THE SPECTROSCOPIC BINARY ETA-ANDROMEDAE - DETERMINATION OF THE ORBIT BY OPTICAL INTERFEROMETRY

We have determined the following orbital elements of the double-lined spectroscopic binary eta Andromedae (combined spectral class G8 IIIb) using measurements with the Mark III long baseline optical interferometer on Mt. Wilson: Period P = 115.72 +/- 0.02d, major axis a = 10.37 +/- 0.03 milliarcseconds, eccentricity e = 0.006 +/- 0.002, epoch of periastron T = JD 2448013 +/- 2, inclination i = 30.5-degrees +/- 0.4-degrees, argument of ascending node OMEGA = 69.4-degrees +/- 0.6-degrees, and argument of periastron omega = 215-degrees +/- 7-degrees. The magnitude differences of the two components measured at lambdalambda 800 nm, 550 nm, and 500 nm are 0.50m +/- 0.06m, 0.55m +/- 0.08m, and 0.56m +/- 0.09m, respectively. Using orbital elements from spectroscopy by Gordon (1946) we derive component masses of (2.59 +/- 0.30) M. and (2.34 +/- 0.22) M., and a distance of (76 +/-2)pc. Monte-Carlo simulations have been performed to estimate parameter uncertainties. We discuss the evolutionary state of eta Andromedae.