Seminars 2025

Spectacular high-resolution imagery from the last decade has revealed that planet formation begins during the turbulent early stages of star formation, when stellar interactions profoundly shape protoplanetary disc structure. Observational evidence increasingly suggests that even apparently isolated stars bear the signatures of past stellar flybys — encounters that leave lasting imprints on disc morphology and may trigger the onset of planet formation. With the majority of stars residing in binary or higher-order multiple systems, these interactions become even more significant. In such environments, circumbinary discs can become misaligned with the binary orbital plane, creating distinctive conditions for circumbinary (P-type) planet formation. Binary companions also truncate their circumstellar discs, directly affecting the formation efficiency of circumstellar (S-type) planets. Recent observations of circumtriple disc architecture further raise intriguing questions about the viability of circumtriple planet formation. I will demonstrate that stellar multiplicity is not merely incidental but fundamental to understanding disc evolution and the diverse pathways of planet formation throughout the galaxy.

 [CAG-ES joint colloquium] Venue: Auditorium S102

Stellar flare and coronal mass ejection (CME) activity plays an important role in the habitability of planets. Previous studies proposed that star-planet interaction could enhance the flare activity on the exoplanet host stars. However, observations in the recent decade have not supported this theoretical prediction. In this talk, I will introduce the relationship along stellar activity, space weather, and exoplanet atmosphere, and then present the observational study of flare activity in M-type binary systems and compare the chromospheric active levels in active/inactive late-type stars, including the exoplanet hosts.

We introduce a set of recommendations and topics for discussions for young scientist to plan their careers, based on our experience in the International School for Young Astronomers (ISYA). We will touch upon a selection of topics ranging from career paths, job searches, project focus, networking, mental higiene, bias awareness and support systems. A general introduction will be followed by ample discussion time.

 [CAG-ES joint colloquium] Venue: Auditorium S102

Most stars are born with one or more stellar companions. Observational advances over the last decade have enabled high-resolution, interferometric studies of forming multiple systems and statistical surveys of multiplicity in star-forming regions. These have yielded new insights into how such systems form and how multiplicity affects disk evolution and planetary architectures. I will review recent observational discoveries of the youngest multiple systems. I will present the results of star cluster simulations modeling the formation and evolution of multiple systems, and I will discuss the role of dynamics and environment in setting stellar multiplicity.  Finally, I will highlight remaining numerical and observational challenges.

 [CAG special seminar] Venue: Conference Center (General Building) B1 Conference Room ; Time: 11:00-12:30 

In 2024, DESI (Dark Energy Spectroscopic Instrument) reported the time variation of the Hubble parameter based on BAO (Baryon Acoustic Oscillation) observations, showing that it contradicts the predictions of standard cosmology.

One interpretation of these discrepancies is to introduce an appropriate time variation to dark energy without changing the cosmological model, but our interpretation is based on observations from infrared galaxy surveys indicating that the Milky Way galaxy is located in a low-density region on a 300 Mpc scale.

In this talk, i will begin with a brief explanation of DESI, BAO, and the Hubble tension, and then consider the effects of the non-uniformity of matter distribution on cosmological scales on the expansion of the universe. I will explain how this allows us to interpret the DESI results and simultaneously explain the Hubble tension.

The climate and environmental crisis poses an ever-increasing threat to our societies. It is therefore more important than ever to seek to limit greenhouse gas emissions as much as possible. The academic world, as much as, if not more than, the rest of society, is strongly challenged. What attitude should we adopt to face these colossal challenges? Can we reduce the greenhouse gas emissions associated with our own activities? During the presentation, I will briefly outline the current state of knowledge on global warming and the main sources of emissions in our societies. I will specifically describe the efforts that have been made in France to quantify emissions from research laboratories and the specific problems encountered in reduction efforts.

 [CAG-ES joint colloquium] Venue: Auditorium S102

When photons, gravitational waves, and massive particles such as neutrinos pass close to a black hole their paths can significantly deviate from the paths in a flat space vacuum. This leads to the formation of multiple images or signals which can potentially be observed by telescopes and detectors on Earth. Observational examples for such lensing effects are the shadow of black holes and the images of accretion disks. In my talk I will now provide an overview on how we can probe the nature of the spacetime of an astrophysical black hole candidate and thus gravity in the strong field regime using gravitational lensing. For this purpose I will assume that we deal with particles in the classical high-frequency regime, namely photons and gravitational waves travelling along lightlike geodesics and massive particles travelling along timelike geodesics. I will first briefly outline how we can solve the equations of motion for black hole spacetimes analytically exact using elementary and elliptic functions and integrals. Then I will write down a lens equation and calculate the redshift and the travel time. I will show examples for different black hole spacetimes from general relativity and discuss how we can use these quantities in a multimessenger context to probe gravity in the strong field regime. I will also briefly outline how one can derive a magnification factor that relates a messenger signal in the reference frame of the observer to the same signal in the reference frame of its source. Finally, I will conclude my talk by outlining the steps we need to take to develop a systematic multimessenger approach for probing gravity in the strong field regime.

Understanding the possible observational features of black holes in quantum theory is a promising avenue to bridge the theoretical formulation of quantum gravity and astrophysical observations. In this talk, I will discuss the observational features of models of compact objects in the presence of horizon-scale quantum corrections, such that the object has no horizon. In particular, the models are characterized by strong redshifts, which, as I will demonstrate, render them perfect as black hole mimickers from observational points of view, including shadow images and gravitational wave ringdowns. However, despite the strong redshifts, the absence of a horizon always results in an excess intensity within the inner shadow of the images in a model-dependent manner. The excess intensity within the inner shadow could be a model-independent image feature to probe horizon-scale quantum effects.

Trans-Neptunian objects (TNOs) with large perihelion distances (q > 60 au) and semi-major axes (a > 200 au) provide insights into the early evolution of the Solar System and the existence of a hypothetical distant planet. These objects are challenging to observe, and thus their detections are still rare, yet they play a crucial role in constraining models of Solar System formation. Here we report the discovery of a Sedna-like TNO, 2023 KQ₁₄, nicknamed ‘Ammonite’, with q = 66 au, a = 252 au and inclination i = 11°. The orbit of Ammonite does not align with those of the other Sedna-like objects and fills the previously unexplained ‘q-gap’ in the observed distribution of distant Solar System objects. Simulations demonstrate that Ammonite is dynamically stable over 4.5 Gyr. Our analysis suggests that Ammonite and the other Sedna-like objects may have shared a primordial orbital clustering around 4.2 Ga. Furthermore, the stable orbit of Ammonite favours larger orbits (~500 au) rather than closer ones for a large hypothetical planet in present-day trans-Neptunian space.

The Canadian Hydrogen Intensity Mapping Experiment (CHIME) is designed to probe dark energy by mapping the large-scale structure of the Universe using the 21 cm line of neutral hydrogen. With its wide field of view and daily coverage of most of the northern sky, CHIME offers unprecedented survey power in the redshift range 0.8 < z < 2.5 and has already achieved 21 cm detections through cross-correlation studies. Beyond cosmology, CHIME has also become a leading instrument for the study of fast radio bursts, pulsars, and 21 cm absorption systems. In this talk, I will discuss CHIME’s design, data, recent discovery and
scientific progress, especially the first detection of the 21 cm auto power spectrum.

Venue: NTNU Gongguan campus B413

The release of the first black hole image by the Event Horizon Telescope (EHT) marked a new era in studying gravity and astrophysics in the strong-field regime. In this talk, I will introduce the key astrophysical concepts behind black hole imaging, explore how horizon-scale observations of M87 deepen our understanding of black hole astrophysics, and share some of my personal contributions to this rapidly evolving field.

[PHY colloquium] Venue: S101; Time: 14:20

[ES colloquium] Venue: S102; Time: 14:20