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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: From the reminder of the Spring semester 2020 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, Mikael Granvik or Clif Kirkpatrick if you want to present a paper.

Present Program: The next talk will be on the 2nd of April - Organised remotely on Zoom

Presenter: Teemu Willamo:
Paper title: Waldmeier Effect in Stellar Cycles
Authors: Garg, S., Karak, B.B., Egeland, R. et al.
Reference: 2019, ApJ, 886, 132

Abstract: One of the most robust features of the solar magnetic cycle is that the stronger cycles rise faster than the weaker ones. This is popularly known as the Waldmeier Effect, which has been known for more than 100 yr. This fundamental feature of the solar cycle has not only practical implications, e.g., in predicting the solar cycle, but also implications in understanding the solar dynamo. Here we ask whether the Waldmeier Effect exists in other Sun-like stars. To answer this question, we analyze the Ca II H and K S-index from Mount Wilson Observatory for 21 Sun-like G─K stars. We specifically check two aspects of Waldmeier Effect, namely, (1) WE1: the anticorrelation between the rise times and the peaks and (2) WE2: the positive correlation between rise rates and amplitudes. We show that, except for HD 16160, HD 81809, HD 155886, and HD 161239, all stars considered in the analysis show WE2, while WE1 is found to be present only in some of the stars studied. Furthermore, the WE1 correlation is weaker than the WE2. Both WE1 and WE2 exist in the solar S-index as well. Similar to the solar cycles, the magnetic cycles of many stars are asymmetric about their maxima. The existence of the Waldmeier Effect and asymmetric cycles in Sun-like stars suggests that the dynamo mechanism which operates in the Sun is also operating in other stars. 

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