ZIBAL (ZETA ERIDANI). One of the longest constellations in the
sky, Eridanus (the River) winds from
bright-third-magnitude Cursa (Beta Eri)
near Rigel in southwestern Orion to the
west, then south, then east, then west again, then finally plunging
back south, where it ends at zero-magnitude Achernar. In the middle of the first
western flow, right between Epsilon and
Eta (Azha), lies faint fifth magnitude (4.80) Zibal, now (but not
always) Zeta Eridani. A set of dim stars between Achernar and Fomalhaut (including Phoenix) were given the Arabic name "al-ri'al," "the
young of the ostriches," which was in more modern times misread as
"al zibal, which went to "Zibal," and later was mis-applied to Zeta
Eri, which is way out of the original area. If nothing else, the
star demonstrates the tricky problems associated with proper star names.
While Zibal may appear to be modest in brightness relative to much
of the rest of the River, it steps forward as one of the chemical
curiosities of the stellar population. Listed as a white class A
(A5) star (that is almost assuredly a hydrogen-fusing dwarf,
confirmed below) with a temperature of 7660 Kelvin, the class
carries the suffix "m" (making it A5m), revealing it to be a
"metallic line star," albeit a modest one. In such stars the
abundances of some of the different "metals" (in the jargon of
astronomy any element other than hydrogen and helium), including
those of the "rare-earths" such as europium, are high compared to
the solar mix, while others like calcium are down (the iron content
here more or less normal or a bit high). Zeta Eri's class then
depends on the criteria used, and runs from A4 to A9. The cause is
chemical diffusion, some elements falling under the action of
gravity, others lofted up through absorption of stellar radiation.
From temperature and a very well-determined distance of 110 light
years (plus or minus just 1!), we find a luminosity of 10.5 times
that of the Sun, from which we calculate a
radius 1.8 times solar. A projected equatorial rotation speed of
72 kilometers per second gives a rotation period of under 2.5 days,
a tenth that of the Sun, consistent with an atmosphere not all that
greatly stirred up and conducive to the star being in the "modest
metallic" class. Application of stellar structure theory then
yields a mass of 1.7 Suns and an age of 875 million years, Zeta Eri
clearly a dwarf about halfway through its projected hydrogen-fusing
lifetime, after which it will swell and brighten as a giant with dead helium core. Two
other features make the star stand out. Not alone, our Zibal has
a
spectroscopically-detected stellar companion that orbits every
17.922 days at (if, as expected, of low mass) an average separation
of 0.16 Astronomical Units, a modest eccentricity taking it between
0.14 and 0.18 AU. The binary is also set into a large cold (86
Kelvin) disk of infrared-
radiating dust some 40 AU out, a feature common among class A "Vega" type stars. Whether any planets are
embedded, as one is in Fomalhaut's disk, is not known. (Thanks to
Kunitzsch and Smart, who elucidated Zibal's naming problem.)
Written by Jim Kaler 1/13/12. Return to STARS.