Seminars 2026

Magnetars are neutron stars powered by their extremely strong magnetic fields. Their X-ray outbursts, short bursts, timing irregularities, and radio activity provide unique opportunities to study neutron-star interiors, magnetospheres, and fundamental physics under extreme conditions. In this talk, I will review how high-cadence X-ray observatories, especially the Neutron Star Interior Composition Explorer (NICER), improved our understanding of magnetar activity. I will highlight observations of several magnetars, including one that may represent a missing link between magnetars and rotation-powered pulsars, as well as pulse-peak migration that may indicate tectonic motion in the neutron-star crust. I will then focus on SGR 1935+2154, the first Galactic magnetar known to emit a fast radio burst (FRB). Its activity in 2020 confirmed the magnetar-FRB connection, and broadband spectroscopy suggests that the FRB-associated X-ray burst may be spectrally distinct from the regular bursts observed during the burst storm. High-cadence monitoring of its 2022 outburst revealed two glitches, an episode of rapid spin-down, intense bursting activity, and rapid spectral softening around the time of an FRB. These observations suggest that energetic flares may change the magnetospheric environment and create conditions favorable for coherent radio emission. Finally, I will discuss recent evidence for vacuum birefringence from X-ray polarimetry, as well as searches for gravitational waves from magnetars.

Feedback from massive stars is one of the greatest sources of energy and momentum in the interstellar medium (ISM). Ionizing radiation, stellar winds and supernovae collectively shape the molecular clouds, the birthplace of stars, as well as their host galaxies. They play a significant role in regulating the entire star formation process.

In this colloquium, I will provide an overview of how feedback interacts with the ISM, and how astrophysicists examine their impact using state-of-the-art computational simulations. Specifically, I will discuss why the highly-asymmetrical nature of supernova remnants may impose a problem to our current feedback models. I will demonstrate that supernovae could actually induce a higher dynamical impact to its local ISM than what was previously expected. 

In the standard cosmological model, the matter content of the Universe is dominated by dark matter—an invisible component that governs the formation and evolution of cosmic structures. While dark matter cannot be observed directly, its gravitational influence can be detected through the deflection of light from background sources, a phenomenon known as gravitational lensing. In this talk, I will introduce the gravitational lensing effect in galaxy clusters and explain how it serves as a powerful probe for detecting dark matter and constraining its physical properties. I will also present a new approach by using radial acceleration relation to constrain the dark matter self-interacting cross-section. 

[CAG-ES joint colloquium] Venue: Room S101, 1F, Research Building  (S101 教學研究大樓)

In my talk, I want to discuss three ideas on how one could use quantum sensors for neutrino physics. I will discuss first how to use SQUIDs or similar devices to constrain neutrino charges. In the second half of my talk, I will discuss the detection prospects to use laser and atom interferometers to look for the Cosmic Neutrino Background. I will also briefly mention Dark Matter in that part.

A scalar field non-minimally coupled to the spacetime curvature could become unstable in a wide class of black hole and cosmological spacetimes, leading to potentially observable effects. One naturally asks if a similar situation happens for the vector field counterpart. We will show that the instability is well-controlled in all Petrov type D spacetimes filled with comoving perfect fluid, including common cases such as FLRW, $\Lambda$LTB, Kerr-Newman-(A)dS. Explicitly, at the linear order in the eikonal limit, one cannot trigger tachyonic instability without ghost and/or gradient instabilities.

How did our Moon form, and when? What evidence supports its origin? While it is often said that the Moon formed from a giant impact, many mysteries remain unsolved. Even today, no single explanation fully aligns with both astronomical and geophysical data. I will review what we currently know about the Moon’s formation, explore the different possible scenarios and their challenges, and discuss the critical new data needed in the future to better constrain the origin of our Moon. I will focus on the question of its composition and show that many mysteries are still standing with potentially  important implications for the formation of terrestrial planets.

*About the speaker: Sebastien Charnoz is a professor of planetary science at Paris University and IPGP (Institut of the physics of the Earth). He is a theoretician, specialist of planet formation and satellite formation. He was a member of the Cassini-Huygens missions, in the camera team, and is now a member of the MMX space mission to Phobos, and HERA space mission to  binary asteroids Didymos. 

 [CAG-ES special lecture]

Time: 03:30 pm

Venue: Room E301, College of Science Building, Gongguan Campus

Star formation activities characterize the evolutionary stages of galaxies. As stars are born from the cold and dense molecular clouds, star formation is governed by the amount of molecular gas and how efficiently molecular gas is converted into stars (i.e., the star formation efficiency, SFE). However, the physical mechanisms that alter the level of star formation in galaxies are not well understood, due to a lack of precise knowledge in molecular gas mass and SFE variations across galaxies at different evolutionary stages.

In this talk, I will present a series of work that tackle these challenges, using state-of-the-art telescopes including ALMA, JWST, and the Green Bank Telescope. We started by establishing an accurate, physics-grounded prescription for the CO-to-H2 conversion factor (α_CO) — a factor that has been causing major uncertainties in current molecular gas and SFE measurements. By re-evaluating molecular gas mass and SFE with our new prescription across ~150 galaxies ranging from starbursts to quiescent galaxies, we have revealed systematic differences in SFE which were obscured in previous studies due to limited α_CO knowledge. Our results show evidence that SFE variation is a dominant factor driving both the enhancement and regulation of star formation in galaxies. I will also talk about my ALMA and JWST programs in progress, which aim to uncover the exact molecular gas conditions and feedback processes that lead to galaxy quenching. 

Discovering rings around exoplanets is among the next major milestones of extra-solar exploration. Yet, despite technical feasibility with existing instruments, no Saturn-like exoring has been confirmed to date. While part of the explanation is observational biases, the absence of detection also comes from our profound lack of knowledge about what are rings, how they form, and what are their lifetimes. With the MRT project (CAG, NTNU), we aim to answer these questions from the perspective of rings demographics in the Solar System. The existence of ring systems around small Solar System bodies gives us the opportunity of gathering many instances of rings, enabling statistical studies of their properties and natural evolution processes. To do so, the MRT project will run a network of telescopes dedicated to stellar occultations distributed across Taiwan’s territory, while benefiting from theoretical advances brought by the companion French project WRAPS: “Where do Rings Appear in Planetary Systems?”

[CAG-ES joint colloquium] Time:14:20 pm, Venue: Room S101, 1F, Research Building  (S101 教學研究大樓)

Are we alone? You often hear this question because the formation of a terrestrial planet had been, theoretically, considered difficult if not impossible. This was changed after the NASA Kepler and K2 missions found that rocky planets are ubiquitous. Afterwards, it has become popular to resolve the natal environment of rocky planets, the protoplanetary disks, and then investigate the planet-formation activities observationally/theoretically. Without a surprise, most of those studies concluded that the protoplanetary disks we resolved are efficiently forming rocky planets, which, however, yield planets that are too wet and too carbonaceous to compare with our own Earth. Are we not alone? This is the issue I have been tackling. In this talk, I will provide an overview of this research field and outline our contribution/destruction.

The present-day Solar System is thought to have formed a few billion years ago from the solar nebula—a giant cloud of dust and gas in interstellar space. During the transformation of a gaseous nebula into a star–disk system, several fundamental physical mechanisms come into play simultaneously. This yet remains one of the most challenging frontiers in the field, as it requires disentangling the physical processes across a wide range of spatial and temporal scales.

In this talk, I will discuss the different evolutionary stages of star formation, starting from the collapse of a magnetized molecular cloud core to the formation of a star–disk system, where planets may also form and follow Keplerian rotation.  In addition, I will briefly discuss the influence of magnetic fields on fragmentation scales both at the molecular clump and disk scales, which may, in turn, influence the prestellar core properties and prospects of giant planet formation, respectively. I will present the results from the global numerical simulations of the magnetized gravitational collapse of a prestellar cloud core, encompassing a wider range of equation-of-states that might provide insights on the direct mass infall onto the central protostar during the earliest stages of core collapse, a key physical process that drives the protostar’s mass growth. Afterwards, I will present our results based on both theoretical and observational grounds that show the existence of a distinct transition zone at the envelope-disk interface (EnDTranZ), through which infalling gas motions from the collapsing envelope gradually settle down into Keplerian motions within the young disk. Lastly, I will talk about results on the long-term PPD evolution from the global disk simulations, including the occurrence of accretion bursts, dust growth, and prospects for first-generation planetesimal formation. Together, these results might offer insights into the interplay of magnetic fields, thermal pressure, self-gravity, mass accretion, and angular momentum transportation that governs the journey from collapsing cores to planet-forming disks.

The turbulent region upstream of the quasi-parallel bow shock, called the foreshock, consists of back-streaming particles and waves that interact with each other and create different transients that impact their surroundings and the downstream plasma. We focus on the compressive transients detected in the subsolar foreshock region. Using spacecraft data from the Magnetospheric Multiscale Spacecraft (MMS) mission, we conduct a statistical analysis on the characteristics and the occurrence rate of the different types of compressive structures, such as plasmoids, SLAMS and mixed structures. We find that the fast or very dense solar wind increases the probability of appearance of all the structures, as well as, the modification of the solar wind in the foreshock. Additionally, we present a multi-spacecraft analysis, based on MMS, THEMIS and OMNI measurements, to explore the relationship between the foreshock compressive structures and the magnetosheath jets. Based on the results obtained, we propose that the dominant formation mechanism of the jets in the magnetosheath is bow shock reformation, driven by steep and rapid changes in the upstream magnetic field orientation. Our findings demonstrate that the appearance of the magnetosheath jets is not directly connected with the foreshock compressive structures. Although the presence of plasmoids and SLAMS is not essential, they may contribute as a part of the random fluctuations involved in the creation of magnetosheath jets. In this respect, the level of the solar wind modification in the foreshock, rather than the pristine solar wind itself, governs the occurrence rate of the transients around the bow shock.