Doppler imaging of Speedy Mic using the VLT Fast spot evolution on a young K-dwarf star

We study the short-term evolution of starspots on the ultrafast-rotating star HD 197890 ("Speedy Mic" = BO Mic, K0 − 2V ,Prot = 0.380 d) based on two Doppler images taken about 13 stellar rotations apart. Each image is based on spec- tra densely sampling a single stellar rotation. The images were reconstructed by our Doppler imaging code CLDI (Clean-like Doppler imaging) from line profiles extracted by spectrum deconvolution. Our Doppler images constructed from two indepen- dent wavelength ranges agree well on scales down to 10 ◦ on the stellar surface. In conjunction with nearly parallel V-band photometry our observations reveal a significant evolution of the spot pattern during as little as two stellar rotations. We suggest that such a fast spot evolution demands care when constructing Doppler images of highly active stars based on spectral time se- ries extending over several stellar rotations. The fast intrinsic spot evolution on BO Mic impedes the determination of a surface differential rotation; in agreement with earlier results by other authors we determine an upper limit of |α| < 0.004 ± 0.002.