Rana

The Star

Delta Eridani is a orange-red subgiant (K0 IVe) that has about 2.3 to 2.7 times Sol's diameter and about 2.8 times its luminosity. Based on spectral type, a mass of 79 percent of Sol's mass has been derived (Murdoch et al, 1993, page 13), but a more recent analysis based on stellar evolution by Professor Kaler of Rana provides an estimates of 1.2 Solar-masses. It may be at least as enriched as Sol in elements heavier than hydrogen ("metals"), as it has between 54 and 214 percent of Sol's abundance of iron (Cayrel de Strobel et al, 1991, page 9).

According to Professor Jim Kaler at the University of Illinois' Department of Astronomy, Rana started life as a main sequence F8 dwarf (somewhat hotter and brighter than Sol with slightly greater mass) around 7.5 billion years ago, but core hydrogen fusion has ceased causing the star to expand and cool as an active subgiant before becoming much brighter and larger "as a true giant star" through core helium fusion. Rana was been designated as a "RS Canum Venaticorum" variable, perhaps erroneously, and as New Suspected Variable star NSV 1246. Some alternative useful star catalogue numbers for the star are: 23 Eri, HR 1136, Gl 150, Hip 17378, HD 23249, BD-10 728, SAO 130686, FK5 135, LHS 1581, LTT 1753, and LFT 304.

Delta Eridani B?

Astrometric measurements over 38 years were inconclusive (Lippincott and Worth, 1980). However, the high luminosity of the primary star, intense Ca II H and K emission (Smith and Dominy, 1979), and radial velocity variations somewhat larger than can be accounted for by the expected uncertainties suggested that Delta Eridani might be an RS CVn-type binary and therefore a photometric variable like most members of that class. Photometry collected in 1982 appeared to support a binary companion with an orbital period of 10 days, but total radial velocity might be "quite small" because of a small orbital inclination (Fisher et al, 1983).

Hunt for Substellar Companions

A recent 2.5-year search analyzing radial velocities failed to find a large Jupiter or brown dwarf within 10 AUs of Delta Eridani (Murdoch et al, 1993, pages 2 and 10). Since Delta Eridani has become a subgiant, it is possible that any planet that held Earth-like conditions earlier in the system's past has now become too hot to support Earth-type life, but that a colder Mars-type planet has become more Earth-like. Today, the distance from Delta Eridani where an Earth-type planet would be "comfortable" with liquid water is centered around 1.7 AU -- beyond the orbital distance of Mars in the Solar System. As a subgiant star subject to pulsations which affect careful measurements of radial velocity, astronomers would find it very difficult to detect any Earth-type planet arond this star using present methods.