OMI AQL (Omicron Aquilae). Within the Milky
Way, fifth magnitude (5.11) Omicron Aquilae sits in Aquila 1.5 degrees almost due north of
first magnitude Altair, making it quite
easy to find. A class F (F8) dwarf, Omi Aql is almost
sunlike, though with some notable exceptions, one of major
importance. The distance of 62.6 light years is known to remarkable
accuracy, within just 0.4 light years. Though in the Galactic disk, the star's
closeness precludes significant dimming by interstellar dust, and a
well-determined temperature of 6125 Kelvin places almost all the
radiation in the visible band of the electromagnetic spectrum.
From these considerations, we get a total luminosity of 2.73 times
that of the Sun and a radius of 1.47 times
solar. Oddly, there is no measure of rotation speed. Theory then
gives a mass of 1.2 to 1.25 Suns. With an age of 3 billion years,
the star has less than a billion to go before it gives up core hydrogen
fusion and turns into a subgiant on its way to gianthood.
There's nothing much unusual here, so why bother with it? First,
Omi Aql is not metal-poor (a common condition), but metal-rich, with
an iron-to-hydrogen ratio 25 percent greater than the Sun's,
suggesting that it came from somewhere in the Galactic interior where
more metals are available to birth clouds thanks to a denser stellar
population and more supernovae. Yet the star's
velocity of 25 kilometers per second is not all that high, just double
normal. Metal-rich stars are breeding grounds for planets, but
Omicron Aquilae does not even seem to have a debris disk around it that would
at least be evidence for a planetary system
(none at this time obviously detected or inferred). It's also
quintuple. OK, not quite. Omi Aquilae C, D, and E, respectively
of 14th, 13th, and 12th magnitudes, are separated from bright "A"
by 22, 50, and 82 seconds of arc. Alas, their motions relative to
Omi Aql A over only short timelines reveal all to be just in the line
of sight and not actual companions. But then there is
Omi Aql B. Now 19.5 seconds away from "A," the separation has changed
by only a second of arc over the past century even though the AB pair
has shifted relative to its surroundings by nearly a full half minute
of arc, some 30 times the stars' separation. Moreover, Omi B's
absolute brightness is consistent with it being classified as an M3
red dwarf, so the two stars are almost certainly partners. If so,
the companion is at least 375 AU away from its much brighter mate,
and would take 6000 years or more to make a full circuit. Assuming
the above distance, from Omicron A, Omi B would appear five times
brighter than our Venus, while from Omi B, Omi A would be would shine
as a point of light glowing with 10 full
Moons. By far the most absorbing feature of Omicron Aquilae is that
in 1979-80 it burst forth with two superflares in which it brightened by
0.09 magnitudes, nearly nine percent! The first flare could have
lasted up to five days, while the second stayed up for nearly two
weeks. Red dwarfs are supposed to behave this way, not those more
or less like the Sun. Omi Aql thus joins an exclusive club that
includes 5 Serpentis and Groombridge 1830 (HR 4550). Reasoning
from ordinary solar and red dwarf flares, imagine what the X-ray intensity must have been.
Imagine too what it would be like if the Sun were to join the group.
Why such stars go off this way is a mystery, as is how frequently
such flares go off. Without a planet, or so it appears anyway, there
is nobody there to report back, which is probably a good thing, as
it sure would be a dangerous place to be.
Written byJim Kaler 8/08/14. Return to STARS.