Chemical surface inhomogeneities in late B-type stars with Hg and Mn peculiarity I. Spot evolution in HD 11753 on short and long time scales

Aims. Time series of high-resolution spectra of the late B-type star HD 11753 exhibiting HgMn chemical peculiarity are used to study the surface distribution of different chemical elements and their temporal evolution. Methods. High-resolution and high signal-to-noise ratio spectra were obtained using the CORALIE spectrograph at La Silla in 2000, 2009, and 2010. Surface maps of Y II, Sr II, Ti II, and Cr II were calculated using the Doppler imaging technique. The results were also compared to equivalent width measurements. The evolution of chemical spots both on short and long time scales were investigated. Results. We determine the binary orbit of HD 11753 and fine-tune the rotation period of the primary. The earlier discovered fast evolution of the chemical spots is confirmed by an analysis using both the chemical spot maps and equivalent width measurements. In addition, a long-term decrease in the overall Y II and Sr II abundances is discovered. A detailed analysis of the chemical spot configurations reveals some possible evidence that a very weak differential rotation is operating in HD 11753.