Upcoming talks

Accretion disks power many of the universe’s most luminous phenomena, acting as intermediaries that enable matter to shed angular momentum and accrete onto stars or compact objects. While angular momentum transport in disks has been extensively studied, especially in the context of magneto-rotational turbulence, significant challenges remain. These include reconciling simulation results with observed accretion rates and understanding state transitions in cataclysmic variables, x-ray binaries, and quasars.

In this talk, I explore how strongly magnetised disks — where azimuthal magnetic fields dominate, with energies exceeding the plasma’s thermal energy — may help resolve these issues. Interest in this regime is motivated by recent “hyper-refined” cosmological simulations, in which such a disk forms self-consistently around a black hole and supports super-Eddington accretion rates. Using local shearing-box simulations, we identify two distinct turbulent states: the previously known “high-β” state with modest accretion stresses (α << 1) and weak magnetic fields, and a new “low-β” state with strong, self-sustaining azimuthal magnetic fields, supersonic turbulence, and rapid accretion (α ≈ 1). The transition between these states is abrupt and occurs when sufficiently strong azimuthal fields are present, allowing the system to sustain a Parker-instability-driven dynamo. While many aspects of this behaviour remain uncertain, it offers a promising pathway to reconcile simulations and observations, with interesting implications for quasars and other rapidly accreting systems.

• Speaker: Jonathan Squire [Otago, New Zealand]
• Thursday 05 June 2025, 14:00-15:00
• Venue: MR12 DAMTP and online.
• Series: DAMTP Astrophysics Seminars; organiser: Loren E. Held.

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Abstract not available

• Speaker: Irene Shivaei (CAB, Madrid)
• Friday 18 July 2025, 11:30-12:30
• Venue: Ryle Seminar Room, KICC + online.
• Series: Galaxies Discussion Group; organiser: Sandro Tacchella.

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In recent years, free floating planets, i.e. those planets not found to be in a planetary system and with no observable companions, have begun to be found in microlensing and direct imaging surveys. Observations have shown that they have a wide variety of masses, ranging from terrestrial-like to giant planets. Microlensing surveys predict that there could be on order tens of free floating planets per star in the Milky Way. How these planets form and arrive on their observed trajectories remains a very open and intriguing question.

Whilst there are many mechanisms for forming free floating planets, e.g. ejections from planet-planet interactions or gravitational collapse of gas within molecular clouds, very few models have predicted the properties of free floating planets on a global scale. In this talk I will present the outcomes of state-of-the-art circumbinary planet formation models, that naturally produce a large abundance free floating planets per system. I will show the resulting mass and velocity distributions arising from the models, which will then be extended to include stellar populations of both single and binary stars, taking into binary fractions, and separations. The population distributions show clear observable features that can be investigated by future missions such as Roman, where evidence of these features will directly point to the specific formation pathways of specific planets, as well as informing on the processes of the planet forming environment in which they originated.

• Speaker: Gavin Coleman [Queen Mary University London]
• Monday 16 June 2025, 14:00-15:00
• Venue: Venue to be confirmed.
• Series: DAMTP Astrophysics Seminars; organiser: Thomas Jannaud.

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Abstract not available

• Speaker: Jonathan Squire [Otago, New Zealand]
• Thursday 05 June 2025, 14:00-15:00
• Venue: MR12 DAMTP and online.
• Series: DAMTP Astrophysics Seminars; organiser: Loren E. Held.

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According to the classical picture, type II migration is a slow, inward motion of the planet that either follows the disc viscous evolution (disc-dominated regime) or is much slower than that (planet-dominated regime). However, over the last decade, this picture of type II migration has significantly evolved, suggesting faster migration in the disc-dominated regime and even outward migration in the planet-dominated regime. In this talk, I will present recent results exploring the planet-dominated regime via live-planet, long-term simulations of planet migration. These show the existence of a correlation between the “gap-depth parameter” K and the direction of planet migration: planets migrate outward or inward depending on whether K is above or below a critical threshold Klim. This also implies the existence of “stalling radius” where migration halts. Using these results, I will introduce a toy model that allows to predict that massive planets accumulate in a band near the stalling radius (typically between 1–10 au), offering an explanation for the observed distribution of Jupiter-like exoplanets while challenging classical models of hot Jupiter formation.

• Speaker: Chiara Scardoni, Università degli Studi di Milano
• Wednesday 28 May 2025, 13:15-13:40
• Venue: The Hoyle Lecture Theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: Cristiano Longarini.

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We introduce Shamrock, a performance-portable framework written in C++17, targeting CPU and GPUs from any vendors using the SYCL programming standard, designed for numerical astrophysics across a wide range of hardware, from laptops to Exascale systems. Astrophysical schemes often share a common structure: a combination of neighbor searching and the numerical scheme itself. Shamrock embraces such abstractions to provide a common framework for multiple hydrodynamical schemes, namely finite elements, finite volume (with adaptive mesh refinement), and Smoothed Particle Hydrodynamics. To achieve this, at its core, Shamrock features a highly optimized, parallel tree algorithm with negligible construction overhead. This tree structure is coupled with a domain decomposition strategy that enables near-linear weak scalability across multiple GPUs. Shamrock achieves 92% weak scaling efficiency on 1024 AMD M I250x GPUs in large-scale Smoothed Particle Hydrodynamics (SPH) simulations. This corresponds to processing billions of particles per second, with tens of millions of particles handled per GPU , allowing us to perform the first SPH simulations above the billion-particle mark for protoplanetary discs.

• Speaker: Timothée Cléris
• Wednesday 28 May 2025, 13:40-14:05
• Venue: The Hoyle Lecture Theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: Cristiano Longarini.

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We introduce a unified Bayesian anomaly-detection framework for Cosmology, applied to the REACH global 21cm probe and also Type Ia supernovae. This approach embeds data-integrity beliefs directly into the inference process. Rather than excising contaminated or anomalous data points, the method employs a piecewise likelihood constrained by a Bernoulli prior and an Occam penalty, allowing anomalies to be down-weighted automatically while performing numerical sampling for parameter inference. When applied to supernova light curves, the framework yields precise estimates of brightness scaling, stretch, and colour, while also automating supernova sample and band selection. In the context of global 21 cm cosmology, it offers a principled way to mitigate radio-frequency interference (RFI), particularly within the band of interest. We also discuss additional potential applications of this methodology.

• Speaker: Samuel Leeney (University of Cambridge)
• Tuesday 27 May 2025, 11:15-12:00
• Venue: Martin Ryle Seminar Room, Kavli Institute.
• Series: Hills Coffee Talks; organiser: Charles Walker.

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The detection of GW170817 /AT2017gfo inaugurated an era of multimessenger astrophysics, in which gravitational-wave and multiwavelength photon observations complement one another to provide unique insight into astrophysical systems. A broad theoretical consensus exists, in which the photon phenomenology of neutron star mergers largely rests upon the evolution of the small amount of matter left on bound orbits around the black hole or massive neutron star remaining after the merger. Because this accretion disk is far from inflow equilibrium, its subsequent evolution depends very strongly on its initial state, yet very little is known about how this state is determined. Using both snapshot and tracer particle data from a numerical relativity/MHD simulation of an equal-mass neutron star merger that collapses to a black hole, we show how gravitational forces arising in a nonaxisymmetric, dynamical spacetime supplement hydrodynamical effects in shaping the initial structure of the bound debris disk. The work done by hydrodynamical forces is ∼10 times greater than that due to time-dependent gravity. Although gravitational torques prior to remnant relaxation are an order of magnitude larger than hydrodynamical torques, their intrinsic sign symmetry leads to strong cancellation; as a result, hydrodynamical and gravitational torques have a comparable effect. We also show that the debris disk’s initial specific angular momentum distribution is sharply peaked at roughly the specific angular momentum of the merged neutron star’s outer layers, a few r g c, and identify the regulating mechanism.

• Speaker: Yossef Zenati (Open University)
• Friday 13 June 2025, 11:30-12:30
• Venue: Ryle Seminar Room, KICC + online.
• Series: Galaxies Discussion Group; organiser: Sandro Tacchella.

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Abstract not available

• Speaker: Lukas Rossmanith
• Tuesday 03 June 2025, 11:15-12:00
• Venue: Martin Ryle Seminar Room, Kavli Institute.
• Series: Hills Coffee Talks; organiser: David Buscher.

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The ΛCDM model has long served as the standard paradigm in cosmology, offering a remarkably successful description of the Universe’s evolution. Yet, as observational precision continues to improve, persistent tensions have emerged across a range of probes, including the well-known Hubble constant discrepancy. While individual datasets may each align with ΛCDM, their collective interpretation reveals significant discordances that challenge the model’s internal consistency. In this talk, I will review the most prominent tensions in modern cosmology and assess their implications. I will present recent results pointing to hints of dynamical dark energy and interactions within the dark sector. I will also reflect on the growing influence of methodological choices, such as dataset selection and model assumptions, in shaping our cosmological conclusions.

• Speaker: Eleonora Di Valentino (University of Sheffield)
• Monday 02 June 2025, 13:00-14:00
• Venue: CMS, Pav. B, CTC Common Room (B1.19) [Potter Room].
• Series: Cosmology Lunch; organiser: Thomas Colas.

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In this talk I will describe the results of the PI2FA project focused on creating and applying tools for multi-dimensional modeling of partially ionized chromospheric plasma based on the single-fluid and two-fluid multi-species formalism. Scientific questions include clarifying chromospheric heating mechanisms, creating multi-dimensional realistic models of the solar chromosphere incorporating ion-neutral effects, and understanding neutrals’ role in prominence dynamics. The research focused on fundamental mechanisms of energy propagation and exchange in complex plasmas, such as waves, instabilities, and plasma-radiation interactions, seeking the transition from one-dimensional idealized models to multi-dimensional simulations, and observational support. Among the main conclusions, our research unveiled that multi-fluid effects become pronounced for waves with frequencies lower than typical inter-particle collisional frequencies, unlike suggested by theory of waves in homogeneous plasmas; we showed that ambipolar heating is most significant in the quietest regions, characterized by small-scale dynamo fields; we found that multi-fluid effects hold great importance within transition layers between cool and hot materials, such as the solar transition region and prominence-corona interface. Multi-fluid effects operate at scales beyond the resolution capabilities of even our most advanced instrumentation, necessitating specialized observational initiatives. Our initial steps in this direction allowed the detection of subtle differences in velocities between ions and neutrals, in line with theoretical predictions.

• Speaker: Elena Khomenko (IAC Tenerife)
• Monday 09 June 2025, 14:00-15:00
• Venue: MR14 DAMTP and online.
• Series: DAMTP Astrophysics Seminars; organiser: Roger Dufresne.

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I will present results on the properties of faint galaxies and AGN in the early Universe, building upon samples identified using Wide Field Slitless Spectroscopy with NIR Cam on the James Webb Space Telescope (JWST). This mode effectively turns JWST into an efficient redshift machine ideal to map out galaxy over-density. In my talk, I will focus on two topics: 1) The impact of galaxies and AGN on the reionization of the Universe, directly measured by mapping out the correlation between galaxies and ionized regions with quasar and galaxy transmission spectroscopy, and 2) The nature of broad Hα line-selected AGN (the so-called Little Red Dots) that JWST has uncovered in the first few Gyr, including new results based on the deep NIR Cam grism spectroscopy of their large-scale environments, deep high resolution spectroscopy unveiling the prevalence of dense absorbing gas and resolved Lyman-alpha mapping of the circumgalactic medium with VLT /MUSE. Finally, I will synthesize what these observations are learning us in the context of galaxy – SMBH co-evolution, SMBH formation and their role in cosmic reionization.

• Speaker: Dr Jorryt Matthee
• Thursday 29 May 2025, 16:00-17:00
• Venue: Hoyle Lecture Theatre, Institute of Astronomy.
• Series: Institute of Astronomy Colloquia; organiser: Jan Scholtz.

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Detecting signatures of planet formation in protoplanetary disks is essential for understanding how and where planets form. In this talk, I will summarise the various fingerprints of planets on the distribution of gas and dust solids in protoplanetary disk, and present Dust and gas observations of the disk around 2MASS J16120668 -301027, studied as part of the ALMA Large Program ‘AGE-PRO: ALMA Survey Of Gas Evolution in Protoplanetary Disks’, where several indicator of planet formation were recently identified in dust dust continuum emission and for molecular lines

• Speaker: Anibal Sierra (UCL)
• Tuesday 03 June 2025, 13:00-14:00
• Venue: Ryle seminar room + ONLINE - Details to be sent by email.
• Series: Exoplanet Seminars; organiser: Dr Dolev Bashi.

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Cosmological and astrophysical observations provide clear evidence for the existence of dark matter and have begun to map its distribution across vast cosmic volumes, yet key questions about its mass and interaction properties remain unanswered. Clues may lie in measurements that probe structure formation on the smallest scales—including dwarf galaxies, strong gravitational lenses, and stellar streams. These observations are already constraining aspects of the microphysical nature of dark matter, including its free-streaming behavior, decay lifetime, self-interactions, and possible interactions with the Standard Model. The upcoming generation of wide-field imaging surveys—including Euclid, the Vera C. Rubin Observatory, and the Roman Space Telescope—combined with spectroscopic surveys such as DESI and the new Via Project, will accelerate our ability to probe this physics. These efforts may detect, for the first time, dark matter halos below the threshold for star formation, directly testing a fundamental prediction of the standard cosmological model and offering the possibility of uncovering definitive astrophysical signatures of dark matter’s particle properties.

• Speaker: Risa Wechsler (Stanford/KIPAC)
• Thursday 19 June 2025, 16:00-17:00
• Venue: Hoyle Lecture Theatre, Institute of Astronomy (and online - details to be sent by e-mail).
• Series: The Kavli Lectures; organiser: Steven Brereton.

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Fast inward migration of planetary cores embedded in gaseous disks is a common problem in the current planet formation paradigm. Even though dust is ubiquitous in protoplanetary disks, its dynamical role in the migration history of planetary embryos has not been considered until recently. In this talk, I will show that a planetesimal embedded in a dusty disk leads to an asymmetric dust-density distribution that can exert a net torque under conditions relevant to planetary embryos up to several Earth masses. Building on the results or a large suite of numerical simulations for measuring this dust torque under a wide range of conditions, I will present the first study showing that dust torques can have a significant impact on the migration and formation history of planetary embryos.

• Speaker: Martin Pessah [NBI Copenhagen]
• Thursday 29 May 2025, 14:00-15:00
• Venue: MR14 DAMTP and online.
• Series: DAMTP Astrophysics Seminars; organiser: Loren E. Held.

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The Dark Energy Spectroscopic Instrument (DESI) has published BAO measurements from one year of data (DR1) in 2024 and 3 years of data (DR2) in 2025. The DESI collaboration argue that their measurements suggest that dark energy is evolving and that this evidence is stronger using the DR2 data. This result would have major implications for fundamental physics if true. I will present a new way of looking at BAO data which shows that the DR2 data are more consistent with the Planck LCDM cosmology than the DR1 data. The evidence for evolving dark energy from DESI BAO has therefore weakened as the data have improved. I will also discuss the impact of systematic errors if DESI BAO data are combined with Type Ia supernovae. In summary, I find very little evidence to suggest that dark energy is evolving.

KICC Special Seminar

• Speaker: George Efstathiou (IoA/KICC)
• Thursday 22 May 2025, 11:30-12:00
• Venue: Hoyle Lecture Theatre, Institute of Astronomy.
• Series: Kavli Institute for Cosmology Seminars; organiser: Steven Brereton.

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Gaps and rings are ubiquitous in observations of protoplanetary discs, and their existence may be attributed to (proto-)planets interacting with their natal environments. However, constraining protoplanet masses or ages – or even just confirming that protoplanets are the cause of these substructures – in any given observation requires a clear theoretical understanding of large numbers of different gap processes.

While theoretical and semi-analytical works exist for the viscously dominated end stages of gap evolution, due to the near inviscid nature of protoplanetary discs, time-dependent theories that can account for the nature of the mutual evolution between planet and disc are required to correctly interpret observations. I will first present on how planets form gaps in the simplest possible case: that of a low mass planet in an two-dimensional inviscid isothermal disc and show new analytical theory that is able to predict the initial stages of gap evolution in this case. Using both Athena++ numerical simulations and analytical arguments, I will then discuss how this picture is modified in the cases of viscous, thermodynamically active, or three-dimensional discs. I will show that the treatment of disc thermodynamics has significant effects on the planet disc interaction whereas viscosity – at the levels expected in protoplanetary discs – does not have a significant impact at the early stages of gap formation.

• Speaker: Amelia J. Cordwell (DAMTP)
• Tuesday 27 May 2025, 13:00-14:00
• Venue: Ryle seminar room + ONLINE - Details to be sent by email.
• Series: Exoplanet Seminars; organiser: Dr Dolev Bashi.

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In weakly ionized regions of protoplanetary disks, hydrodynamic instabilities likely play a key role in the development of turbulence, the formation of structures, and the transport of angular momentum. Among these, the vertical shear instability (VSI) stands out as a robust mechanism, requiring only baroclinic stratification and short thermal relaxation timescales to operate. In this talk, I will present results from axisymmetric radiation-hydrodynamical simulations of the VSI in passive, irradiated T Tauri disks, focusing on angular momentum redistribution, the emergence of secondary instabilities, and their role in VSI saturation. I will also discuss how dust and molecular cooling shape the regions where the VSI can operate, and compare these results with current observations of protoplanetary disks.

• Speaker: David Melon-Fuksman [MPIA Heidelberg]
• Tuesday 27 May 2025, 14:00-15:00
• Venue: MR4 DAMTP and online.
• Series: DAMTP Astrophysics Seminars; organiser: Loren E. Held.

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Abstract: High Mass X-ray Binaries (HMXBs) are thought to be one of the key contributors to the X-ray background during the Cosmic Dawn (CD) and Epoch of Reionization (EoR). However, in traditional semi-numerical simulations of the CD-EoR, the LX-SFR relation is assumed to be fixed across cosmic time, which may not be accurate, especially for low star-forming regions. To mitigate this problem, we model the total luminosity in a stochastic manner and implement it in our 21-cm simulation from the CD-EoR to see its imprints on the 21-cm signal statistics like the Power Spectrum and 21-cm brightness temperature maps. We find the effects of XRB stochasticity in the small-scale PS (k > 0.9) and in the 21-cm maps that may have the potential for detection via the lunar based observations.

• Speaker: Saswata Dasgupta
• Monday 19 May 2025, 13:15-13:40
• Venue: The Hoyle Lecture Theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: Cristiano Longarini.

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Hercules X-1 is one of the most complex X-ray binary systems. It is known to harbor a strongly magnetized, highly accreting neutron star. Thanks to the high inclination of the binary, a warped accretion disk precessing every 35 days, and the neutron star rotating every 1.27 seconds, the system exhibits a very broad range of timing and spectral phenomena. These include X-ray pulsations, eclipses, absorption dips, cyclotron lines, accretion disk winds and emission lines observed from various parts of the accretion flow. The unique properties of Her X-1 allow us to reveal the physics of accretion in X-ray binaries through means that are impossible in other systems. In September 2024, we carried out a large observational campaign on Hercules X-1 led by the new XRISM observatory. With over 200 ks of XRISM exposure time and a spectral resolution of better than 5 eV in the Fe K energy band (R>1000 – more than 10 times better than previous instruments), we are for the first time able to resolve and separate the various evolving emission and absorption components spectrally, and in time. In this talk, I will present the first results from this campaign, and the novel insights it provides into our understanding of accretion disk wind physics, and of X-ray pulsar accretion.

• Speaker: Peter Kosec
• Wednesday 21 May 2025, 13:40-14:05
• Venue: The Hoyle Lecture Theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: Cristiano Longarini.

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