Orbital
Distance

(a=AUs)
Orbital
Period

(P=years)
Orbital
Eccentricity

(e)
Orbital
Inclination

(i=degrees)
Mass
Estimate

(Solar)

Diameter

(Solar)

Density

(Earths)
Surface
Gravity

(Earths)

Metallicity
(Solar)
Aab-Babc? Mass Center0.0........................
Aab Mass Center10.659.90.412121.2...............
Alula Australis Aa0.4661.830.53911.050.97......0.98
Alula Australis Ab1.2241.830.53910.4............
Disrupted H.Z. Aab?1.31.5091...............
Babc? Mass Center10.659.90.412121.2...............
Alula Australis Ba0.34<=1??0.9.........0.76
Alula Australis Bb0.060.0030?0.15............
Alula Australis Bc?0.76<=1??>=0.5............
Disrupted H.Z. Ba?1.11.20?...............


NOTE: This animation attempts to relate the complicated orbits (and possible habitable zones) of star groups Aab and Babc? in the Alula Australis (Xi Ursae Majoris) system to their respective centers of mass. To enlarge the display, the orbits have been arbitrarily rotated by 45 degrees. Although the actual inclinations of the Aab and Babc? orbital system and of their component orbits (from the perspective of an observer on Earth) are as displayed, the orbital inclination of any planet that may be discovered someday in this star system would likely be different from those of the habitable zone orbits depicted here. In fact, the apparent noncoplanarity of the orbits of star pairs Aab and Babc? reduces the likelihood of stable planetary orbits at increasing distances from each star or binary pair Alan Hale, 1994. (For the purposes of this animation, the masses of Stars or brown dwarfs Aa, Ab, Ba, Bb, and Bc are assumed to be 1.05, 0.4, 0.90, 0.04, and 0.5 Solar, respectively.)

The wide star groups Aab and Babc? are separated by an "average" distance of about 21.2 AUs (of a semi-major axis of 2.533" at 27.3 ly) in a elliptical orbit (e= 0.412) of 59.9 years, so that the two star groups get as close as 12.5 AUs and as far away as 39.9 AUs (Wulff D. Heintz, 1996; revising earlier earlier estimates, including Mason et al, 1995). The orbital inclination of the two pairs from Earth fluctuates from 122.1 from 1935 to 1995 to 121.2 from 1995-2034.

Star Aa is a New Suspected Variable designated NSV 5165, and spectroscopic and astrometric analyses reveal a companion Ab. First detected as a periodic orbital perturbation in 1905 by N. E. Norlund, this companion has been cited by the Sixth Catalog of Orbits of Visual Binary Stars as being separated "on average" from Star Aa by a semi-major axis of 0.054", which may be a misinterpretation of Heintz (1996). In any case at a distance of about 27.3 ly, with a period of 1.834 years and a combined mass for Aab of around 1.45 Solar-mass, the semi-major axis should lie around 1.69 AUs. The two stars have a highly elliptical orbit, which radial velocity analyses suggest is 0.53 rather than 0.61 (Griffin, 1998). Hence, Aa and Ab may move as close as 0.8 and as far as 2.6 AUs, at an inclination from the perspective of an observer on Earth of 91 (Heintz, 1996).

Star Ba may have a brown dwarf companion (Bb or HD 98230 b) in a "torch orbit," with an average separation of 0.06 AU in a highly circular orbit (e=0.00) whose period is completed within four days (see Extrasolar Planets Encyclopaedia). A recent analysis of radial velocities, however, discusses the possibility of a Bb companion as a orange-red (late K-type) dwarf star based on suspected mass ratios among the binary pairs without any mention of the 1996 brown dwarf finding (Griffin, 1998, pp. 293-294). Past calculations of orbital elements and system mass ratios based on astrometry (and other visual observations) and the spectral type of Star Ba (G0-5) indicate that HD 98230 b is not massive enough to fully account for subsystem B, and suggest the existence of a stellar companion (i.e., Bc).

Analysis of only one of 27 speckle interferometric observations (obtained with Kitt Peak and Canada-France-Hawaii telescopes) uncovered a fifth visual component to this multiple system (Mason et al, 1995). This object, however, apparently "never [has] had any effect whatsover upon the astrometric and radial-velocity behaviour of the observable components whose existence is reliably established" (Griffin, 1998, pages 275-276). The star may be an orange-red, main sequence dwarf of spectral and luminosity type K2-3 V and have an orbital period with star Ba or 2.2 to 2.9 years. Subsequently, Heintz (1996, page 411) suggested that such a companion to Star Ba would have to have a mass of at least half Sol's to reach detectable brightness, and that, among other orbital requirements, Bc's period would have to be less than an Earth year in order to account for the absence of effects on Ba's radial velocities and positions.


 

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