Differential rotation in rapidly rotating F-stars

We obtained high quality spectra of 135 stars of spectral types F and later and derived ``overall'' broadening functions in selected wavelength regions utilizing a Least Squares Deconvolution (LSD) procedure. Precision values of the projected rotational velocity $v \sin{i}$ were derived from the first zero of the Fourier transformed profiles and the shapes of the profiles were analyzed for effects of differential rotation. The broadening profiles of 70 stars rotating faster than $v \sin{i} = 45$ km s$^{-1}$ show no indications of multiplicity nor of spottedness. In those profiles we used the ratio of the first two zeros of the Fourier transform $q_2/q_1$ to search for deviations from rigid rotation. In the vast majority the profiles were found to be consistent with rigid rotation. Five stars were found to have flat profiles probably due to cool polar caps, in three stars cuspy profiles were found. Two out of those three cases may be due to extremely rapid rotation seen pole on, only in one case ($v \sin{i} = 52$ km s$^{-1}$) solar-like differential rotation is the most plausible explanation for the observed profile. These results indicate that the strength of differential rotation diminishes in stars rotating as rapidly as $v \sin{i} \ga 50$ km s$^{-1}$.