Upgrading the Solar-Stellar Connection: News about activity in Cool Stars


In this splinter session, ten speakers presented results on solar and stellar activity and how the two fields are connected. This was followed by a lively discussion and supplemented by short, one-minute highlight talks. The talks presented new theoretical and observational results on mass accretion on the Sun, the activity rate of flare stars, the evolution of the stellar magentic field on time scales of a single cycle and over the lifetime of a star, and two different approaches to model the radial-velocity jitter in cool stars that is due to the granulation on the surface. Talks and discussion showed how much the interpretation of stellar activity data relies on the sun and how the large number of objects available in stellar studies can extend the parameter range of activity models.


Coronal magnetic activity is what makes Cool Stars special and distinguishes them from hotter stars without magnetic dynamos. Coronal activity also shapes the environment of many extrasolar planets, giving rise to a completely new perspective why we care about magnetic phenomena. While coronal magnetic activity has been studied for a long time, there are new and exciting insights from recent solar and stellar missions, and this session provided a forum to bring the stellar and solar astronomers together and discuss synergies from both fields.

In two key areas there is significant observational progress in the last few years:

  • New solar observations, especially IRIS (Interface Region Imaging Spectrograph) and SDO/AIA (Solar Dynamics Observatory, Atmospheric Imaging Assembly) have significantly improved the spatial and temporal resolution of solar observations, as well as their spatial/temporal/temperature coverage.

  • The sample size of stellar activity surveys has exploded, specifically due to the data from Kepler and COROT. We are now at a crucial stage to understand and recap the wealth of such observations, especially in the light of future high-energy missions like Athena+, which was recently selected by ESA for the L2 launch opportunity.

There is an intimate connection between the activity of our Sun and cool stars. Almost all interpretation of coronal activity starts from the Sun, where observations with a spatial and temporal resolution that is unmatched in stellar astrophysics are possible. On the other hand, active stars provide for more energetic events than commonly observed in our Sun, so that the stellar perspective can help to extend models to a wider range, e.g. to explain the so-called super-flares.

This splinter session consisted of ten major talks that presented both observational and theoretical studies of stellar activity. The content of these presentations will be summarized in the following sections. Each presentation was followed by a lively debate with the audience. In addition, several short, one-minute presentations allowed conference attendees to highlight relevant posters or advertize other work. Slides from the presentions are can be retrieved from Zenodo or from the conference website if the speakers decided to make them available.

In addition to this summary, presentations given in this splinter session may also be discussed as separate articles in this volume.

In the following sections, we summarize the results presented in this splinter session, roughly ordered by the timescale of the variability phenomenon, although all these phaenomena are of course related. Section \ref{sect:sun} presents infall on the Sun as a template for accretion in young stars, Section \ref{sect:flares} looks at stars with a high occurence rate of flares, Section \ref{sect:cycles} presents studies of phaenomena that change over the solar or stellar cycle, such as the stellar radius, angular momentum loss, and activity level, and Section \ref{sect:evolution} describes changes of the activity level over the lifetime of a star. Finally, we summarize this splinter session in Section \ref{sect:summary}.