Observing the changing surface structures of the active K giant σ Geminorum with SONG^★,★★

¹ European Southern Observatory (ESO), Alonso de Córdova 3107, Vitacura, Santiago, Chile
e-mail: heidi.korhonen@eso.org
² Yale Center for Astronomy and Astrophysics, Department of Physics, Yale University, New Haven, CT 06520, USA
³ Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, Yunnan Province, PR China
⁴ Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming 650216, PR China
⁵ School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 101408, PR China
⁶ Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
⁷ Center of Excellence in Information Systems, Tennessee State University, Nashville, TN 37209, USA
⁸ DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 328, 2800 Kgs. Lyngby, Denmark
⁹ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
¹⁰ Universidad de La Laguna, Departamento de Astrofísica, 38206 La Laguna, Tenerife, Spain

Received: 30 June 2020
Accepted: 11 December 2020

Abstract

Aims. We aim to study the spot evolution and differential rotation in the magnetically active cool K-type giant star σ Gem from broadband photometry and continuous spectroscopic observations that span 150 nights.

Methods. We use high-resolution, high signal-to-noise ratio spectra obtained with the Hertzsprung SONG telescope to reconstruct surface (photospheric) temperature maps with Doppler imaging techniques. The 303 observations span 150 nights and allow for a detailed analysis of the spot evolution and surface differential rotation. The Doppler imaging results are compared to simultaneous broadband photometry from the Tennessee State University T3 0.4 m Automated Photometric Telescope. The activity from the stellar chromosphere, which is higher in the stellar atmosphere, is also studied using SONG observations of Balmer Hα line profiles and correlated with the photospheric activity.

Results. The temperature maps obtained during eight consecutive stellar rotations show mainly high-latitude or polar spots, with the main spot concentrations above latitude 45°. The spots concentrate around phase 0.25 near the beginning of our observations and around phase 0.75 towards the end. The photometric observations confirm a small jump in spot phases that occurred in February 2016. The cross-correlation of the temperature maps reveals rather strong solar-like differential rotation, giving a relative surface differential rotation coefficient of α = 0.10 ± 0.02. There is a weak correlation between the locations of starspots and enhanced emission in the chromosphere at some epochs.