Rotation- and temperature-dependence of stellar latitudinal differential rotation

More than 600 high resolution spectra of stars of spectral type F and later have been 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 significantly hotter than the convection boundary exhibits signatures of differential rotation. Four late A-/early F-type stars close to the convection boundary and at v sin i ≈ 100 km s−1 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 is connected to a narrow range in surface velocity; the four stars are very similar in Teff and v sin i. Detection frequencies of differential rotation α = ∆Ω/Ω > 0 are 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, ∆Ω, and differential rotation, α, are examined as functions of stellar effective temperature and rotation period. The largest values of ∆Ω are found at rotation periods between two and three days. In slower rotators, the strongest absolute shear at a given rotation rate, ∆Ωmax, is approximately given by ∆Ωmax ∝ P−1, i.e., αmax ≈ const. In faster rotators, both αmax and ∆Ωmax are diminishing 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 ∆Ω with temperature is consistent with the temperature scaling law found in Doppler Imaging measurements.