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, 28 of them are
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~100 km/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. Detection
frequencies of differential rotation 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
and differential rotation are examined as functions of the stellar effective
temperature and rotation period. The strongest shear is found at rotation
periods between two and three days. In slower rotators, the strongest shear at
a given rotation rate is given approximately by DOmega_max ~ P^{-1}. In faster
rotators, alpha_max and DOmega_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, the upper boundary in
absolute shear DOmega with temperature is consistent with the temperature
scaling law found in Doppler Imaging measurements.Comment: 15 pages, accepted for publication in A&A, typos correcte
