Epsilon Indi, at just fifth magnitude (4.69), is the easternmost bright star in the modern southern constellation of Indus, the Indian. A class K4.5 V dwarf, ε Ind is one of the intrinsically least luminous stars visible to the unaided eye. The star can be seen without a telescope only because it is close to us, merely 11.8 light years away, making it the 17th nearest star system.
Properties
With a cool temperature of 4620 K, Epsilon Indi shines with a luminosity of just 22% that of the Sun; its radius is 75% solar. This low wattage is the result of a low mass, about 70% solar. While the star's metal content is close to solar, the velocity relative to the Sun is high, the two stars moving past each other at 90 km/sec, five times normal. Like the Sun, the star is magnetically active with a chromosphere and X-ray-radiating corona heated to roughly 5 million K. From associated rotational variations we find a sunlike rotation period of 23 days, consistent with the star's measured rotation speed of 0.7 km/sec and an axial tilt against the plane of the sky of 26 degrees.
Companions
Epsilon Indi's claim to fame is that it hosts a tightly orbiting pair of brown dwarfs that together are called ε Ind B. It is the nearest brown dwarf binary to the Earth, making the star a triple system. Brown dwarfs are "substars" below about 0.075 solar masses that are so undermassive that they cannot run full hydrogen-to-helium fusion. Such "stars," the coolest known, with temperatures around 1000 K, are characterized by methane in their spectra. Of new classes L and T, they are of immense importance as they are transition bodies between stars and planets; planets and brown dwarfs quite likely overlap, perhaps formed in different ways.
ε Ind B lies a good distance from ε Ind A: 6.7', or at least 1460 AU. The two brown dwarfs themselves are separated by 0.73", or at least 2.65 AU. Both are spectral class T, too cool to radiate in the visual spectrum, and therefore have no visual magnitudes. The more massive, ε Ind Ba (class T1), has a temperature of 1276 K, a total luminosity of 0.000019 suns, a mass 47 times that of Jupiter (0.045 solar masses), and a radius 0.091 times solar (about the size of Jupiter). The lesser body, ε Ind Bb (class T6) is about 854 degrees K, 0.0000045 solar luminosities, 28 Jupiter masses (0.027 solar masses), and 0.096 solar radii. The two brown dwarfs orbit each other with a period of at least 16 years, while the pair orbits ε Ind A with a period of at least 63,000 years.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]