Chi1 Orionis 2

Chi1 Orionis A

This star is a yellow-orange main sequence dwarf star of spectral and luminosity type G0 V, with roughly the same mass as Sol (Irwin et al, 1992), as much as 1.45 times its diameter (George G. Gatewood, 1994, page 143), and less than 1.1 times its luminosity. It may be 89 to 178 percent as enriched as Sol with iron (Cayrel de Strobel et al, 1991, page 12).

The star, however, is a "Barium-dwarf" (s-process element rich but comparatively carbon deficient) candidate star, that was probably enriched by an asymptotic branch giant (AGB) star (see Gacrux) but is now a very dim, white dwarf companion. That white dwarf is now the dimmer component "B" of the HD 147513 AB (G0 V and DA2 /VII) system, in the same Ursa Major stellar moving group. This hypothesis suggests that all three stars may have formed a multiple system until their orbital stability was disrupted when the once, brighter and bigger AGB star (HR 6094 B) shed most of an estimated original mass of 2.6 Solar to reveal its white dwarf core about 30 million years ago (Porto de Mello and da Silva, 1997). Useful star catalogue numbers and designations for Chi1 Orionis A include: Chi1 Ori, 54 Ori, HR 2047, Gl 222 A, Hip 27913, HD 39587, BD+20 1162, SAO 77705, and LTT 11743.

Spectroscopic, astrometric and radial-velocity analyses reveal a companion with an average separation of 6.4 AUs (a semi-major axis derived from a photocentric estimate of 0.0969" times [1 + (mass A of 1.00 / mass B of 0.15)] -- and a HIPPARCOS parallax of 0.11543 +/- 0.00108"), varying between 3.5 and 9.3 AUs. The two stars move in an elliptical (e= 0.45) orbit that takes about 14.2 years to complete and is inclined at about 93° from the perspective of an observer on Earth (George G. Gatewood, 1994 and Irwin et al, 1992). (See an animation of the orbits of Stars A and B and their potentially habitable zones, with a table of basic orbital and physical characteristics.)

NASA -- larger image

Chi1 Orionis B is probably a dim red dwarf star, like
Gliese 623 A (M2.5V) and B (M5.8Ve) at lower right.

Chi1 Orionis B

Analysis of radial velocity variations suggest that this probable red dwarf star has about 15 percent of Sol's mass (Irwin et al, 1992).

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 Chi1 Orionis A (Cumming et al, 1999). The distance from Chi1 Orionis A where an Earth-type planet would be "comfortable" with liquid water is centered within 1 AU -- around inside the orbital distance of Earth in the Solar System. Astronomers would find it very difficult to detect an Earth-type planet around either of these stars using present methods.