Cool stars 21 Splinter session : Characterising stellar activity in the era of extreme radial velocity surveys of low-mass planets orbiting F-M stars

Important message from the main conference organisation


"We regret to announce that the Cool Stars, Stellar Systems, and the Sun 21st Conference, originally planned to be held on June 22-26, 2020, in Toulouse, France, has to be postponed due to the current Covid-19 pandemic. The decision was discussed within the SOC and LOC, and agreed by all members of the organizing committees. We plan to reschedule CS21 in Summer 2021.

Preliminary dates (tbc) for CS21 would be 5-9 July 2021, at the same location in Toulouse, France. We foresee that the program that was designed for CS21 this year, including the planned invited talks and the scheduled splinter sessions, will still be largely relevant next year, unless other hot topics emerge till then. We will continue to provide updated information on the current CS21 website."




Nadège Meunier1, Xavier Delfosse1, Annelies Mortier2, Chris Watson3, Raphaëlle Haywood4, Heather Cegla5


1 Institut de Planétologie et d’Astrophysique de Grenoble, Université Grenoble Alpes, France

2 Kavli Institute & Cavendish Laboratory, University of Cambridge, UK

3 Astrophysics Research Centre, Queen’s University Belfast, UK

Harvard-Smithsonian Center for Astrophysics, USA

Observatoire de Genève, Université de Genève, Switzerland



Key information


1) Abstract submission for this splinter session is closed (see here for details). Depending on the details of the conference postponement, this will open again in 2021. We will keep you updated.


2) For details of the purpose of the session, and more information on our invited speakers, please read on! Our schedule can be found here.
We look forward to hopefully seeing you in Toulouse!




A wide variety of complementary techniques have been used for a long time to characterise stellar variability, from both ground-based observatories and space missions. Examples of such techniques include studies of: chromospheric emission, X-ray emission, brightness variability (at various wavelengths), astrometry, Zeeman-Doppler imaging, and polarimetry. The interpretation of these observables is complex because they are disk-integrated and subject to several sources of degeneracy. 

Stellar variability, in turn, also manifests itself in radial-velocity (RV) measurements. On the one hand this can be beneficial, and the field of asteroseismology exploited this principle long before the advent of space-based photometric missions such as CoRoT and Kepler. On the other hand, soon after the discovery of 51 Peg b it became clear that stellar variability can have significant consequences for the exoplanet field: stellar signals can hide the signals of true low-mass planets, mimic their existence, and affect the mass characterisation of the detected planets.

While the number of exoplanets detected using RVs has dramatically increased over the last two decades (largely thanks to the stability of modern spectrographs), current measurements still face a limit on the obtainable RV precision of around 1 m/s. This limit is largely due to stellar variability, and it has been shown that RVs are sensitive to many processes occurring in the photospheres of stars arising from either magnetic activity, photospheric flows (occurring on various scales), or a combination of both. As a consequence, the interpretation of an RV time series is complex, but it also includes information on processes that are not observable using other techniques, which makes it very interesting for our understanding of stellar physics.

Significant attention has, therefore, been devoted by a large community to the understanding of RV signals generated by stellar activity. This international effort has employed a variety of novel approaches (encompassing stellar and solar observations as well as simulations) to understand and model stellar activity signals, with the ultimate goal of improving planet detection limits. These studies may prove critical to advance our understanding of low-mass planet formation, evolution, and ultimately life - and provides the focus for this splinter session.


The organisation of this splinter session is timely for two main reasons:

  • Exoplanet science is pushing us to examine the surfaces of stars at an unprecedented level of detail, since it is necessary to mitigate stellar RV signal contributions in order to detect very small planetary signals. In turn, these exquisite solar and stellar RV observations will teach us about stellar magnetic fields, velocity flows and convection processes. Characterising these surface processes across a range of spectral types and rotation rates will provide much needed constraints to theories and simulations of stellar interior structure dynamos, in addition to their impact on exoplanet characterisation. The challenge is crucial for Sun-like stars and M stars alike: in both fields, a better knowledge of the star is needed to propose new stellar RV mitigation methods based on a good understanding of physical processes.
  • Recent efforts have permitted the development of new spectrographs, to extend the wavelength coverage to the infrared in addition to the traditional observations in the visible part of the spectrum (CARMENES since 2016, GIANO-B since 2017, SPIRou since 2019), and to push technological limits to reach 10-20 cm/s stability (ESPRESSO/VLT, EXPRES/DCT). In addition, several facilities have implemented, or plan to implement, long-term RV monitoring of disk-integrated solar spectra (HARPS-N since 2015, HARPS since 2018). These datasets are providing a wealth of new information on solar/stellar activity effects in RV measurements. In particular, the recent solar results emerging from these projects show that there is even more to be learnt about the Sun than had previously been realised.

Scientific motivation


The issues outlined above are at the interface between stellar physics, solar physics and exoplanet science, and are of interest to a large community with various interests. In this splinter session, we wish to focus on the spectroscopic manifestation of stellar variability, but covering a large range of spectral types (F-M), where similar processes are present but with different relative impacts on radial velocities. We will also focus on attempting to significantly improve our understanding of RV variability to enable the detection of very low-mass planets. We have therefore two main objectives:


  • The communities involved in these approaches have developed different techniques, with little exchanges between them (in particular, between solar-type stars and M dwarfs), although some processes are similar. The characterisation of stellar activity is also incomplete in both cases. For example, spot contribution is believed to be dominant in M dwarfs, with properties (lifetime in particular) that may be different from solar-type stars; however, there are indications that the inhibition of the convection in plages due to magnetic fields could play a role on long timescales even for such stars. On the other hand, the inhibition of the convective blueshift appears to play a dominant role for a star like the Sun, but many other processes cannot be neglected in the 0.5-1 m/s range, even at long timescales. Hence, it is crucial to cover a wide range of spectral types to have a more complete view of these processes, focusing here on main sequence stars, to exchange on these processes as well as on the techniques that have been developed so far.
  • Given the current challenges, it is important to have a community effort to identify the next steps and define what we need to improve our understanding of RV stellar variability. For example, future magnetohydrodynamic (MHD) simulations of these processes (3D MHD simulations of dynamos, structures such as spots or plages, convection…) will help to constrain models used to interpret RV data and build mitigating techniques based on physical conditions. We will also need inputs from other techniques (photometry for example) and/or simultaneous observations to be able to lift certain degeneracies.

We will encourage attendees to develop collaborations across the various observational and theoretical techniques. There is much to be gained from further exploring those synergies and comparing the Sun to other stars.


Main conference website


For any matters relating the main conference, including registration and the code of conduct, we refer you to the main Cool Stars 21 website:

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