PHI DRA (Phi Draconis) -- otherwise known as a "nice mess," though that is a description, not the star's name. But it should be. At first things seem simple. Phi Dra is just one more star on the gentle curves that make Draco, the celestial Dragon, as it winds between the Big and Little Dippers. Though it is not much of a distinction, fourth magnitude (4.22) class A (A0) Phi stands out a bit as the third-most-northerly Greek-letter-named star within the constellation (the others Chi and Tau). Seventy one degrees north of the celestial equator, it is invisible to anyone south of the Tropic of Capricorn. The difficulties begin with the recognition that Phi Dra is triple, with fourth-magnitude (4.5) Phi-A separated (for the moment) from sixth magnitude (5.9) Phi-B by just half a second of arc. Phi-A is then an inseparable spectroscopic double. Combine these pairings with the lack of spectral class for the two lesser stars (Phi-B and Phi Ab) and that the brightest of them (Phi-Aa) is peculiar anyway, and it becomes quite difficult to assess the stars' natures. At least we know that all the stars are hydrogen- fusing dwarfs that lie 303 light years (give or take 9) away. We also have decent orbital information. Phi A and B go around each other every 307.8 years at a mean separation of 90 AU, a severe eccentricity taking them between 22 and 157 Astronomical Units apart. They will be closest in 2116.
Phi Dra Phi Draconis B orbits Phi Draconis A (at the cross, itself a very close double made of Aa and lesser Ab) every 307.8 years at an average separation (as calculated from these data) of 90 AU, a high eccentricity taking them between 157 and 22 AU. The orbit is highly tilted to the sky, by 84 degrees, which, from our perspective, greatly distorts it. The dashed line is the true major axis, the cross the true focus. The scale is in seconds of arc. In reality, the two stars go about a common center of mass. (From the Sixth Catalog of Orbits of Visual Binary Stars, W. L. Hartkopf and B. D. Mason, US Naval Observatory Double Star Catalog, 2006.)
Kepler's Laws then give a sum of the masses for the trio of 7.6 times that of the Sun, which is much too high for the class of Aa alone plus B, which is fainter and assumed to be class A4. The best (imperfect) fit from all the data (which includes a temperature for Aa of 11,300 Kelvin) suggests a luminosity of 185 Suns for Aa, masses of 3.2 and 2.0 for Aa and Ab, and a luminosity of 30 Suns for Phi-B which leads to a mass of 2.2 solar. Kepler's Laws then applied to Aa and Ab, which orbit every 26.7 days, make them just 0.3 AU apart. (Of course, the orbit could be in error. If Ab has minimal mass, then the luminosity of Aa goes up to 185 Suns and the mass to 3.4 solar.) Now add to all this that Phi Dra Aa is also a peculiar "Alpha-2 Canum Venaticorum" star like Cor Caroli, one with magnetic patches that are here enhanced with silicon (caused by separation of elements in a quiet atmosphere), the average field strength some 700 times that of the Earth. Slight variations caused by the magnetic patches moving in and out of sight give a rotation period of 1.72 days, which in turn gives an equatorial rotation velocity of 84 kilometers per second, which fits nicely with the measurement of at least 72 km/s. Not had enough? Off in the distance, 73 seconds of arc away, is 13th magnitude Phi Draconis C. If it really belongs to the system, it has the brightness of a cool class K dwarf, and orbits the inner trio with a period of at least 220,000 years at a distance of at least 6700 AU. Motions though, more strongly suggest that the coupling is just a line-of-sight coincidence. All in all, a nice mess indeed!
Written by Jim Kaler 11/12/10. Return to STARS.