Observing the changing surface structures of the active K giant sigma Gem with SONG

Abstract

Aims: We aim to study the spot evolution and differential rotation in the magnetically active cool K-type giant star sigma 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 H alpha 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 deg. 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 $\alpha$ = 0.10 +/- 0.02. There is a weak correlation between the locations of starspots and enhanced emission in the chromosphere at some epochs.

19 pages, 10 figures, accepted for publication in Astronomy & Astrophysics, in the published version the Tables B1 and B2 will only be available via CDS

References in corpus (17)

• Molecfit: A general tool for telluric absorption correction. I. Method and application to ESO instruments
• No Sun-like dynamo on the active star $\zeta$ Andromedae from starspot asymmetry
• First Results From The Hertzsprung Song Telescope: Asteroseismology Of The G5 Subgiant Star {\Mu}Her
• Doppler Images and Chromospheric Variability of TWA 6
• Magnetic flux transport on active cool stars and starspot lifetimes
• Two-dimensional spectroscopy of a sunspot. III Thermal and kinematic structure of the penumbra at 0.5"
• The Two-Colour EMCCD Instrument for the Danish 1.54m Telescope and SONG
• Spot evolution on the red giant star XX Triangulum. A starspot-decay analysis based on time-series Doppler imaging
• Contemporaneous Imaging Comparisons of the Spotted Giant $\sigma$ Geminorum Using Interferometric, Spectroscopic, and Photometric Data
• Solar differential rotation in the period 1964 - 2016 determined by the Kanzelhöhe data set
• Antisolar differential rotation of the K1-giant sigma Geminorum revisited
• Conducting the SONG: The robotic nature and efficiency of a fully automated telescope
• Rotation--differential rotation relationships for late-type single and binary stars from Doppler imaging
• Anti-solar differential rotation on the active sub-giant HU Virginis
• Short-term evolution and coexistence of spots, plages and flare activity on LQ Hydrae
• Stellar Oscillations Network Group
• Time-series Doppler images and surface differential rotation of the effectively-single rapidly-rotating K-giant KU Pegasi

Cited by in corpus (1)

• Magnetic activities on two single-lined RS Canum Venaticorum binaries IM Pegasi and $\sigma$ Geminorum