ZETA AND (Zeta Andromedae). Named after the eighth magnitude variable
star RS Canum Venaticorum (in Canes
Venatici), RS CVn stars are doubles in which a cool giant is spinning much faster than
expected, the result of tides raised by
a smaller companion that were strong enough to force synchronous
rotation with the orbital period (like the Earth did to the Moon, one
side of which perpetually faces us). The rapid spin then produces
sunlike magnetic activity not usually seen in such giants. It's a
surprise not only to find two relatively bright ones within a constellation, Zeta and Lambda Andromedae, but also to have both
named with Greek letters. Ten
degrees southeast of Alpheratz (Alpha
And), fourth magnitude (4.06) Zeta Andromedae is the southernmost
Greek-lettered star in Andromeda,
while Lambda is almost the
northernmost, just losing to Psi Andromedae by 8 minutes of arc. Zeta
And was originally thought to be an eclipsing binary that varies
between magnitudes 4.06 and 4.20 with a period of 17.77 days, but it's
now seen as an ellipsoidal variable that changes brightness because
it continually presents a changing face to us as a result of rotational
and tidal distortion, the effect modulated by starspots. As expected,
the spectroscopic period closely matches. The primary star is a class
K (K1) giant 189 light years away (give or take 3). After allowance
for a bit of infrared radiation
from a 4870 Kelvin surface, the star is seen to shine at a rate 89.5
times that of the Sun, which in turn implies
a radius of 13.3 times solar. Direct measure of angular diameter gives
15.9 solar radii, which is probably not bad agreement for a distorted
star. Given the synchronous rotation, the period and radius yield an
equatorial rotational speed of 37.6 kilometers per second, close to
an old measured value of 40, the agreement showing that the rotation
axis is nearly vertical to the line of sight and consistent with the
ellipsoidal variation. Such a speed, quite high for a giant, produces
the above-noted magnetic activity that results in emission lines and X-rays similar to those radiated
by the Sun. Theory gives a mass of between
2.75 and 3.0 Suns, the star probably fusing core helium into carbon
and oxygen, as is the case for most K giants. The companion seems to
be a class K (or maybe cool G) dwarf with a mass of perhaps 0.8 Suns.
Kepler's laws then give it an
orbital radius of 0.20 AU, 43 solar radii, or just three times the
radius of the giant. The two are close to being in contact, and thus
affect each other mightily. When the giant later expands with a dead
carbon core it may well swallow the little one. It's hard to know as
there are so many variables. Prominences (in the Sun, cool gases that
float in the hot corona above sunspot zones) and even an activity cycle
might be present. Zeta Andromedae is attended by three faint
companions. At respective separations of 37 and 156 seconds of arc,
15th magnitude Zeta And B and 11th magnitude D are, from their rapid
motions, clearly line of sight coincidences. Zeta-C, however, 97
seconds away, seems to be tracking bright Zeta-A rather well. If it
belongs, it's a red dwarf with an orbital radius of at least 5600 AU
and a period of more than a quarter million years. If nothing else,
it's got a great show to watch from a safe distance as the inner two
whirl around each other.
Written byJim Kaler 12/04/15. Return to STARS.