Séminaires

PyLMT: A transient detection pipeline for the 4-m International Liquid Mirror Telescope
Kumar Pranshu (STAR)

Salle de réunion AGO (local -1/14), Institut d'Astrophysique et de Géophysique
Bâtiment B5c, Quartier Agora, Allée du 6 Août, 19C, B-4000 Liège 1 (Sart-Tilman)

The 4m International Liquid Mirror Telescope (ILMT) is India’s first optical survey telescope, which uses a spinning mercury mirror as its primary reflector. It is the only operational liquid mirror telescope globally, with a 22.3' FoV and limiting magnitude of g-mag ~22. The ILMT captures ~15 GB of imaging data per night, producing nearly 30 science frames. A key goal is discovering optical transients and variables, which is facilitated by an automated detection pipeline called the PyLMT. The pipeline employs image subtraction to generate difference frames, followed by a CNN-based classifier to distinguish real sources from artefacts in the resulting images. A second CNN classifies real sources based on host morphology, and known asteroids and solar system objects are identified for rejection. In a recent modification to the pipeline, new CNN models for transient detection and classification were trained using the transfer learning technique, employed between images from the Zwicky Transient Facility and the ILMT. Operational since November 2023, the pipeline has identified various transients and variables, including AGNs, asteroids, T Tauri outbursts, RR Lyrae, Eclipsing Binaries, and 21 supernova-like transient candidates. Five of the detected supernova candidates, viz. AT 2023yjc, 2024fxn, 2024zsm, 2024agkc, and 2024aifv were reported to the transient name server (TNS) as new discoveries.

The impact of rotational mixing in intermediate-age star clusters with extended main-sequence turn-offs and extended red clumps
Lorenzo Martinelli (University of Newcastle (Australia))

Salle de réunion AGO (local -1/14), Institut d'Astrophysique et de Géophysique
Bâtiment B5c, Quartier Agora, Allée du 6 Août, 19C, B-4000 Liège 1 (Sart-Tilman)

The extended main-sequence turn-offs (eMSTOs) and extended red clumps (eRCs) observed in intermediate-age star clusters challenge the traditional understanding of star clusters as simple stellar populations. In recent years, eMSTOs have been interpreted as signatures of fast stellar rotation, but the role of rotational mixing in shaping the colour–magnitude diagram (CMD) of these clusters remain uncertain. I will present a comparison between observed and synthetic CMDs for two intermediate-age clusters, NGC 419 and NGC 1817. Both clusters show a clear eMSTO and eRC, with turn-off stars showing large v sin i values. We used two grids of rotating stellar models, identical in input physics except for the efficiency of rotational mixing, to build synthetic clusters. Our results show that models with weak rotational mixing best reproduce both the eMSTO and eRC features and match the red clump luminosities and asteroseismic masses in NGC 1817. Strong mixing leads to post main-sequence CMD features that are too bright and inconsistent with observations. Overall, our findings suggest that rotational mixing might play a role in shaping eRCs, but its influence on intermediate-mass stellar evolution is definitely weaker than most stellar models predict.

Non-LTE modelling and stellar abundances of heavy Elements
Sema Caliskan (University of Uppsala)

Salle de réunion AGO (local -1/14), Institut d'Astrophysique et de Géophysique
Bâtiment B5c, Quartier Agora, Allée du 6 Août, 19C, B-4000 Liège 1 (Sart-Tilman)

The cosmic origin of elements heavier than iron remains one of the open questions in astrophysics. These elements are produced through neutron-capture processes in diverse astrophysical sites, but to disentangle their contributions and galactic evolution, we rely on accurate stellar abundances. Deriving these abundances requires reliable non-LTE models of stellar spectra. However, for many heavy elements, non-LTE modelling is still missing due to the scarcity of accurate atomic data. This challenge highlights the interconnected nature of atomic physics and astrophysical interpretation: without reliable atomic data, we cannot fully exploit the wealth of stellar observations.

During my PhD, I have worked at this intersection, aiming to improve atomic data and construct more accurate non-LTE models for heavy elements. I will present results from different stages of this work: from theoretical calculations of atomic structure and transition probabilities, to non-LTE modelling of copper and silver. I will show how improved atomic data, such as hydrogen collision rates and photoionisation cross-sections, impact non-LTE abundance determinations of these elements, and how refined abundance trends in turn sharpen our understanding of nucleosynthetic origins and galactic chemical evolution.