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Summary:
This is the homepage of the of the weekly Astrophysics journal club at the Division of Particle physics and astrophysics at the University of Helsinki. The meetings are of an unofficial nature and the main driver is to get together and discuss recent interesting papers. This meeting differs from the Seminar series in that people are not expected to talk about their own research and in that no credits will be awarded to students. The meetings are open to everyone and the aim is to stimulate discussion about recent results and provide the possibility to
 learn about research that is not necessarily connected to one's own field of expertise.

All topics are welcome ranging from planetary science to larger scales involving stellar
 astrophysics, Milky Way studies, galaxies and cosmology.
The only requirement is that the
 presented papers should be interesting to a wider audience and that they should be presented 
in such a way that also a non-expert can follow the presentation. In the meetings we discuss one paper each week for about 45 minutes.
 All meetings are in English.

Location: In the Spring semester 2021 the Journal club will be organised remotely on Zoom
Normal location: Conference room C310 on the third floor.

Time: Thursdays at 10.15-11.00am during term time.

Speakers: Please contact Peter Johansson or Mikael Granvik if you want to present a paper.

Present Program: The next talk will be on the 28th of January - Organised remotely on Zoom

Presenter: Thomas Hackman
Paper title: Common dynamo scaling in slowly rotating young and evolved stars
Authors: Lehtinen, J. et al.
References: 2020, Nature Astronomy, 4, 658

Abstract:  One interpretation of the activity and magnetism of late-type stars is that these both intensify with decreasing Rossby number up to a saturation level, suggesting that stellar dynamos depend on both rotation and convective turbulence. Some studies have claimed, however, that rotation alone suffices to parametrize this scaling adequately. Here, we tackle the question of the relevance of turbulence to stellar dynamos by including evolved, post-main-sequence stars in the analysis of the rotation-activity relation. These stars rotate very slowly compared with main-sequence stars, but exhibit similar activity levels. We show that the two evolutionary stages fall together in the rotation-activity diagram and form a single sequence in the unsaturated regime in relation only to Rossby numbers derived from stellar models, confirming earlier preliminary results that relied on a more simplistic parametrization of the convective turn-over time. This mirrors recent results of fully convective M dwarfs, which likewise fall on the same rotation-activity sequence as partially convective solar-type stars. Our results demonstrate that turbulence plays a crucial role in driving stellar dynamos and suggest that there is a common turbulence-related dynamo mechanism explaining the magnetic activity of all late-type stars.  


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