Important message from the main conference organisation

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 conference is now scheduled from the 4th to the 8th of July 2022, and our splinter session will be held on the 5th of July, from 2 to 6 pm.

Organisers

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

4 Astrophysics Group, University of Exeter, Exeter EX4 2QL, UK

5  Centre for Exoplanets and Habitability, University of Warwick, Coventry, CV4 7AL, UK2, Department of Physics, University of Warwick, Coventry, CV4 7AL, UK

Key information

Context

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 variability. This international, interdisciplinary effort has employed a variety of novel approaches (encompassing stellar and solar observations as well as simulations) to understand and model intrinsic stellar 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, which was proposed and accepted for the 2020 conference, remains timely for the following 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 still 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, HARPS, EXPRES, NEID). These datasets are providing a wealth of new information on solar/stellar variability 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. Since our proposal in 2019, more instruments are now fully operational and have been gathering data, such as EXPRES and NEID in the visible, and SPIROU in the infrared. This makes this proposal even more timely for 2022. Additionally, we are also closer to the launch of PLATO, for which ground-based RV-follow up, and thus stellar activity understanding, is crucial for the mass measurement of their planet candidates. This in turn will be critical for the success of future direct imaging missions such as HABEX and LUVOIR.
• A joint NASA-NSF initiative focusing on the RV variability of FGK stars ran in 2019-2020, and the final report was published in summer 2021 by Crass et al. (2021), which is therefore very timely for this splinter proposal. The initiative, undertaken by the Extreme Precision Radial Velocity working group, was tasked by NASA and NSF with determining the future requirements needed to enable the detection and characterisation of very low mass planets around such stars. This is in preparation for future missions such as HABEX and LUVOIR and, although this is originally a US initiative, many researchers outside the US are involved -- especially in the stellar variability working group. The final report of this initiative concluded that international efforts are needed to achieve small planet characterisation, such as a coordinated global coordination network, including European involvement as well as other countries, and funding stellar variability research as it was identified as the biggest challenge faced. EPRV is also highlighted in the recently published Decadal survey, with finding and characterizing Earth-like planets in the habitable zone a priority.
  

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 variability 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: https://coolstars21.github.io/