Starspot activity and surface differential rotation on UX Arietis

We present new Doppler images of the K0 subgiant primary component of the RS CVn-type binary UX Arietis (UX Ari), derived from time-series spectra obtained in November–December of 2017 and 2024. Observations demonstrate that some spectral lines of the K0 IV component exhibit rapid changes on timescales of 1–2 hr, which seem not to be resulting from spot activity; meanwhile, other spectral lines show no such fast variations. Through an investigation, we find that the Ca I 6439 Å profile shows variation that follows the rotational modulation of spots. Using this line as a reference, we derive the least-squares deconvolution (LSD) profile from the selected lines of each spectrum so as to generate a more reliable Doppler image, which is consistent with the shape of the corresponding Ca I 6439 Å line. The Doppler images are separately reconstructed from the Ca I 6439 Å and the LSD profiles for each data set, and the surface maps are in good agreement with each other. All of the surface maps show dominant starspot structure at mid-to-high latitudes with appendages extending to the equator, while their locations differ by about 0.5 in the rotational phase between 2017 and 2024. In 2017 November–December, the main starspot group appears to be spatially associated with a large flare event just half a month later. Through the cross-correlation method, we have derived a weak antisolar differential rotation for the primary component of UX Ari, while its equator belt is well tidally locked.