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

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  • H. Korhonen, European Southern Observatory
  • ,
  • R. M. Roettenbacher, Yale University
  • ,
  • S. Gu, Chinese Academy of Sciences, University of Chinese Academy of Sciences
  • ,
  • F. Grundahl
  • M. F. Andersen
  • G. W. Henry, Tennessee State University
  • ,
  • J. Jessen-Hansen
  • V. Antoci, DTU –Space
  • ,
  • P. L. Pallé, Instituto Astrofisico de Canarias, University of La Laguna

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.

TidsskriftAstronomy and Astrophysics
StatusUdgivet - feb. 2021

Bibliografisk note

Funding Information:
cA knowledgements. We would like to thank the anonymous referee for their comments that helped to improve the paper. Based on observations made with the Hertzsprung SONG telescope operated on the Spanish Observatorio del Teide on the island of Tenerife by the Aarhus and Copenhagen Universities and by the Instituto de Astrofisica de Canarias. R.M.R. acknowledges support from the Yale Center for Astronomy & Astrophysics (YCAA) Prize Postdoctoral Fellowship. S.G. would like to acknowledge support from the National Natural Science Foundation of China (grant Nos. 10373023, 10773027, 11333006, and U1531121). G.W.H. acknowledges long-term support from NASA, NSF, Tennessee State University, and the State of Tennessee Centers of Excellence program. V.A. was supported by a research grant (00028173) from VILLUM FONDEN. Funding for the Stellar Astrophysics Center is provided by The Danish National Research Foundation (Grant agreement no. DNRF106). This work has made use of the SIMBAD database at CDS, Strasbourg, France, and NASA’s Astrophysics Data System (ADS) services.

Publisher Copyright:
© ESO 2021.

Copyright 2021 Elsevier B.V., All rights reserved.

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