation/orbit period syncing is having the additional effect of
locking the binary system into its strangely elliptical orbit. Normally, tidal effects on close binary
companions are thought to circularize the orbits rather quickly (10 6 - 107 yrs). The exotic dance
KOI-54 is engaged in seems to be stabilizing and maintaining itself. This universe in which we
find ourselves is a truly amazing and elegant place.