Abstract

The Cesam2k20 stellar evolution code (https://www.ias.u-psud.fr/cesam2k20) was released to the public in 2023 during the first edition of this workshop. Since then, we have succeeded in building a community of users around the code (approximately 30 participants for each edition, including about ten from abroad), as well as a community of developers featuring numerous interns and three Ph.D. students (M. Vidal, D. Lu, and L. Gauvrit) who are actively contributing to the code’s development. Finally, in the summer of 2025, Cesam2k20 was merged with a related planetary evolution code, CEPAM. This code is already being used for comparisons related to the PLATO mission. This now allows Cesam2k20 to model the evolution of the internal structure of gas giants and stars. This now makes the code available to the exoplanet community.

In order to further expand the Cesam2k20 user community, improve users’ ability to work with the code, and support outreach to the exoplanet community, we propose organizing a fourth Cesam2k20 workshop—a two-day event—at the Jules Janssen Center. To incorporate the feedback we received during previous workshops, this year we plan to follow a central theme by organizing the sessions around the 94 Ceti system. This triple star system consists of an F-type star, whose oscillations have been detected, and two M-type dwarfs. A super-Jupiter, 94 Ceti Ab, orbits the primary component. This system will therefore serve as an example for a wide variety of modeling applications. The workshop will be open to both beginners and experienced users. Several sessions are planned, beginning with a presentation by an outside speaker on one of his specific uses of the code, followed by a hands-on exercise for participants based on the 94 Ceti case.

Scientific justifications

The Cesam2k20 stellar evolution code is designed to model the structural evolution of low- and intermediate-mass stars. Its development was initiated by P. Morel (OCA) in 1989, and it has been actively maintained for nearly 30 years, making it a mature code that implements a wide variety of physical processes. It has been validated multiple times through comparisons with other stellar evolution codes (ESTA-CoRoT model comparison 2006–2010, Aarhus Red Giant Working Group 2014–2020, Stellar Evolution Challenge 2005–2006, and currently PLATO WP121 100 comparisons). The code was recently identified by the PNPS as a candidate for community-supported status. This is part of a new charter for community-supported codes certified by the INSU. The goal now is to coordinate the efforts of the stellar physics community in developing and using a limited number of numerical codes. This second workshop is therefore a continuation of that effort.

Cesam2k20 is also heavily involved in preparations for the PLATO mission. The main objective of the PLATO mission is to detect and characterize rocky exoplanets orbiting bright, Sun-like stars, preferably within the habitable zone. In order to best characterize the observed exoplanets, PLATO will conduct photometric monitoring of host stars to obtain asteroseismic data and measure mode frequencies with a precision of 0.3–0.5 microHz. PLATO Science Management (PSM) is committed to using these observations to determine masses, radii, and ages (MRA) with accuracies of 15%, 2%, and 10%, respectively.

Determining a star’s MRA relies heavily on realistic stellar evolution models. The choice of physics and its implementation in the models are critical to obtaining unbiased results. In order to provide the most accurate parameters possible, the WP121 working group must provide grids of stellar models. Three generations of grids are being considered, each increasingly complex. The first grid will include standard physics, as implemented in most stellar evolution codes for decades. The Cesam2k20 stellar evolution code was chosen to calculate it.

The second stellar model grid will need to incorporate more sophisticated physics, particularly realistic transport mechanisms. The code that will be used to calculate this grid has not yet been selected, but Cesam2k20 is already a strong candidate. This new workshop therefore also aims to support the developments that would enable it to be selected. It will provide an opportunity to learn how to use the Cesam2k20 code, with a particular focus on transport mechanisms. This workshop is intended for the national and possibly international community of stellar physicists, as well as for researchers interested in the connections between stars and planets or in the characterization of stellar populations in the Milky Way. At the Paris Observatory, this workshop is of particular interest to colleagues working on Gaia stars and first-generation stars (currently at GEPI), as well as colleagues from the LESIA Star Division.

Preliminary program

–* Morning 1: General introduction to the code, as well as presentations
on the developers’ latest progress.

• Afternoon 1 and Morning 2: The following afternoon and morning will be devoted to exploring the models available in Cesam2k20. Each session will begin with an introductory presentation by an expert Cesam2k20 user on their area of expertise, followed by a hands-on exercise in which participants will apply what they’ve learned to the 94 Ceti system.

• Afternoon 2: The final afternoon will have a more open format. We will ask participants in advance to let us know their needs regarding Cesam2k20. We will select a few examples and explain how to use and configure the code to model the object of their choice. Then each participant will be free to work on their favorite object of study, with the help of the instructors.

Proposed speakers: Y. Lebreton, L. Manchon, M. Deal, J. P. C. Marques, Tristan Guillot, Ducheng Lu, Marin Vidal