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THETA AQL (Theta Aquilae). Project a line fromAltair (the luminary ofAquila, the Eagle) through its southern outlierAlshain (Beta Aquilae), and it passesalmost straight through third magnitude (3.23) Theta Aquilae. Tothe east of the main line of theMilky Way,the star marks a sharp westward bend in theconstellation's classic outline. As a distance of 287 light years, Thisseemingly ordinary class B (B9.5, just to the high temperature sideof Vega-like class A0) giant hides a surprise. It's not just oneB9.5 star, buttwo of them(as best we can tell the classes) very close together. Moreover,as seen below, neither of them is a giant. Thespectrograph reveals the pair intight orbital motion with an orbital period of a mere 17.123 days. Too close together to separate visually at the telescope (anastoundingly small 0.004 seconds of arc), the stars have beenindividually "seen" through the extraordinary power of"interferometry," which makes use of the interfering properties oflight waves.

The interferometer reveals the tight orbit of fifth magnitude ThetaAquilae B as it orbits third magnitude Theta Aql A, whjich lies atthe cross (in reality the two orbiting a common center of massbetween them). The numbered marks on the axes (hard to read here)are just 0.002 seconds of arc apart. A fit of an ellipse to thepartially observations (coupled with spectroscopy) gives an orbitalperiod of 17.123 days and an average orbital size of 0.28 AU (whichfrom theoretical analysis of the stars themselves is probably morelike 0.24 AU). The elliptical orbit is seen as tilted by 36degrees to the plane of the sky. (From an article by C. A. Hummelet al. in theAstronomical Journal, vol. 110, p. 376, 1995,as given in theSixth Catalog of Orbits of Visual BinaryStars, W. I. Hartkopf and B. D. Mason, U.S. Naval Observatory.)

Theta A comes in at the lower end of third magnitude (3.47), whileTheta B is nailed right at exact fifth (5.00). Assuming that thetemperature of 10,800 Kelvin measured for Theta A also applies toB (needed to allow for a bit of ultraviolet light), they haverespective luminosities of 370 and 90 times that of theSun, which yield radii 5.5 and 2.7 solar. Application of theory tells of masses of 3.7 and 2.8 solar and thatTheta A is more properly a subgiant right at, or near, the end ofits hydrogen-fusing lifetime, whereas Theta B is a middle-ageddwarf, the stars about 200 million years old. Interferometricobservation of the orbiting pair gives an average orbitalseparation of 0.28 Astronomical Units, just three quarters ofMercury's distance from the Sun.Kepler's laws then give a sum of masses of 10.2 times solar,considerably more than the 6.5 given by evolutionary theory. Asmall adjustment down to a 0.24 AU orbital radius reconciles thetwo, however. A relatively high eccentricity (rather odd for starsthis close together) takes them between 0.10 and 0.39 AU apart. The projected equatorial rotation speed of 64 kilometers per secondfor Theta A leads to a rotation period under 4.9 days. Assumingthe axial tilt is the same as that of the orbit, the true rotationperiod is but 2.9 days. Unlike many class B stars, this one (ThetaA) is metal-rich, with an iron-to-hydrogen ratio 60 percent greaterthan solar. Nearly two minutes of arc away is 13th magnitude ThetaAql C, which -- if a real relative -- would be a class K dwarf10,000 AU from the main pair and have an orbital period greaterthan 300,000 years. Most likely, though, it is just another line-of-sight coincidence. More interesting, the star seems to be embedded in a large reflection nebula (its dust reflecting the starlight) that -- if the two are indeed related -- is some fivelight years across.

Written byJim Kaler 9/19/08. Return toSTARS.

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