2006A&A...446..267R

More than 600 high resolution spectra of stars with spectral type F and later were obtained in order to search for signatures of differential rotation in line profiles. In 147 stars the rotation law could be measured, with 28 of them found to be differentially rotating. Comparison to rotation laws in stars of spectral type A reveals that differential rotation sets in at the convection boundary in the HR-diagram; no star that is significantly hotter than the convection boundary exhibits the signatures of differential rotation. Four late A-/early F-type stars close to the convection boundary and at vsini≃100km/s show extraordinarily strong absolute shear at short rotation periods around one day. It is suggested that this is due to their small convection zone depth and that it is connected to a narrow range in surface velocity; the four stars are very similar in T_eff and vsin i. Detection frequencies of differential rotation α=ΔΩ/Ω>0 were analyzed in stars with varying temperature and rotation velocity. Measurable differential rotation is more frequent in late-type stars and slow rotators. The strength of absolute shear, ΔΩ, and differential rotation α are examined as functions of the stellar effective temperature and rotation period. The highest values of ΔΩ are found at rotation periods between two and three days. In slower rotators, the strongest absolute shear at a given rotation rate ΔΩ_max is given approximately by ΔΩ_max∝P^–1, i.e., α_max≃const. In faster rotators, both α_max and ΔΩ_max diminish less rapidly. A comparison with differential rotation measurements in stars of later spectral type shows that F-stars exhibit stronger shear than cooler stars do and the upper boundary in absolute shear ΔΩ with temperature is consistent with the temperature-scaling law found in Doppler Imaging measurements.