THE MASS OF THE CLASSICAL CEPHEID SU CYGNI

Velocities of the companion of the classical Cepheid SU Cyg have been measured from 13 IUE high-dispersion spectra. Differential velocities of the companion with respect to the Cepheid for the long-wavelength spectra have an uncertainty smaller than 3.0 km s-1. Absolute velocities from the long-wavelength spectra have a standard deviation of 3.5 km s-1. Because the companion is itself a member of a short-period binary system, its short-period orbit and the orbital amplitude of the center of mass in the long-period system are determined from a triple system solution. The velocity amplitude of the short-period center of mass in the long-period orbit is 32.2 ± 1.6 km s-1. This leads to a mass ratio between the mass of the Cepheid and the sum of the masses of the companions of 1.07 ± 0.05. Combining this mass ratio with the mass function from the Cepheid orbit produces a purely dynamical lower limit to the mass of the Cepheid of 5.9 ± 0.4 M⊙. A preliminary upper limit to the mass from the total mass of the short-period system is 6.2 M⊙. The lower limit to the mass is in good agreement with the evolutionary mass for the Cepheid without mass loss or convective overshoot near the main sequence. This constrains the amount convective overshoot during the main-sequence phase. Pulsation masses from the distance scales published by Caldwell in 1983 and Schmidt in 1984 are both more than 3 σ smaller than the lower limit.