GAMMA EQU (Gamma Equulei). Dim, fifth magnitude (4.69), in the
tiny, obscure constellation (smallest of
the ancients) of Equuleus (the Little
Horse), Gamma Equulei remains one of the more interesting of its
breed as a class F (F0, some say A9) "peculiar" hydrogen-fusing
dwarf. In a faint part of the sky to the southwest of Pegasus, Gamma Equ (also known as 5 Equ),
the star at first appears as a naked-eye double paired with sixth
magnitude 6 Equulei, which lies just 6 minutes of arc away. Though
6 Equ is also known as "Gamma Equ D," the relation is clearly just
"line of sight." Gamma is 118 light years away (plus or minus 2),
while 6 Equ is an ordinary A2 dwarf almost four times farther.
With a modest temperature of 7715 Kelvin, Gamma Equ shines with the
light of 13.4 Suns, giving it a radius of
2.1 times the solar value. The theory of stellar structure and
evolution then reveals it to have a mass of 1.8 solar, and shows it
to be about halfway through its 1.5-billion-year hydrogen-fusing
lifetime. Gamma, however, is best known as an "Alpha-2 Canum Venaticorum star," one with
peculiar chemical abundances that are enhanced in intense magnetic
spots. The observed magnetic field of some 150,000 times that of
Earth varies a bit as the spots rotate in and out of sight, showing
an amazingly long rotation period of 72 years, by far the long-
period record. Such a period is not at all in accord with a listed
rotation speed of 8 kilometers per second (implying under half a
month), which must then be wrong. The chemical anomalies are not
trivial. Strontium, the rare earth Europium, even iron, are up
over solar (compared to hydrogen) by factors of 50 or so. Even
though a class F dwarf, such stars are still generically called
"Ap" stars ("p" for "peculiar"). Gamma Equ goes one better as a
"rapidly oscillating Ap" or "roAp" star. Among the first half-
dozen of the breed to be found, it varies by under a thousandth of
a magnitude with multiple periods of around 12 minutes, the chief
one 12.21, which from theory give a stellar mass of 1.8 solar,
exactly that found from luminosity and temperature! While not
paired with 6 Equ, Gamma does indeed seem to have a binary
companion, of 9th magnitude set 2.5 seconds of arc away. If real,
it would have to be a G9-K0 dwarf with an orbital radius of at
least 54 Astronomical Units and a period of at least 250 years (and
probably a lot bigger and a lot longer). Another line-of-sighter,
12th magnitude Gamma-C, hovers a minute of arc away. In the 19th
century, the closer pair was called "exquisite," proximity and
magnitude difference making the brighter appear yellow, the fainter
blue, rather than the more true white and yellow-orange.
Written by Jim Kaler 11/19/10. Return to STARS.