The gap of differential rotation in early F-type stars

Ammler-von Eiff Matthias, ammler@tls-tautenburg.de, Thueringer Landessternwarte Tautenburg, Germany
Reiners Ansgar, Institute for Astrophysics Goettingen

Abstract
In previous years, stellar differential rotation similar to solar was detected using various methods. We present new measurements of the rotational profile of 56 nearby stars of spectral types A-F. The overall broadening profile is derived from spectral line shape by averaging hundreds of spectral lines. The effect of differential rotation on the average broadening profile is measured by the Fourier transform method, which is limited to stars with vsini greater than 12km/s. Rates of relative differential rotation rates of 5% and more are detected. Together with earlier work, we now know of 33 stars in which sun-like differential rotation was identified by line profile analysis. The frequency of differential rotators decreases towards high effective temperature and rapid rotation. We find evidence of two populations of differential rotators, one group of rapidly rotating A stars at the granulation boundary with strong horizontal shear, and another group of mid- to late-F type stars with moderate rates of rotation and less shear. An upper bound to horizontal shear in F-type stars is identified, and this may explain the apparent decrease of differential rotators towards hotter and rapidly rotating stars. However, this effect can only partially explain the gap we find between A and F stars, in which the stars appear to exhibit very little shear. Apparently, the physical conditions of differential rotation change at early-F spectral types.