Apsidal motion of the eclipsing binary as Camelopardalis: discrepancy resolved

We present a spectroscopic study of the eclipsing binary system AS Camelopardalis, the first such study based on phase-resolved CCD échelle spectra. Via a spectral disentangling analysis we measure the minimum masses of the stars to be M Asin 3 i = 3.213 ± 0.032 M ☉ and M Bsin 3 i = 2.323 ± 0.032 M ☉, their effective temperatures to be T eff(A) = 12, 840 ± 120 K and T eff(B) = 10, 580 ± 240 K, and their projected rotational velocities to be v Asin i A = 14.5 ± 0.1 km s–1 and v Bsin i B ≤ 4.6 ± 0.1 km s–1. These projected rotational velocities appear to be much lower than the synchronous values. We show that measurements of the apsidal motion of the system suffer from a degeneracy between orbital eccentricity and apsidal motion rate. We use our spectroscopically measured e = 0.164 ± 0.004 to break this degeneracy and measure $\dot{\omega }_{\rm {obs}}=0\fdg 133 \pm 0\fdg 010$ yr–1. Subtracting the relativistic contribution of $\dot{\omega }_{\rm {GR}}=0\mbox{$.\!\!^\circ $}0963\,{\pm}\, 0\mbox{$.\!\!^\circ $}0002$ yr–1 yields the contribution due to tidal torques: $\dot{\omega }_{\rm {cl}}=0\mbox{$.\!\!^\circ $}037\,{\pm}\, 0\mbox{$.\!\!^\circ $}010$ yr–1. This value is much smaller than the rate predicted by stellar theory, 0fdg40-0fdg87 yr–1. We interpret this as a misalignment between the orbital axis of the close binary and the rotational axes of its component stars, which also explains their apparently low rotational velocities. The observed and predicted apsidal motion rates could be brought into agreement if the stars were rotating three times faster than synchronous about axes perpendicular to the orbital axis. Measurement of the Rossiter-McLaughlin effect can be used to confirm this interpretation