IOTA ARI (Iota Arietis). The sky is filled with
cool (3800-4800 Kelvin) class K orange, helium-fusing giants, in part the result of
their being generally so bright, their apparent population all out
of proportion to their real number. Remove them from the sky and
our constellation patterns would be
profoundly altered. In contrast, the naked-eye class K hydrogen-
fusing dwarfs are faint enough that they make little impact on the
nightly sky. The best known are probably fifth magnitude 61 Cygni (a double that contained the first
star to have had a distance measure through direct parallax), the
dimmer component of Alpha Centauri,
and 40 Eridani (which has both red dwarf
and white dwarf
companions). It's exciting then to seem to come on to another one,
fifth magnitude (5.10) Iota Arietis, which can be found just under
two degrees southeast of Gamma
Arietis, the southernmost of the trio that forms the flat
triangle that makes most of classical Aries, the zodiacal
Ram. Listed as a "peculiar" class K1 dwarf, it certainly is, as
measure of its distance of 520 light years, and subsequent
calculation of its absolute brightness, clearly reveal a case of
mistaken identity. Using a poorly-defined temperature of 4630
Kelvin (to estimate the amount of infrared radiation), the
luminosity comes in at 294 times that of the Sun, the radius at 27 solar, clearly making the
star a true giant (even though the color is still more that of a
dwarf). A slow projected equatorial rotation speed of 3.3
kilometers per second gives a rotation period as long as 400 days.
Theory then also shows the star to carry a mass of 3.5 to 4 Suns,
depending on the exact state of evolution. But the
misclassification, or at best the oddness, carries along through
the literature, and even gets more bizarre, one source making the
star out to be a class G supergiant, which it is not. The star
does have going for it that it is a spectroscopic double with a period
of 1567.7 days, 4.29 years. Another source gives a one magnitude
difference between the stars. If so, Iota Ari A's luminosity goes
down to 210 Suns, the mass to 3 solar), and the companion is a
class A dwarf with a luminosity of 113 Suns and a mass of 2.8
solar. The magnitude difference is probably much more however,
voiding the interpretation. Given a minimum mass for the
companion, it is at least 4.8 AU from Iota-A. Little else is known
except that Iota Ari proper has only about half the metals found in
the Sun (iron used as a general proxy), but that conclusion might
be voided by the poorly-known temperature. The star begs to be re-
observed, if only to understand the reason for the mis-
identifications, which might prove interesting.
Written by Jim Kaler 9/30/11. Return to STARS.