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.