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Dynamics of nearly isolated, lenticular vortices: From waves to bulk instabilities?

In geophysical and astrophysical settings, rotating stratified flows often exhibit large-scale, nearly isolated vortices. Examples include the Mediterranean eddies in the Atlantic ocean, or the Great Red Spot (GRS) in Jupiter. These vortices have been widely studied using shallow-water or quasi-geostrophic models for decades. In particular, these models have successfully explained why these vortices maintain nearly lenticular shapes through time. However, prior reduced models have a blind spot when it comes to predicting the internal dynamics of such vortices, despite the fact that they are far from being motionless in their bulk (e.g. as observed for the GRS ). Various instabilities may sustain small-scale turbulence and accelerate the decay of large-scale vortices on long time scales.

Here, I will present a reduced model accounting for the bulk dynamics of large-scale pancake-like vortices. This model, which is developed in the framework of an interdisciplinary collaboration between pure and applied mathematics, is largely inspired by some ideas and methods pioneered by astrophysicists (e.g. S. Chandrasekhar or N. Lebovitz). First, I will describe the properties of the normal modes, because wave motions are often key to understanding the transition to turbulence in geophysical flows. As in the rotating non-stratified case, it will be shown that the wave spectrum solely consists of eigenvalues, and that the eigenvectors are all smooth. Moreover, it will be explained why some low-frequency waves/modes, which are governed by a mixed hyperbolic-elliptic problem for the velocity, can exist below the usual cutoff frequency of inertia-gravity waves. Next, by combining local and global stability methods, I will discuss whether some bulk instabilities could sustain small-scale bulk turbulence in strongly deformed stratified vortices.

• Speaker: Jérémie Vidal (ENS Lyon)
• Monday 20 October 2025, 16:00-17:00
• Venue: MR14 DAMTP and online.
• Series: DAMTP Astrophysics Seminars; organiser: Mattias Brynjell-Rahkola.

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Hydrogen-deficient binary stars with magnetic braking

Hydrogen-deficient binary stars comprise one star which has been stripped of its hydrogen through mass transfer to a binary companion. Observations show that the companion is able to accrete several solar masses without spinning up to critical rotation, and so there must be a mechanism to drain spin angular momentum from the accretor. We test magnetically coupled winds and magnetic star-disc coupling as possible mechanisms and find that, while the disc coupling is negligible, the winds are sufficient to allow the accretor to gain mass without spinning up to critical rotation. However, in order to fully replicate observations, time-dependent scalings of the dynamo-generated magnetic field are needed.

• Speaker: David Bour / IoA
• Wednesday 22 October 2025, 13:15-13:40
• Venue: Hoyle Lecture theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: Xander Byrne.

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Hydrogen-deficient binary stars with magnetic braking

Hydrogen-deficient binary stars comprise one star which has been stripped of its hydrogen through mass transfer to a binary companion. Observations show that the companion is able to accrete several solar masses without spinning up to critical rotation, and so there must be a mechanism to drain spin angular momentum from the accretor. We test magnetically coupled winds and magnetic star-disc coupling as possible mechanisms and find that, while the disc coupling is negligible, the winds are sufficient to allow the accretor to gain mass without spinning up to critical rotation. However, in order to fully replicate observations, time-dependent scalings of the dynamo-generated magnetic field are needed.

• Speaker: David Bour / IoA
• Wednesday 22 October 2025, 13:15-13:40
• Venue: Hoyle Lecture theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: Xander Byrne.

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arXiv:2509.21822v2 Announce Type: replace Abstract: We investigate the spatial distribution, kinematics, and metallicity of stars in the Draco dwarf spheroidal galaxy using data from the Dark Energy Spectroscopic Instrument (DESI). We identify 155 high probability members of Draco using line of sight velocity and metallicity information derived from DESI spectroscopy along with {\it Gaia} DR3 proper motions. We find a mean line of sight velocity of $ -290.62\pm0.80$ km s$^{-1}$ with dispersion = $9.57^{+0.66}_{-0.62}$ km s$^{-1}$ and mean metallicity $\rm{[Fe/H]}$ = $-2.10\pm0.04$, consistent with previous results. We also find that Draco has a steep metallicity gradient within the half-light radius, and a metallicity gradient that flattens beyond the half-light radius. We identify eight high probability members outside the King tidal radius, four of which we identify for the first time. These extra-tidal stars are not preferentially aligned along the orbit of Draco. We compute an average surface brightness of 34.02 mag $\rm arcsec^{-2}$ within an elliptical annulus from the King tidal radius of 48.1 arcmin to 81 arcmin.

arXiv:2509.21822v2 Announce Type: replace Abstract: We investigate the spatial distribution, kinematics, and metallicity of stars in the Draco dwarf spheroidal galaxy using data from the Dark Energy Spectroscopic Instrument (DESI). We identify 155 high probability members of Draco using line of sight velocity and metallicity information derived from DESI spectroscopy along with {\it Gaia} DR3 proper motions. We find a mean line of sight velocity of $ -290.62\pm0.80$ km s$^{-1}$ with dispersion = $9.57^{+0.66}_{-0.62}$ km s$^{-1}$ and mean metallicity $\rm{[Fe/H]}$ = $-2.10\pm0.04$, consistent with previous results. We also find that Draco has a steep metallicity gradient within the half-light radius, and a metallicity gradient that flattens beyond the half-light radius. We identify eight high probability members outside the King tidal radius, four of which we identify for the first time. These extra-tidal stars are not preferentially aligned along the orbit of Draco. We compute an average surface brightness of 34.02 mag $\rm arcsec^{-2}$ within an elliptical annulus from the King tidal radius of 48.1 arcmin to 81 arcmin.

arXiv:2510.10021v1 Announce Type: new Abstract: Extracting cosmological information from the Euclid galaxy survey will require modelling numerous systematic effects during the inference process. This implies varying a large number of nuisance parameters, which have to be marginalised over before reporting the constraints on the cosmological parameters. This is a delicate process, especially with such a large parameter space, which could result in biased cosmological results. In this work, we study the impact of different choices for modelling systematic effects and prior distribution of nuisance parameters for the final Euclid Data Release, focusing on the 3$\times$2pt analysis for photometric probes and the galaxy power spectrum multipoles for the spectroscopic probes. We explore the effect of intrinsic alignments, linear galaxy bias, magnification bias, multiplicative cosmic shear bias and shifts in the redshift distribution for the photometric probes, as well as the purity of the spectroscopic sample. We find that intrinsic alignment modelling has the most severe impact with a bias up to $6\,\sigma$ on the Hubble constant $H_0$ if neglected, followed by mis-modelling of the redshift evolution of galaxy bias, yielding up to $1.5\,\sigma$ on the parameter $S_8\equiv\sigma_8\sqrt{\Omega_{{\rm m}} /0.3}$. Choosing a too optimistic prior for multiplicative bias can also result in biases of the order of $0.7\,\sigma$ on $S_8$. We also find that the precision on the estimate of the purity of the spectroscopic sample will be an important driver for the constraining power of the galaxy clustering full-shape analysis. These results will help prioritise efforts to improve the modelling and calibration of systematic effects in Euclid.

arXiv:2510.10021v1 Announce Type: new Abstract: Extracting cosmological information from the Euclid galaxy survey will require modelling numerous systematic effects during the inference process. This implies varying a large number of nuisance parameters, which have to be marginalised over before reporting the constraints on the cosmological parameters. This is a delicate process, especially with such a large parameter space, which could result in biased cosmological results. In this work, we study the impact of different choices for modelling systematic effects and prior distribution of nuisance parameters for the final Euclid Data Release, focusing on the 3$\times$2pt analysis for photometric probes and the galaxy power spectrum multipoles for the spectroscopic probes. We explore the effect of intrinsic alignments, linear galaxy bias, magnification bias, multiplicative cosmic shear bias and shifts in the redshift distribution for the photometric probes, as well as the purity of the spectroscopic sample. We find that intrinsic alignment modelling has the most severe impact with a bias up to $6\,\sigma$ on the Hubble constant $H_0$ if neglected, followed by mis-modelling of the redshift evolution of galaxy bias, yielding up to $1.5\,\sigma$ on the parameter $S_8\equiv\sigma_8\sqrt{\Omega_{{\rm m}} /0.3}$. Choosing a too optimistic prior for multiplicative bias can also result in biases of the order of $0.7\,\sigma$ on $S_8$. We also find that the precision on the estimate of the purity of the spectroscopic sample will be an important driver for the constraining power of the galaxy clustering full-shape analysis. These results will help prioritise efforts to improve the modelling and calibration of systematic effects in Euclid.

Stars often fall into black holes, and now it seems the opposite can also occur, producing an extra long-lasting explosion as the star is consumed from within

Miscibility in sub-Neptunes

Sub-Neptunes are found around 50% of Sun-like stars in our galaxy. Despite their ubiquity, we lack a comprehensive understanding of their interior structure. I will present the first evolving interior structure model for sub-Neptunes that accounts for the expected miscibility between silicate magma and hydrogen. I will discuss the concept of “binodal surfaces”, which represent phase transitions within sub-Neptunes and provide a physically/chemically informed boundary between a planet’s “interior”’ and “envelope”. I will discuss the many implications of miscibility, and a potential observational route to testing its prevalence.

• Speaker: James Rogers / IoA
• Wednesday 15 October 2025, 13:15-13:40
• Venue: Hoyle Lecture theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: Xander Byrne.

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Miscibility in sub-Neptunes

Sub-Neptunes are found around 50% of Sun-like stars in our galaxy. Despite their ubiquity, we lack a comprehensive understanding of their interior structure. I will present the first evolving interior structure model for sub-Neptunes that accounts for the expected miscibility between silicate magma and hydrogen. I will discuss the concept of “binodal surfaces”, which represent phase transitions within sub-Neptunes and provide a physically/chemically informed boundary between a planet’s “interior”’ and “envelope”. I will discuss the many implications of miscibility, and a potential observational route to testing its prevalence.

• Speaker: James Rogers / IoA
• Wednesday 15 October 2025, 13:15-13:40
• Venue: Hoyle Lecture theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: Xander Byrne.

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TBC

Abstract not available

• Speaker: Dr Ander Biguri & Dr Josh Kaggie
• Thursday 27 November 2025, 14:00-15:00
• Venue: Room A (East 1)/West Hub.
• Series: Data Intensive Science Seminar Series; organiser: Sri Aitken.

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TBC

Abstract not available

• Speaker: Dr David "Davidad" Dalrymple
• Thursday 20 November 2025, 15:00-16:00
• Venue: Room A (East 1)/West Hub.
• Series: Data Intensive Science Seminar Series; organiser: Sri Aitken.

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TBC

Abstract not available

• Speaker: Dr Matt Kenzie
• Thursday 13 November 2025, 14:00-15:00
• Venue: Room A (East 1)/West Hub.
• Series: Data Intensive Science Seminar Series; organiser: Sri Aitken.

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TBC

Abstract not available

• Speaker: Featurespace
• Thursday 06 November 2025, 14:00-15:00
• Venue: Room A (East 1)/West Hub.
• Series: Data Intensive Science Seminar Series; organiser: Sri Aitken.

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TBC

Abstract not available

• Speaker: Dr Stephen-John Sammut
• Thursday 30 October 2025, 14:00-15:00
• Venue: Room A (East 1)/West Hub.
• Series: Data Intensive Science Seminar Series; organiser: Sri Aitken.

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TBC

Abstract not available

• Speaker: Dr Chi Wang
• Monday 27 October 2025, 13:00-14:00
• Venue: Room A (East 1)/West Hub.
• Series: Data Intensive Science Seminar Series; organiser: Sri Aitken.

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TBC

Abstract not available

• Speaker: EF
• Thursday 23 October 2025, 14:00-15:00
• Venue: Room A (East 1)/West Hub.
• Series: Data Intensive Science Seminar Series; organiser: Sri Aitken.

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TBC

Abstract not available

• Speaker: Dr Boris Bolliet
• Thursday 16 October 2025, 14:00-15:00
• Venue: Room A (East 1)/West Hub.
• Series: Data Intensive Science Seminar Series; organiser: Sri Aitken.

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Strong cosmic censorship for de Sitter black holes

We discuss modern formulations of the strong cosmic censorship conjecture (SCCC) and possible resolutions supported by rigorous non-linear results for the spherically symmetric Einstein-Maxwell-scalar field system. We show that the presence of a positive cosmological constant suggests a violation of the SCCC at a fundamental level of regularity. Indeed, the blueshift mechanism occurring at the Cauchy horizon can be counterbalanced by the dispersive effects encoded in the exponential Price law along (cosmological) black hole event horizons. On the other hand, we show that, if non-smooth black hole solutions are allowed, then the aforementioned violations are non-generic in a positive co-dimension sense.

• Speaker: Flavio Rossetti (Gran Sasso Science Institute)
• Friday 24 October 2025, 13:00-14:00
• Venue: Potter Room / Zoom .
• Series: DAMTP Friday GR Seminar; organiser: Daniela Cors.

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arXiv:2510.09153v1 Announce Type: new Abstract: The Euclid mission aims to measure the positions, shapes, and redshifts of over a billion galaxies to provide unprecedented constraints on the nature of dark matter and dark energy. Achieving this goal requires a continuous reassessment of the mission's scientific performance, particularly in terms of its ability to constrain cosmological parameters, as our understanding of how to model large-scale structure observables improves. In this study, we present the first scientific forecasts using CLOE (Cosmology Likelihood for Observables in Euclid), a dedicated Euclid cosmological pipeline developed to support this endeavour. Using advanced Bayesian inference techniques applied to synthetic Euclid-like data, we sample the posterior distribution of cosmological and nuisance parameters across a variety of cosmological models and Euclid primary probes: cosmic shear, angular photometric galaxy clustering, galaxy-galaxy lensing, and spectroscopic galaxy clustering. We validate the capability of CLOE to produce reliable cosmological forecasts, showcasing Euclid's potential to achieve a figure of merit for the dark energy parameters $w_0$ and $w_a$ exceeding 400 when combining all primary probes. Furthermore, we illustrate the behaviour of the posterior probability distribution of the parameters of interest given different priors and scale cuts. Finally, we emphasise the importance of addressing computational challenges, proposing further exploration of innovative data science techniques to efficiently navigate the Euclid high-dimensional parameter space in upcoming cosmological data releases.