Strong latitudinal shear in the shallow convection zone of a rapidly rotating A-star

Abstract

We have derived the mean broadening profile of the star V102 in the region of the open cluster IC4665 from high resolution spectroscopy. At a projected equatorial rotation velocity of vsini = (105 +- 12)km/s we find strong deviation from classical rotation. We discuss several scenarios, the most plausible being strong differential rotation in latitudinal direction. For this scenario we find a difference in angular velocity of DeltaOmega = 3.6 +- 0.8 rad/d (DeltaOmega/Omega = 0.42 +- 0.09). From the Halpha line we derive a spectral type of A9 and support photometric measurements classifying IC4665 V102 as a non-member of IC4665. At such early spectral type this is the strongest case of differential rotation observed so far. Together with three similar stars, IC4665 V102 seems to form a new class of objects that exhibit extreme latitudinal shear in a very shallow convective envelope.