IOTA LEP (Iota Leporis) which, along with RX LEPORIS, gives us
a two-for-one special. Echoes from the nineteenth century call
Iota Leporus, in Lepus, the Hare, "A
fine and delicate double star
in the Hare's left ear" (Smythe and Chambers) with the brighter
fourth magnitude (4.45) star given as "white," the fainter 11th
magnitude (10.8) component contrasting as "pale violet." (RX is
not part of this system and is taken up below.) The color is about
right for the main member, Iota A, a class B8 dwarf. That for Iota
B, though, is a result of its proximity (just 12 seconds of arc
away) to A and its faintness; in reality it is a yellow-white class
G8 or G9 dwarf. From a distance of 240 light years, Iota A shines
with a luminosity of 160 Suns from a hot,
12,900 Kelvin, surface, from which we deduce a radius of 2.5 times
solar. Spinning rapidly with an equatorial velocity of at least
190 kilometers per second, the star rotates in under 16 hours,
rather typical of the class. Luminosity and temperature together
with the theory of stellar structure and evolution give us a
substantial mass of 3.5 times solar and tell that the star is very
young. Indeed, it is classed as a very youthful "post T-Tauri
star." (T Tauri and its kind are "protostars" that are still in
the act of formation, are still
accreting matter from their surrounding disks, which will most
likely spawn planets, and have yet to turn on their hydrogen
fusion; see the region of T Tauri on the Anticenter page.) As a G8 dwarf, Iota Lep B,
just as young, is of far lower mass. From its temperature of 5240
Kelvin (540 K cooler than the Sun) and luminosity of just 40
percent solar, we find a mass of 90 percent solar. The system
presents a bit of a mystery by the presence of an additional
companion to Iota A (called Aa) that was measured to have a
separation from Iota A of only 0.4 seconds of arc. Nothing else is
known about it. From the first measures in 1783, there has been so
little movement between Iota A and B that no orbital path can be
constructed. They are separated by at least 890 Astronomical
Units, which (assuming Aa has a solar mass) gives an orbital period
of at least 11,500 years. (Iota Aa would then be perhaps 30 AU
from Iota A and take some 75 years to make a turn.) With a
hydrogen-fusing lifetime of just 250 million years, Iota A will
quickly turn into a 0.75 solar mass white dwarf; weakly glowing
Iota B will outlast it by 16 billion years.
RX LEP: Once it is admired for its duplicity, Iota Leporis then
provides a fine gateway to RX
Leporis, a sixth magnitude variable
star that lies just a quarter of a degree to the west. It's a
cool class M6 giant that varies irregularly (with approximate
periods of 60 and 90 days) between magnitudes 5.0 and 7.0. Iota
provides a nice comparison with which to watch the action. At a
distance of 450 light years, RX clearly has no physical relation
with Iota. With a temperature of around 3300 Kelvin, the star
radiates at a rate of somewhere between 1500 and 4500 solar
luminosities (depending on the estimate of the amount of infrared
radiation). The radius must then be between 0.5 and 1 AU (the size
of Earth;s orbit). With a mass between 1 and 4 or so solar, RX is
most likely brightening as an advanced giant with a dead
carbon-oxygen core and does not have much time left to it. Losing
its outer envelope through winds (at a rate of a tenth of a
millionth of a solar mass per year, some 10 million times the flow
rate of the solar wind), the star is already surrounded by a shell
of its own making as it prepares to turn itself into a white dwarf.
Iota Leporis A will someday do the same thing.
Written by Jim Kaler 2/13/09. Return to STARS.