Luyten 726-8 AB (UV Ceti)

Luyten 726-8 A

This cool and dim, main sequence red dwarf (M5.6 Ve) may have about 10 to 11 percent of Sol's mass (Geyer et al, 1988; and RECONS estimate), 14 percent of its diameter (Johnson and Wright, 1983, page 649), and less than 6/100,000th of its luminosity (Worley and Behall, 1978). Although less dramatic in its brightness variations than its binary companion B, Luyten 726-8 A has also been identified as a flare star. Star A and its companion UV Ceti have a combined mass of about 20 percent of Sol's but less than 11/100,000th of its brightness. The two stars may be separated "on average" by only about 5.5 times the Earth-Sun distance (AUs of a semi-major axis) in a highly elliptical orbit (e= 0.62), at an inclination of 127.3° degrees. Based on orbital data that generates the mass estimates cited previously, the two stars may swing between 2.1 and 8.8 AUs apart in an orbital period lasting 26.5 years (Geyer et al, 1988). (See an animation of the orbits of the two stars and their potentially habitable zones, with a table of basic orbital and physical characteristics.) Like its companion, Star A is also a variable and flare star, designated BL Ceti. Some useful star catalogue numbers are: BL Cet, Gl 65 A, G 272-61, LHS 9, and LDS 868.

UV Ceti (L 726-8 B)

Arnold O. Benz, Institute of Astronomy, ETH Zurich

High resolution and jumbo images (Benz et al, 1998).

A flaring UV Ceti was resolved as at least two evolving
spatial components that were separated by four to five
stellar radii, in this VLBA/VLA radio image at 3.6 cm
wavelength. The components changed in appearance
over about six hours of observation but stayed aligned
along the axis of the binary orbit, as shown by the arrow.

This often even dimmer, main sequence red dwarf star (M6.0 Ve) may have only 10 percent of Sol's mass (Geyer et al, 1988; and RECONS estimate), 14 percent of its diameter (Johnson and Wright, 1983, page 649), and less than 4/100,000th of its luminosity. However, UV Ceti is an extreme example of a flare star that can boost its brightness by five times in less than a minute, then fall somewhat slower back down to normal luminosity within two or three minutes before flaring suddenly again after several hours. In 1952, UV Ceti was observed flaring to 75 times its normal brightness in only 20 seconds. Useful catalogue numbers include: L 726-8 B, Gl 65 B, and LHS 10.

Hunt for Substellar Companions

There may have been an unconfirmed detection of two planetary candidates around Luyten 726-8 A, one with a mass one-tenth more than Jupiter's and a second with about 40 percent more. A recent search for faint companions using the Hubble Space Telescope, however, found no supporting evidence for a large Jupiter or brown dwarf sized object, although the observed positions of Luyten 726-8 AB differed from published orbital elements (Schroeder et al, 2000). In any case, the distance from either Luyten 726-8 A or UV Ceti where an Earth-type planet would be comfortable with liquid water may be centered around 0.0071 AUs based on visible light alone, with a "year" of less than 17 hours -- 0.04 to 0.07 AUs when accounting for infrared radiation. At that distance, the rotation of the planet would probably be tidally locked with the star so that one side would have perpetual daylight while the other would be in darkness. (For an illustrated discussion, see Christoph Kulmann's web page on the potential habitable zone around a red dwarf, flare star like UV Ceti.)

Life Around a Flare Star

Many dim, red (M) dwarf stars exhibit unusually violent flare activity for their size and brightness. These flare stars are actually common because red dwarfs make up more than half of all stars in our galaxy. Although flares do occur on our Sun every so often, the amount of energy released in a solar flare is small compared to the total amount of energy that Sol produces. However, a flare the size of a solar flare occurring on a red dwarf star (UV Ceti) that is more than ten thousand times dimmer than our Sun would emit about as much or more light as the red dwarf does normally.

Flare stars erupt sporadically, with successive flares spaced anywhere from an hour to a few days apart. A flare only takes a few minutes to reach peak brightness, and more than one flare can occur at a time. Moreover, in addition to bursts of light and radio waves, flares on dim red dwarfs may emit up to 10,000 times as many X-rays as a comparably-sized solar flare on our own Sun, and so flares would be lethal to Earth-type life on planets near the flare star. Hence, Earth-type life around flare stars may be unlikely because their planets must be located very close to dim red dwarfs to be warmed sufficiently by star light to have liquid water (about 0.0075 AU for UV Ceti), which makes flares even more dangerous around such stars. In any case, the light emitted by red dwarfs may be too red in color for Earth-type plant life to perform photosynthesis efficiently.