IOTA LEO (Iota Leonis). Leo, the Lion, is such a prominent figure that the stars that fall outside the main outline are rarely recognized. Iota Leonis, of no proper name, deserves better. We tend to be fascinated by Sunlike stars, as we can in a sense see ourselves from a distance and wonder what might be circling them. Even more intriguing perhaps is to see one in a double system. Iota Leonis is a fourth magnitude (3.97) binary made of a fourth magnitude (4.06) white class F (F4) subgiant (but see below) only a couple seconds of arc from a seventh magnitude (6.71) VERY Sunlike yellow-white class G3 dwarf (the Sun G2). In the late nineteenth century, Smythe and Chambers respectively called them "pale yellow and light blue," showing how contrast effects can fool the eye. At a distance of 77 light years (give or take 2), they orbit each other every 186 years at an average separation of 45 Astronomical Units, a rather high eccentricity taking them between 69 and 21 AU apart.
Iota Leo The Sunlike star Iota Leonis B orbits warmer, more massive, Iota Leo A (at the cross) every 186 years at an average separation of 45 Astronomical Units. They are now getting farther apart. In reality they both orbit a common point between them. The scale is in seconds of arc. From the Sixth Catalog of Orbits of Visual Binary Stars , W. I. Hartkopf and B. D. Mason, US Naval Observatory Double Star Catalog, 2006.
The two were last closest in 1948 and will be farthest apart in 2042. Kepler's Laws then give a combined mass of 2.66 times that of the Sun. How does that fit with what we get from luminosity and temperature? Iota Leo A's temperature is a well-determined 6700 Kelvin, while we can only estimate Iota B's temperature of 5650 from its class (which is plenty good enough, since in either case most of the light is visual, not ultraviolet or infrared). The luminosity of the brighter then comes in at 2.2 times that of the Sun, while that of Iota B is right on solar, at 1.0! Radii are then 2.4 and 1.1 times solar. The projected rotation speed of Iota A is measured at 18 kilometers per second, giving a rotation period of under 6.7 days. Theory then gives respective masses of 1.6 and 1.0 (!) times that of the Sun, the sum of 2.6 solar fitting almost perfectly with that derived from the binary orbit, showing that all parameters are well determined. Theory also suggests that Iota A is not quite ready to be a subgiant (with a dead or dying hydrogen-fusing core), though as an old dwarf about 2 billion years old, it is close. There is some indication that Iota A is a "Delta Scuti" type rapid variable with a variation of around 5 percent, but the class is uncertain and not supported by other observations. Off in the great distance, 332 seconds of arc away, is 11th magnitude Iota Leo C. If it is a true companion, which is uncertain, it must be a class M1 red dwarf at least 7900 AU away with an orbital period the order of half a million years. What must it be like to live in such a system, with Iota B as your Sun but with a more luminous companion that turns night into day, and with a possible red dwarf component that shines almost as brightly as our Venus? We would probably wonder in vain, as infrared observations reveal no debris disk that survived the formation of planets. The best guess then is that nobody's home. And that's a bit too bad, because any residents would (after a very long time) be treated with an amazing sight when Iota A's hydrogen-burning core finally expires, sending the star to gianthood. Fortunately for the Sunlike G dwarf, the two are far enough apart that Iota A will do no damage.
Written by Jim Kaler 5/07/10. Return to STARS.