EPS IND (Epsilon Indi). Most stars that make our constellations are intrinsically luminous, that is, in terms of power output they are far brighter than the Sun. They have to be luminous to appear so bright in our skies and to be seen over such great distances. Naked-eye stars intrinsically fainter than the Sun are rare, the 61 Cygni pair and Epsilon Eridani fine examples. Among this set is Epsilon Indi, at just-fifth magnitude (4.69) the easternmost reasonably bright star in the modern southern constellation of Indus, the Indian, where it lies close to the border with Tucana. A class K (K4.5) dwarf, Eps Ind is one of the intrinsically least-luminous stars visible to the unaided eye. It also harbors a striking, recently revealed, secret (the story told below). The star can be seen without the telescope only because it is close to us, a mere 11.8 light years away, making it the 17th nearest star system (such systems including single stars, doubles, and triples). With a cool temperature of 4620 Kelvin, Eps Ind shines with a luminosity that is just 22 percent that of the Sun, its radius 75 percent solar. This low wattage is the result of a low mass, about 70 percent solar as determined from luminosity, temperature, and theory. 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 kilometers per second, five times normal. (Most such high- speed stars have lower metal abundances, a result of their coming to us from a different part of the Galaxy.) Like the Sun, the star is magnetically active with a chromosphere and X-ray-radiating corona heated to roughly 5 million Kelvin. From associated rotational variations we find a sunlike rotation period of 23 days, consistent with the star's measured rotation speed of 0.7 kilometers per second and an axial tilt against the plane of the sky of 26 degrees. Now to the real story. Epsilon Indi's claim to fame is that it hosts a tightly orbiting pair of brown dwarfs that together are called Epsilon Indi B. It is the nearest brown dwarf binary to the Earth, making the star a triple system. Brown dwarfs are "substars" below about 0.073 solar masses that are so undermassive that they cannot run full hydrogen-to-helium fusion. Such "stars," the coolest known, with temperatures around 1000 Kelvin, 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 (perhaps formed in different ways) quite likely overlapping. Eps Indi B lies a good distance from Epsilon Indi A, 6.7 minutes of arc, or at least 1460 Astronomical Units, while the two brown dwarfs are separated by a mere 0.73 seconds of arc (at least 2.65 AU). Both spectral class T, they are too cool to radiate in the visual spectrum, and therefore have no visual magnitudes. The more massive, Epsilon Indi Ba (class T1), has a temperature of 1276 Kelvin, a total luminosity of 0.000019 times that of the Sun, a mass 47 times that of Jupiter (0.045 solar), and a radius 0.091 solar (10 times that of Earth and about the size of Jupiter). The lesser body, Epsilon Indi Bb (class T6), respectively comes in at 854 Kelvin (1080 degrees F), 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 Epsilon A with a period of at least 63,000 years. Astronomers will be watching the brown dwarf binary very closely to map out the bodies' mutual orbits. Application of gravitational theory to learn their true masses will then lead to better understanding of how such substars relate to more massive stars like our Sun and to stars -- like Epsilon Eridani -- that harbor what appear to be true planets. (Brown dwarf data from M. Audard et al. in Astronomy and Astrophysics.)
Written by Jim Kaler. Return to STARS.