EF Eridani is an ultra-short period binary system consisting of a white dwarf and a substellar companion of unknown type. In a study by T.E. Harrison et al. (2004), astronomers used the NIRI on Gemini-North (7:21 to 9:00 UT on 24 December 2002) and NIRSPEC on Keck II (6 September 2003) to learn more about this unusual binary system. Estimates indicate that the white dwarf has ~60 percent the Sun’s mass, while the substellar companion has only ~5 percent the Sun’s mass. Both objects are separated by merely ~400,000 km and they whiz around each other in 81 minutes.
Figure 1: Artist’s impression of the EF Eridani system as it might appear today. Image credit: Gemini Observatory.
The white dwarf in EF Eridani is a dense, burnt-out remnant of a Sun-like star. It is roughly the same size as Earth and has a modelled surface temperature of about 9500 K. Circling around the white dwarf is a substellar companion of unknown type. It is thought that ~500 million years ago, the companion was a typical star that began losing mass to the smaller but more massive white dwarf. Over time, the companion lost so much mass to the white dwarf that it has since regressed into a cool ember, believed to be roughly the size of Jupiter. With only ~5 percent the Sun’s mass, the companion is not massive enough to sustain nuclear fusion in its core, making it far too low in mass to be a star. For an object to be a star, it needs to have at least ~8 percent the Sun’s mass to sustain nuclear fusion.
Although an object less than ~8 percent the Sun’s mass is classified as a brown dwarf, observations reveal that the composition of the substellar companion in EF Eridani does not match any known brown dwarf. This is the case even though the object has an estimated temperature of 1700 K, equivalent to a cool brown dwarf. As a result, the substellar companion is neither star nor brown dwarf, and it represents an object that does not match any known category.
Figure 2: Comparison of the spectrum of the substellar companion in EF Eridani to brown dwarf model spectra. The temperature of each model is listed, and below the temperature are the abundances of carbon, nitrogen and oxygen (shown on a log10 scale in comparison to the Sun’s abundance). Source: T.E. Harrison et al. (2004).
T.E. Harrison et al. (2004), “Phase-Resolved Infrared H- and K-band Spectroscopy of EF Eridani”, arXiv:astro-ph/0409735