The Pulsation Mode and Distance of the Cepheid FF Aquilae

The determination of pulsation mode and distance for field Cepheids is a complicated problem best resolved by a luminosity estimate. For illustration a technique based on spectroscopic luminosity discrimination is applied to the 4.47 day s-Cepheid FF Aql. Line ratios in high dispersion spectra of the variable yield values of MV = –3.40 ± 0.02 s.e. (±0.04 s.d.), average effective temperature T eff = 6195 ± 24 K, and intrinsic color (B – V)0 = +0.506 ± 0.007, corresponding to a reddening of E B – V = 0.25 ± 0.01, or E B – V (B0) = 0.26 ± 0.01. The skewed light curve, intrinsic color, and luminosity of FF Aql are consistent with fundamental mode pulsation for a small-amplitude classical Cepheid on the blue side of the instability strip, not a sinusoidal pulsator. A distance of 413 ± 14 pc is estimated from the Cepheid's angular diameter in conjunction with a mean radius of R = 39.0 ± 0.7 R ☉ inferred from its luminosity and effective temperature. The dust extinction toward FF Aql is described by a ratio of total-to-selective extinction of RV = AV /E(B – V) = 3.16 ± 0.34 according to the star's apparent distance modulus.