Epsilon Indi B

Epsilon Indi is a nearby star system in the constellation Indus. Two of its three stars belong to Class T, a group which consists of brown dwarfs. Epsilon Indi Bb is a late T star, and Epsilon Indi Ba is an early T star.

Both Epsilon Indi Ba and Epsilon Indi Bb have radii that are 0.08 of a solar radius. With a mass of 0.066 solar mass units, Epsilon Indi Ba is more dense than Epsilon Indi Bb, which has a mass of 0.047. These stars aren’t exactly super gravitationally attractive; their surface gravities are 5.50 and 5.25, respectively, and each is less than Earth’s!

The Epsilon Indi Bs aren’t exactly the hottest stars out there or the brightest. Epsilon Indi Ba has an effective temperature in the 1300-1340 K range, and Epsilon Indi Bb’s is lower still, somewhere between 880 and 940. The luminosities of these stars are  1.9 × 10−5 and 4.5 × 10−6 times that of the Sun, respectively.

It seems that in this star system, Epsilon Indi A is the star of the show; this star has the third highest proper motion of any star visible to the unaided eye. Its friends Epsilon Indi Ba and Epsilon Indi Bb were discovered in 2003, when it was revealed that the two brown dwarfs are binary with an orbital period of approximately 15 years.

This star system is located in the constellation Indus with coordinates RA 22hr03min21sec D -56deg47arcmin09arcsec, and if you look outside, you can see it since it’s visible (though most of the light is from Epsilon Indi A) with an apparent magnitude of 4.831.

Shortly after the Epsilon Indi Bs were discovered, astronomers at La Silla Observatory in Chile captured an infrared spectrum of the stars. The spectrum revealed that due to the stars’ broad absorption features (indicating upper atmospheric water steam and methane), they are T Class brown dwarfs.  

Because the Epsilon Indi Bs are so near and bright, they are excellent candidates for detailed observation on brown dwarfs. What astronomers can learn about brown dwarfs from these stars will remain applicable for many more stars of the same class. *


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