Kappa Ceti

INES, LAEFF, ESA




Larger illustration.





Also commonly referred as
HD 20630 by many astronomers,
Kappa Ceti is a main-sequence
star that is a little cooler and
redder than Sol.

As Kappa Ceti has become one of the top 100 target stars for NASA's planned Terrestrial Planet Finder (TPF), images of this star and its position relative to the Milky Way in Earth's night sky are now available from the TPF-C team.

The Star

Kappa Ceti is a main sequence, yellow-orange dwarf (G5 Ve). It may have roughly the same mass as Sol, a similar diameter at 107 +/- 12 percent of Sol's (Campbell and Garrison, 1985), but only 85 percent of its luminosity. The star may be as or more enriched in elements heavier than hydrogen ("metals") because it has between 98 and 240 percent of Sol's abundance of iron (Cayrel de Strobel et al, 1991, page 8). The European Space Agency has used ultraviolet spectral flux distribution data to determine stellar effective temperatures and surface gravities, including those of Kappa Ceti.

Much younger than Sol, Kappa Ceti may only be around 800 million year old (Guinan et al, 1999). It is a New Suspected Variable star designated NSV 1100, and observations announced in 1998 suggest that Kappa Ceti emits a new gargantuan class of stellar mass ejection associated recently with Sol-type stars of spectral class F8 to G8 called a "superflare" (Schaefer et al, 2000). Some useful star catalogue numbers for Kappa Ceti are: Kap Cet, Kap1 Cet, 96 Cet, HR 996, Gl 137, Hip 15457, HD 20630, BD+02 518, SAO 111120, FK5 1095, and LTT 11094.

Kappa Ceti B?

Past spectroscopic analysis (Hoffleit and Jaschek, Yale Bright Star Catalog, 1982) suggested that Kappa Ceti might have a stellar companion (Duquennoy and may, 1991, page 5), which speckle interferometry failed to resolve (Hartkopf and McAlister, 1984, pages 3 and 5). Subsequently, more recent radial velocity analysis also failed to find supporting evidence (Young et al, 1987, page 5) for such an object.

Hunt for Substellar Companions

Using the radial velocity technique pioneered by Geoffrey Marcy and Paul Butler, the Lick Planet Search for substellar companions has thus far failed to find a brown dwarf or large Jupiter- or Saturn-mass object in a "torch" orbit around Kappa Ceti (Cumming et al, 1999). Given the regular eruption of superflares, however, it is unlikely that Earth-type life could survive for long on any inner rocky planet. The distance from Kappa Ceti where an Earth-type planet would be "comfortable" with liquid water is centered around only 0.92 AU -- between the orbital distances of Earth and Venus in the Solar System. At that distance from the star, such a planet would have an orbital period of almost 324 days -- nearly an an Earth year.

Astronomers are hoping to use NASA's Terrestrial Planet Finder (TPF) and the ESA's Darwin planned groups of observatories to search for a rocky inner planet in the so-called "habitable zone" (HZ) around Kappa Ceti. As currently planned, the TPF will include two complementary observatory groups: a visible-light coronagraph to launch around 2014; and a "formation-flying" infrared interferometer to launch before 2020, while Darwin will launch a flotilla of three mid-infrared telescopes and a fourth communications hub beginning in 2015.

Super Flares

According to one recent hypothesis, unusually intense stellar flares from a sun-like ("Sol-type") star could be caused by the interaction of the magnetic field of a giant planet in tight orbit with that star's own magnetic field (Rubenstein and Schaefer, 2000). Some Sol-type stars of spectral class F8 to G8 have been found have been observed to undergo enormous magnetic outbursts to produce "superflares" (coronal mass ejections) that release between 100 and 10 million times more energy than the largest flares ever observed on the sun, making them brighten briefly by up to 20 times. These superflares last from one hour to one week and increase the normal luminosity of a star as much as one thousand times. If our sun were to produce a large superflare, Earth's ozone layer would be destroyed, and ice on the daylight side of moons as far out as those of Jupiter or even Saturn would be melted, producing vast floodplains that refreeze after the flare subsides. No traces of past superflares have been detected in our Solar System.

NASA (Krist, et al, 1999)


Until the discovery of superflares by stars like
Kappa Ceti, enormous coronal mass ejections releasing
energy from 100 to 10 million times greater than a
typical Solar flare by stars of spectral classes F8
to G8 were thought to be limited to very young stars
-- like this magnetically-driven outburst by the
million-year-old binary pair of pre-main-sequence
stars, XZ Tauri AB.