14 AUR (14 Aurigae=KW Aurigae). Toward the lower part of Auriga (the Charioteer) is a short, more or less north- south string of stars that looks like a celestial exclamation point that has fifth magnitude (5.02) 14 Aurigae (the name a "Flamsteed number") as its easily-found lower "point." (Like Auriga's "Little Kids," the "Exclamation Point" could be a new asterism!) At a distance of 286 light years (give or take 20), this remarkable star system (14 Aur, not the asterism, which is made of stars unrelated to each other) is at least quintuple and is a treasure chest that presents a fine display of stellar evolution in action and that includes a hot white dwarf. The main star, the one visible to the naked eye, "14" itself, is a class A (on the cool side, A9, almost F) subgiant, implying that it has recently shut down core hydrogen-fusion. Though Auriga is filled with dark clouds, 14 Aur (and the Exclamation Point) lies along a relatively clear pathway and is undimmed by interstellar dust. With a temperature of 7670 Kelvin, it shines with the light of 59 Suns. Some 4.4 solar diameters across, it's a modest spinner with a projected equatorial rotation velocity of 27 kilometers per second, which gives it a rotation period of under 8 days. Carrying a mass of 2.4 solar masses, as predicated by its spectral class, 14 Aur has indeed just quit core fusion and is beginning to evolve into a giant. It's also a modest variable of the "Delta Scuti" class, one that varies subtly with multiple periods, though only one, of 2.11 hours, has been identified (the star varying between magnitudes 4.94 and 5.10). As a result, it also bears the variable star name KW Aurigae.

Now the real fun begins. First, 14 Aur has a very close-in companion that is detected only spectroscopically and orbits in a mere 3.794 days. Nothing else is known about it (though there is reference to low-mass class K). Then, just over 12 seconds of arc away lies 10th magnitude 14 Aurigae B. Though they at first look companionable, "B" is widely agreed to be just an optical line-of- sighter. Not so, however, for 14 Aur C, a 7.3 magnitude class F (F3) dwarf separated from 14 Aur A by 14.6 seconds of arc. It too is a spectroscopic double with a period of 2.99 days. Assuming the companion to be inconsequential, 14 Aur C (with a temperature of 6900 Kelvin) has a luminosity of 7 Suns, a radius and mass both of 1.9 solar. So now we are up to four components. Observations from space, however revealed ultraviolet radiation coming from a very hot source estimated at a visual magnitude of 14 and a temperature of 42,000 Kelvin, implying that one of the spectroscopic components was a hot white dwarf, which was then narrowed to 14 Aurigae C. But the hot source showed no velocity variations. An ultraviolet image finally taken with the Hubble Space Telescope (at 1930 Angstroms) then revealed the actual white dwarf two seconds of arc away, making 14 Aur C a triple, 14 Aur Ca and Cb separable, with 14 Aur Ca the F dwarf spectroscopic double, Cb the white dwarf.
14 Aur In this ultraviolet Hubble image, 14 Aurigae A hovers near the upper left corner. Near lower right are 14 Aur Ca and Cb, which orbit each other at least every 1600 years, while the C-pair takes a full 20,000 years or more to make it around 14 Aur A. The Cb component, a hot white dwarf, appears to be as bright as the class F dwarf 14 Aur Ca because of its intense ultraviolet radiation. Optically, it would appear much fainter and be difficult to detect. In addition, both A and Ca are close spectroscopic doubles that are not noticeable here, making the system quintuple. At one time, the white dwarf dominated the little group. (The alphanumeric code at lower left denotes the filter used to take the image.) From a paper by M. A. Barstow, H. E. Bond, M. R, Burleigh, and J. B. Holberg in Monthly Notices of the Royal Astronomical Society, vol. 322,. p. 891, 2001.
Given its magnitude and luminosity, the radius of Cb comes out to be about the size of Earth, confirming white dwarf status. At visual wavelengths, Cb is too faint to be seen easily from the ground. From the masses and Kepler's Laws (assuming the spectroscopic companions to be of low mass), 14 Aur A and Aa (the close companion) are separated by just 0.07 AU, while the components of 14 Aur Ca are just 0.05 AU apart. Given that dead Cb, the white dwarf, had to be the first to evolve and therefore was initially the most massive star of the system, we might guess an initial mass of 3 solar, which leads to a white dwarf mass of 0.7 solar (the star having lost mass as it aged as a giant). Ca and Cb (the white dwarf) then take at least 1600 years to orbit each other, while triple 14 Aur C and double 14 Aur A take at least 20,000 years to make their mutual circuit. Off in the distance, some three minutes of arc away, is 14 Aur D! But relax, as it's probably just another line-of-sight coincidence. Fourteen Aurigae thus provides a powerful lesson, not just in hierarchical multiple systems, but also in stellar evolution. Just a few hundred million years ago, it was dominated by the a three solar mass class B star that had a bunch of fainter companions. For a time, the system held what is now Cb as a brilliant red giant radiating thousands of solar luminosities. The most massive is now among the faintest, having lost most of its mass and expiring as a dim white dwarf about the size of Earth. Aa is now following in its Cb's footsteps.
Written by Jim Kaler 2/05/10. Return to STARS.