17 and IQ Aurigae

17 (AR) AND IQ AUR: (17=AR Aurigae and IQ Aurigae). Auriga, in the heart of the Milky Way towards the anticenter of the Galaxy, is dominated by zeroth-magnitude (0.08) Capella, the seventh brightest star in the sky, and made of a G0-G8 pair of . To the southwest of Capella lies the charming thin triangle that makes up the "Kids," which contains one of the weirdest stars in the sky, the eclipsing binary Epsilon Aurigae. Roaming through the constellation, we then come upon three open clusters, M36, M37, and M38, named after the French astronomer Charles Messier (1730-1817). Attention is then drawn to the dense clump of half a dozen or so stars that resides near the southern end of the pentagon. the makes the constellation's main figure, and that at first looks like it might be another naked-eye open cluster, a distant Hyades perhaps. It isn't. It's more like the "Coathanger" in Vulpecula, just a random, though striking, set of stars that fall in the line of sight. At the group's center is a pair of stars some seven minutes of arc apart. Oddly, at magnitude 6.14 the fainter western of the two made it into Flamsteed's catalogue as 17 Aur, while the brighter eastern one, at magnitude 5.41, did not. It is, however, variable, so instead we use its variable-star name, IQ Aurigae.

The complex set of stars in southern Auriga harbors an eclipsing binary as well as other interesting objects.

But 17 Aur is ALSO variable and tagged AR Aurigae. While the little group is just a random collection, there is one exception: 17 and IQ constitute a "proper motion pair," its stars travelling through space together across the line of sight at respective speeds of 19.8 km/s and 19.4 km/s for 17 and IQ, and both moving away from us at 28.6 and 25.4 km/s. Moreover, they are at similar distances of 399 and 413 light years (give or take 23 and 19). The origins of their variability are very different. AR (17) is an eclipsing spectroscopic double made of two class B stars (B9 and B9.5) that orbit in just 4.13169 days, each alternately getting in the way of the other. From the spectroscopic and eclipse data (light curve), they are separated by 18.2 Astronomical Units, have luminosities of 38.0 and 37.5 times that of the Sun, masses of 2.98 and 2.29 Suns, radii of 1.78 and 1.82 times solar. We can also analyze the star via the theory of stellar structure and evolution, which yields a total luminosity of 60 Suns (notably less than the previous value) and masses of 2.25 Suns each, which are at least in the neighborhood.

The light curve of the primary eclipse of AR Aurigae in blue and yellow (visual band) light on an arbitrary scale. "Phase" is the relative fraction of the orbital period of 4.13169 days. The curved bottom of the eclipse's light curve shows that the eclipse is partial.

IQ Aur is very different. It's a single "Ap" (A0p, "p" for "peculiar") star in the class of "Alpha-2 Canum Venaticorum" stars," noted for its magnetic field, which is two to three thousand times the strength of Earth's, and strong silicon absorption. With a fairly slow projected equatorial rotation speed of 51 km/s, the chemical elements tend to separate, some falling inward under the force of gravity, others pushed outward by radiation. The magnetic fields of Ap stars are concentrated into spots that cause the star to vary in brightness as it rotates, which here gives a rotation period at 2.466 days. From distance and an average temperature of 14,550 Kelvin (needed to derive that amount of ultraviolet radiation), the star has a luminosity of 256 times that of the Sun, a radius of 2.52 times solar, and a mass of 4.0 Suns. Radius and temperature give about the same rotation period. The problem with such deviant stars is that there are no real spectroscopic standards. The class of "A0" is based upon one spectral criterion; another might give a different class altogether. To add to the mix, IQ Aurigae appears to be the bluest known Ap star.

Written byJim Kaler 06/16/17. Return to STARS.