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.