Luyten 726-8 AB (UV Ceti)

AB Binary Star System

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).

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	Orbital Distance (a=AUs)	Orbital Period (P=years)	Orbital Eccentricity (e)	Orbital Inclination (i=degrees)	Mass (Solar)	Diameter (Solar)	Density (Earths)	Surface Gravity (Earths)	Metallicity (Solar)
AB Mass Center	0.0	...	...	...	...	...	...	...	...
Luyten 726-8 A	2.5	26.5	0.62	127.3	0.10-0.11	0.15	...	...	...
Inner HZ Edge A?	~0.02	0.01	0	127.3	...	...	...	...	...
Outer H.Z. Edge A?	~0.05	0.04	0	127.3	...	...	...	...	...
UV Ceti	3.0	26.5	0.62	127.3	0.10	0.14	...	...	...
Inner HZ Edge B?	~0.02	0.01	0	127.3	...	...	...	...	...
Outer H.Z. Edge B?	~0.05	0.04	0	127.3	...	...	...	...	...

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; RECONS; and NASA Star and Exoplanet Database, derived from Henry and McCarthy, 1993), 14 percent of its diameter (Johnson and Wright, 1983, page 649), and less than 6/100,000th of its luminosity (Worley and Behall, 1978). With less than 20 percent of Sol's mass, Both Stars A and B are so small that they can transport core heat to its surface only through convection, unlike larger red dwarf stars like Gliese 752 A -- also known as Wolf 1055 A or Van Biesbroeck's Star (more).

Jeffrey L. Linsky, JILA, STScI, NASA




Larger image.




Like Gliese 752 B, both Stars A and B are so
small, with less than 20 percent of Sol's mass,
that it can transport core heat only through
convection, unlike larger larger red dwarf
stars like Gliese 752 A (more).

Although less dramatic in its brightness variations than its binary companion B, Luyten 726-8 A has also been identified as a flare star. Like its companion, Star A is also a variable and flare star, designated BL Ceti. Some useful star catalogue numbers and designations are: BL Cet, Gl 65 A, G 272-61, L 726-8 A, LP 768-27, LFT 144, LTT 892, LHS 9, LDS 868, and 2MASS J01390120-1757026 A.

UV Ceti (L 726-8 B)

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.

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.

UV Ceti, however, 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. In addition to variable star designations UV Ceti and New Suspected Variable (NSV) star 577, useful catalogue numbers and designations include: UV Cet, L 726-8 B, Gl 65 B, LP 768-26, LFT 145, LTT 893, LHS 10, NSV 577, and MASS J01390120-1757026 B.

Habitable Zone around A or B

With a spectral type of M5.5, Proxima Centauri can be used as a rough proxy for either Stars A or B (M5.6 and M6.0, respectively). Accounting for infrared radiation, the distance from Proxima where an Earth-type planet could have liquid water on its surface is around 0.022 to 0.054 AU (Endl and Kürster, 2008; and Endl et al, 2003, in pdf) -- much closer than Mercury's orbital distance of about 0.4 AU from Sol -- with a corresponding orbital period of 3.6 to 13.8 days (Endl and Kürster, 2008), while the NASA Star and Exoplanet Database has calculated a slightly farther out habitable zone between 0.033 and 0.064 AUs around Proxima. In any case, the rotation of such a planet would probably be tidally locked so that one side would be in perpetual daylight and the other 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.)

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).

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.

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