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Complete CV

Research works

Since the beginning of my PhD, I have been specializing in the study of massive binary and triple systems through detailed stellar models simulations, under the supervision of Dr Sylvia Ekström and Dr Patrick Eggenberger. These multiple systems are common among massive stars: around 70% of stars of more than 8 solar mass have one or more companions. I am particularly interested in the numerical methods used to predict the evolution of these systems (fast and detailed stellar evolution codes), as well as in the impacts of the assumptions made for the physics of the stellar components (mass loss, convective core boundary mixing, angular momentum transport in radiative zones, etc.) on these predictions.

Published papers

About eight months after the start of my thesis, I realized that most recent simulations that have aimed to use Jean-Paul Zahn's (1975, 1977) dynamical tidal formalism to predict the effect of these tides on the rotation and orbital evolution of close massive (≥ 8 M☉) systems have done so in a way that is inconsistent with this formalism. In my first publication, a letter to the editor published in January 2024 in the journal Astronomy & Astrophyics, I describe these inconsistencies with Zahn's formalism and discuss their consequences for the predictions of binary system models.

After a visit in Dr. Silvia Toonen's group in Amsterdam in June 2023 (and a second visit in June 2024), we began a collaboration that resulted in an extension of the triple system evolutionary code (TRES) developed in her group. I coupled TRES to the detailed code MESA, which allows me to perform state-of-the-art simulations of triple systems. These systems are particularly common among massive stars, as it is estimated that about 50% of them are in triple or higher multiplicity systems. Among very massive stars (≥ 50 M☉), observations tend to show that the proportion of triple or higher order systems could be even larger, up to 80%. However, most recent studies aiming at describing the evolution of these systems by numerical simulations use simplified methods for stellar evolution, (fast stellar codes). Despite the high performance of these fast codes, their reliability significantly decreases beyond 50 M☉, as this mass corresponds to the limit of the grid of stellar models on which they are based. In my second publication, an article published in June 2025 in the section "Stellar structure and evolution" of the journal Astronomy & Astrophyics, I compare the predictions of the fast code SeBa (used by default by TRES) with those of MESA in the mass range 8-120 M☉. I show that divergences between the two codes sharply increase beyond 50 M☉, especially when significant mass loss is taken into account. I demonstrate how these divergences in the predictions of the two codes lead to divergences in the predicted evolutionary pathways of triple systems. A particularly interesting result concerns the minimum value of the period avoiding mass transfer in the inner binary system, which is reduced by three orders of magnitude when SeBa is replaced by MESA. This result has important implications for gravitational wave emission in the triple compact objects scenario. The code developments necessary for the accomplishment of this project have been made open source at the same time as the publication of the article via two pull-requests (a PR of the AMUSE environment , a PR of the TRES code  ).

Ongoing research

Paper II: Chemical evolution of close massive binaries - tidally-enhanced or tidally-suppressed mixing?

In this ongoing project, I focus on the evolution of binary systems simulated with the Geneva Evolutionary Code (GENEC) and on the impact of tides raised by a companion on the predictions of stellar models. Specifically, I try to understand how chemical evolution is altered by tides, and how this depends on the chosen physical assumptions for the angular momentum transport (purely hydrodynamic models or magneto-hydrodynamic models). Stay tuned!

Paper III: Grids of single and binary models with GENEC - Impacts of the systems configurations and physical assumptions on the apsidal motion constant evolution

In this future project carried out in collaboration with Dr Sophie Rosu, we aim to provide a grid of GENEC models in the mass range 8-45 M☉, at solar metallicity, and to investigate how the internal structure constant (or apsidal motion constant/k2) reacts to different physical assumptions (overshoot, angular momentum transport) and initial conditions (velocity for the single star, orbital period and eccentricity for binary systems). The structure constant depends on the internal density profile of the components of close binary systems, and determines the precession of the line of apsides, a motion that can be detected in some close binaries. This constant naturally establishes a direct link between an observable phenomenon and the interior of binary components, making it a unique quantity to compare models to observations. There are available stellar model grids on the market that predict the value of k2, but these grids systematically ignore (or greatly simplify) the impact of rotation and binarity on the evolution of k2. By publishing this grid of models, we aim to fill this gap, and discuss the impacts of the systems configurations and physical assumptions on the predicted evolution of the internal structure constant. Stay tuned!

Co-author publications

Griffiths, A., Aloy, M.-Á., Hirschi, R., Reichert, M., Obergaulinger, M,. Whitehead, E. E., Martinet, S., Sciarini, L., Ekström, S., Meynet, G. 2025A&A...693A..93G.

Sibony, Y., Shepherd, K. G., Yusof, N., Hirschi, R., Chambers, C., Tsiatsiou, S., Nandal, D., Sciarini, L., Moyano, F. D., Bétrisey, J., Buldgen, G., Georgy, C., Ekström, S., Eggenberger, P., and Meynet, G. 2024A&A...690A..91S.

Tsiatsiou, S., Sibony, Y., Nandal, D., Sciarini, L., Hirai, Y., Ekström, S., Farrell, E., Murphy, L., Choplin, A., Hirschi, R., Chiappini, C., Liu, B., Bromm, V., Groh, J., and Meynet, G. 2024A&A...687A.307T.