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Stellar astrophysics group at University of Helsinki

Last modified by Thomas Hackman on 2025/11/24 12:37

The University of Helsinki astrophysics research group focuses on observations of stellar magnetic activity and its relation to exoplanets. The goal is to use these observations for a better general understanding of the magnetic activity of solar type stars, and in particular the Sun's activity. We also investigate the influence of a stars activity on the detectability and space climate of exoplanets. We use the following observations:

  • High resolution spectroscopy from e.g. the Nordic Optical Telescope (NOT) and telescopes of the European Southern Observatory (ESO)
  • Spectropolarimetry from e.g. the NOT and ESO
  • Ground based photometry from e.g. Automated Photometric Telescopes
  • Space based high precision photometry from e.g. the Transiting Exoplanet Survey Satellite
  • Ground-based Ca II H&K monitoring surveys
  • Radiocarbon measurements from tree rings

Methods we are using include Doppler imaging, Zeeman-Doppler imaging (ZDI) and time series analysis. Radiocarbon measurements are used to detect historical solar energetic particle events. An example of a ZDI-map and the Stokes I&V LSD-profiles for V889 Her (6/2021, publication in prep.) using NOT observations can be found here: ZDI-map and Stokes I&V profiles.

The research team consists of:

  • Thomas Hackman, PhD, University researcher
  • András Haris-Kiss, MSc, Doctoral researcher
  • Jyri Lehtinen, PhD, external member
  • Emilia Rintamäki, Doctoral researcher
  • Nora Routamo, MSc
  • Mikko Tuomi, PhD, University researcher
  • Teemu Willamo, PhD, external member
  • Joonas Uusitalo, PhD, Postdoc researcher (Laboratory of Chronology, University of Helsinki)

The group has extensive cooperation with a large number of national and international research institutes, e.g.: Aalto University (Maarit Korpi-Lagg), University of Oulu (Eija Tanskanen), Uppsala University (Oleg Kochukhov), AIP Potsdam (Ilya Ilyin), Konkoly Thege Miklós Astronomical Institute (Zsolt Kővári), Tennessee State University (Gregory Henry).

Recent presentations at seminars and scientific conferences

Posters at the  21st Cambridge Workshop: Cool Stars, Stellar Systems and the Sun in Toulouse 4-9 July, 2022: 

Recent publications by team members

Hackman, T., Kochukhov, O. , Viviani, M., Warnecke, J. , Korpi-Lagg, M.J., Lehtinen, J.J., 2024, "From convective stellar dynamo simulations to Zeeman-Doppler images", A&A 682, A156

https://www.aanda.org/articles/aa/pdf/2024/02/aa47144-23.pdf

Haris, A., Tuomi, M., Hackman, T., 2024: "Homogeneous search for spot transits in Kepler and TESS photometry of K ﹣ M-type main-sequence stars", A&A, in press.

https://arxiv.org/abs/2502.18129

Kochukhov, O., Hackman, T., Lehtinen, J.J, 2023, Surface distribution of small-scale magnetic field on the active cool star LQ Hydrae, A&A 680, L17

https://www.aanda.org/articles/aa/pdf/2023/12/aa47930-23.pdf

Lehtinen, J.J., Käpylä, M.J., Hackman, T., Kochukhov, O., Willamo, T., Marsden, S.C., Jeffers, S.V., Henry, G.W., Jetsu, L., 2022, ”Topological changes in the magnetic field of LQ Hya during an activity minimum”, A&A 660, A141

https://www.aanda.org/articles/aa/pdf/2022/04/aa36780-19.pdf

Nikbakhsh, S., Tanskanen, E., Hackman, T., 2025, Magnetic evolution of complex solar active regions, A&A, subm.

Tuomi, M , Lehtinen, J.,J., Henry, G.,W., Hackman, T., 2024, "Characterising the stellar differential rotation based on largest-spot statistics from ground-based photometry", A&A 689, A262

https://doi.org/10.1051/0004-6361/202449861

Uusitalo, J, 2024, "Astronomical and terrestrial anomalies in high-latitude tree-rings", PhD thesis, University of Helsinki, Report series in astronomy 34

https://helda.helsinki.fi/items/65e513f9-efec-400f-8ca4-5971f4bd80ff

Uusitalo, J, Arppe L., Helamaa, S. et al., 2022, "From lakes to ratios: 14C measurement process of the Finnish tree-ring research consortium", NIMPB 519, 37

https://doi.org/10.1016/j.nimb.2022.03.013

Uusitalo, J., Golubenko, K., Arppe, L., Brehm, N., Hackman, T., Hayakawa, H., Helama, S., Mizohata, K., Miyake, F., Mäkinen, H., Nöjd, Tanskanen, E., Tokanai, F., Rozanov, E., Wacker, L., Usoskin, Oinonen, M, 2024, "Transient Offset in 14C after the Carrington Event Recorded by Polar Tree Rings, Geophysical Research Letters 51, e2023GL106632

https://doi.org/10.1029/2023GL106632

Willamo, T., 2022, "Stellar activity : starspots, magnetic fields and activity cycles", PhD thesis, University of Helsinki, Report series in Astronomy 30

https://helda.helsinki.fi/handle/10138/342285

Willamo, T., Hackman, T., Lehtinen, J.J., Korpi-Lagg, M., Kochukhov, Oleg, 2022, ”V889 Her: abrupt changes in the magnetic field or differential rotation?”, OJAp 5, 10

https://arxiv.org/pdf/2203.13398v2.pdf

Willamo, T., Lehtinen, J.J., Hackman, T., Käpylä, M. J., Kochukhov, O., Jeffers, S.V., Korhonen, H., Marsden, S.C, 2022, ”Zeeman-Doppler imaging of five young solar-type stars”, A&A 659, A71

https://www.aanda.org/articles/aa/pdf/2022/03/aa41649-21.pdf