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

TL;DR

This study presents detailed Doppler imaging of SZ Psc's K star, revealing starspot distribution, evolution, and differential rotation, along with orbital parameters of the tertiary component, based on high-resolution spectroscopic data from 2014 to 2018.

Contribution

First detailed Doppler images of SZ Psc's K star showing starspot distribution and evolution, and measurement of its differential rotation and tertiary orbit.

Findings

Stable non-axisymmetric polar spot identified.

Starspots evolve within one month.

Measured differential rotation parameters and tertiary orbit.

Abstract

We present a series of 9 Doppler images of the magnetically active K component of the RS CVn-type binary SZ Psc, based on the high-resolution spectroscopic data collected from 2014 to 2018. We apply least-squares deconvolution to all spectra to extract the average profiles with high signal-to-noise ratios (SNRs) for Doppler imaging. The surface maps of the K subgiant show starspots widely distributed along latitude and longitude. A prominent, non-axisymmetric 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 in a time scale of one month. We have determined the surface shear rate of the K component from the starspot maps reconstructed 10 days apart in 2017 Nov--Dec, through the cross-correlation method. The surface differential rotation parameters are $\Omega_{eq} = 1.591 \pm 0.002$ rad d$^{-1}$ and $\Delta \Omega = 0.035 \pm 0.003$ rad d$^{-1}$. The absorption lines contributed from the tertiary component are detected in all LSD profiles of SZ Psc, and we measure the radial velocity of the binary system and the tertiary component to derive an elliptical orbit with a period of $1530 \pm 3$ days and a mass of $0.75 \pm 0.06$ M$\odot$ for the tertiary component.