FF AQL (FF Aquilae). Aquila is known for many things, among them a beautiful part of the Milky Way and the first-known Cepheid variable, Eta Aquilae, discovered by Edward Pigott in 1786. If things were fair, Cepheids (class F and G supergiants that typically vary by a magnitude or so over periods of several days) should be called Aquilids instead of being named after the second to be found, Delta Cephei (by John Goodricke shortly thereafter). But things aren't fair and the variables remain "Cepheids," which are critically important to the establishment of the cosmic distance ladder and the structure of the Universe. A Cepheid's variation period is strongly linked to its luminosity, or absolute magnitude, the magnitude a star would have at a standard distance of 32.6 light years. Comparison of absolute and apparent magnitudes then gives the precious distance of the Cepheid and of the assembly to which it belongs. The relation was discovered in 1912 at Harvard by Henrietta Leavitt. While fourth magnitude Eta Aql, eight degrees south of Altair, dominates Aquila's Cepheid scene, it's not the only one visible to the naked eye. In the far northwestern corner of the constellation lies the fifth magnitude (averaging 5.38) class F (nominally F8) supergiant FF Aquilae, known best by its two-letter variable-star name. The star's a bit of a curiosity as it varies by only a few tenths of a magnitude (5.20 to 5.55, the class going from F5 to G0) over a period of 4.471 days (increasing at a rate of 0.07 seconds per year), the size of the variation notably less than usual. Perhaps that's why it took so long to find its variability, the name way down the list in discovery order within the constellation. While most Cepheids have light curves (plots of magnitude vs. time) characterized by a rapid rise in brightness followed by a longer fall, FF's curve is more sinusoidal, making it into an "s-Cepheid" that does not fit the usual period-luminosity relation.

One of the serious problems in the use of Cepheids to get distances, or to calibrate the period-luminosity relation using Cepheids of known distance, is the contamination of their light from companions or stars in the line of sight. FF Aql has three of them, a visual ninth magnitude neighbor at a separation of 6.8 seconds of arc, a sixth magnitude companion at around 0.2 seconds, and a spectroscopic one (ninth magnitude?) with a period of 3.93 years. After removing their light and correcting for 0.73 magnitudes of dimming by interstellar dust, FF Aql's visual magnitude is actually 4.78. Two disparate distances, 1545 light years from Hipparcos satellite parallax and 1160 from Hubble's Fine Guidance Sensor, give the adopted distance of 1355 light years. A temperature of 6195 Kelvin indicates little correction for infrared or ultraviolet. The star's mean luminosity then sits right at 1600 Suns, which together with temperature yields a radius of 35 times solar. A direct measure of angular radius shows 39 times solar, not a bad match given the uncertainty in distance and the variable radius of the pulsating star. Application of theory yields a mass of six Suns. The removal of the contamination reveals the asymmetry of a classical, non-s-Cepheid. The standard period luminosity relation is then applicable and yields an absolute magnitude of -3.25, which with the corrected apparent visual magnitude shows the star to be 1315 light years away, very close to the above average and a highly satisfactory result. Meanwhile, back at the system, the three companions from inside out should be close to dwarf classes M2, K2, and M1 with masses of 0.8 solar for the K dwarf and 0.5 for the M stars. We might expect a separation of 5 AU from FF proper for the spectroscopic companion, at least 83 AU and a period of 280 years for the next one, and at least 2800 AU and 120,000 years for the visual neighbor. This star is relatively nearby. Just think of the contamination problems facing the astronomer trying to get a distance from a Cepheid in another, far more distant, galaxy. (Thanks to D. G. Turner et al. for discussion of FF Aql in a paper that appeared in Astrophysical Journal Letters, 772:L10, 2013.) 1

Written byJim Kaler 10/24/14. Return to STARS.