Beta Comae Berenices

JPL, CalTech, NASA


Larger illustration


Astronomers have identified Beta
Comae as a prime target for the
Terrestrial Planet Finder (TPF),
now planned for launch between
2014 and 2020.

The Star

Beta Comae Berenices is a main-sequence yellow-orange dwarf star of spectral and luminosity type F 9.5-G0 V. The star has about 1.05 times Sol's mass, about 99 to 110 percent of its diameter (Perrin and Karoji, 1987; Leggett et al, 1986; and Johnson and Wright, 1983), and about 1.42 times its luminosity. It appears to be more enriched than Sol in elements heavier than hydrogen ("metals") with between 1.05 to 2.29 times Sol's abundance of iron (Cayrel de Strobel et al, 1991, pages 294-295). While one study suggests that the star may be about 4.4 billion years old (Edvardsson et al, 1993, page 123), another suggests that its age may be as much as 10 percent younger than Sol -- at around 4.1 billion years (Gray et al, 1996). Moreover, based on one analysis of isochrones, the star may only be 1.7 billion years old (Guinan et al, 1999).

Beta Comae rotates about twice as fast as Sol. As a result of faster rotation and up-and-down convection in its outer gas layers, the star is probably also more magnetically active than our Sun. It has a long-term activity cycle of 16.6 years (roughly six years longer than Sol), and possibly secondary cycle of 9.6 years. Useful star catalogue numbers for Beta Comae Berenices include: Bet Com, 43 Com, HR 4983, Gl 502, Hip 64394, HD 114710, BD+28 2193, SAO 82706, FK5 492, LHS 348, LTT 13815, and LFT 978.

Beta Comae Berenices B?

According to the Yale Bright Star Catalogue, 1991 5th Revised Edition notes entry for HR 4983, analysis of spectroscopy and velocity variations over at least five years initially suggested that Beta Comae Berenices may have an unseen companion, which was unresolved by speckle interferometry (Hartkopf and McAlister, 1984, page 109). Analysis of astrometry over 53 years (Wulff Dieter Heintz, 1986) and more precise measurement of radial velocities (Campbell et al, 1988, pages 906 and 919) suggest otherwise.

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 Beta Comae Berenices (Cumming et al, 1999). The distance from Beta Comae Berenices where an Earth-type planet would be "comfortable" with liquid water may be centered around 1.2 AU -- between the orbital distances of Earth and Mars in the Solar System with an orbital period of 1.29 Earth Years. 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 Beta Comae. 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.