The Pulsation Mode and Distance of the Cepheid FF Aquilae

TL;DR

This paper uses spectroscopic luminosity discrimination to determine that FF Aquilae is a fundamental mode Cepheid with a distance of approximately 413 parsecs, refining its pulsation mode and luminosity estimates.

Contribution

It introduces a spectroscopic technique for accurately determining pulsation mode and distance of field Cepheids, applied here to FF Aquilae.

Findings

FF Aql pulsates in the fundamental mode.

Distance to FF Aql is estimated at 413 parsecs.

The star's properties are consistent with a small amplitude classical Cepheid.

Abstract

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 4.47d 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 Teff=6195+-24 K, and intrinsic color (<B>-<V>)o = +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 Rsun inferred from its luminosity and effective temperature. The dust extinction towards 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.