Further study of starspot activity and measurement of differential rotation for SZ Piscium

We present a series of nine Doppler images of the magnetically active K component of the RS CVn-type binary SZ Piscium (SZ Psc), based on high-resolution spectroscopic data collected from 2014 to 2018. We apply least-squares deconvolution (LSD) to all spectra to extract the average profiles with high signal-to-noise ratios for Doppler imaging. The surface maps of the K subgiant show starspots widely distributed along its latitude and longitude. A prominent, nonaxisymmetric polar spot around phase 0 is revealed by all images with sufficient phase coverage, which may be a stable feature on the K component. The starspots evolve over a timescale of 1 month. We have determined the surface shear rate of the K component from starspot maps reconstructed 10 days apart in 2017 November–December through the cross-correlation method. The surface differential rotation parameters are Ω_eq = 1.591 ± 0.002 rad day⁻¹ and ΔΩ = 0.035 ± 0.003 rad day⁻¹. The absorption lines contributed from the tertiary component of SZ Psc are detected in all LSD profiles, and we measure the radial velocity of the binary system and the tertiary component to derive an elliptical orbit with a period of 1530 ± 3 days and a mass of 0.75 ± 0.06 M ⊙ for the tertiary component.