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.)
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Written byJim Kaler 10/24/14.
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