The pulsation mode and distance of the Cepheid FF Aquilae

Abstract

<p>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 &lt;<em>M</em>_V&gt; = &minus;3.40 &plusmn; 0.02 s.e. (&plusmn;0.04 s.d.), average effective temperature <em>T</em>_eff = 6195 &plusmn; 24 K, and intrinsic color (&lt;<em>B</em>&gt;&minus;&lt;<em>V</em>&gt;)₀ = +0.506&plusmn;0.007, corresponding to a reddening of E_B&minus;V = 0.25 &plusmn; 0.01, or E_B&minus;V (B0) = 0.26 &plusmn; 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 &plusmn; 14 pc is estimated from the Cepheid&rsquo;s angular diameter in conjunction with a mean radius of &lt;<em>R</em>&gt; = 39.0 &plusmn; 0.7 R_☉&nbsp;inferred from its luminosity and effective temperature. The dust extinction toward FF Aql is described by a ratio of total-to-selective extinction of R_V = A_V /E(B &minus; V) = 3.16 &plusmn; 0.34 according to the star's apparent distance modulus.</p>