Also known as HD 27442, Epsilon Reticuli is a binary system located at a distance of 59 light years in the southern constellation of Reticulum, the Net (a name associated with the crosshairs or “reticule” of a telescopic sight). The brighter of the two stars is an orange subgiant of spectral type K2 IV, whose metallicity is high even for an exoplanetary host, with [Fe/H] = 0.42. The dimmer companion has been confirmed as a white dwarf.

The two stars are currently separated by about 236 AU, but the parameters of their shared orbit have not been precisely determined (Mugrauer et al. 2007). Mugrauer and colleagues suggest a provisional eccentricity value of 0.5, as this is typical for binary orbits. Period and semimajor axis are unknown.

A single planet more massive than Jupiter has been detected in a circular orbit around the orange subgiant (Epsilon Reticuli A). The Catalog of Nearby Exoplanets provides a semimajor axis of 1.27 AU, an eccentricity of 0.06 (similar to that of Saturn), and a period of 428 days. The minimum mass of this planet (Epsilon Reticuli b) is given as 1.56 MJUP.

Nevertheless, key system parameters remain unresolved.

For the white dwarf (Epsilon Reticuli B), Mugrauer and colleagues suggest a mass of 0.58 MSOL. This is very close to the average mass of white dwarfs in the Solar neighborhood (0.587 MSOL) and similar to the value of 0.602 MSOL determined for the white dwarf companion of the nearby subgiant Procyon (Girard et al. 2000). Mugrauer’s group also find a cooling age of 220 million years, indicating that star B’s explosive contraction to its current mass was an astronomically recent event (Mugrauer et al. 2007).

Ironically, no consensus has yet emerged for the mass and age of Epsilon Reticuli A, despite many decades of observation. Mugrauer’s group favor a mass of 0.98 MSOL and an age of 10 billion years, further noting that star A is only slightly less evolved than star B. They predict that the subgiant will follow its smaller companion through the red giant phase and collapse into a white dwarf within a few hundred million years (Mugrauer et al. 2007). Given its present state of evolution, star A retains its original mass. Therefore, if we accept the mass provided by this group, its progenitor was either a G or an early K-type star. Its companion’s progenitor would then be a slightly more massive G or F-type star. Thus both members of the binary were originally Sun-like stars, which have a main sequence lifetime similar to the overall age attributed to the Epsilon Reticuli system.

A very different analysis is presented in a recent study by Takeda and colleagues. They suggest a likely mass of 1.59 MSOL for the orange subgiant, out of a possible range of 1.45-1.68 MSOL (Takeda et al. 2007). This much larger estimate would require the main sequence progenitor of Epsilon Reticuli A to be an extremely massive F-type or, more likely, an A-type star. Accordingly, Takeda’s group place the system age within the range of 2.48-3.44 billion years, with 2.84 billion as their preferred value. In their analysis, the progenitor of the white dwarf would have to be an A-type star of still higher mass, since it reached its evolutionary endpoint faster. If we accept this interpretation, the system of Epsilon Reticuli becomes a more evolved (and much more metallic) version of the Procyon binary, in which the subgiant star has a mass of 1.5 MSOL and [Fe/H] = -0.05, and the overall system age falls in the range of 1.42-2.02 billion years (Eggenberger et al. 2005).

Last update June 2007