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URL: http://talks.cam.ac.uk/show/index/54089
Updated: 36 min 58 sec ago

Tue 06 Feb 13:00: Photoevaporation from Exoplanet Atmospheres: Understanding the Role of Stellar Winds and Considering Water-rich Atmospheres

Mon, 29/01/2024 - 11:37
Photoevaporation from Exoplanet Atmospheres: Understanding the Role of Stellar Winds and Considering Water-rich Atmospheres

The atmospheres of close-in exoplanets are extremely vulnerable to the effects of stellar UV to X-ray radiation. Photoevaporation can significantly alter planetary atmospheres or even strip them entirely, potentially rendering a planet uninhabitable. Understanding how these atmospheres evolve, persist, or fade away remains a fundamental challenge. In this talk, I will discuss two distinct but interconnected areas of photoevaporative research.

Firstly, I will discuss the interaction between the stellar wind and photoevaporating atmospheres. I will present 3D magnetohydrodynamic simulations of the interaction between the stellar wind and the photoevaporating outflow of a planet orbiting an M dwarf. This analysis reveals a diverse range of magnetosphere morphologies and plasma distributions due to the wind-outflow interaction. I consider how these changing morphologies might impact observable hydrogen Lyman-alpha signatures during planetary transits.

In the second part, I will delve into our current understanding of photoevaporation from water-rich atmospheres. Conventional analytic approaches often oversimplify the process, assuming two scenarios: the escape of only lighter hydrogen, or the dragging of oxygen along with escaping hydrogen. These two scenarios lead to two end cases: a planet that has retained its water-rich atmosphere or a planet which has lost its atmosphere, becoming dry and desiccated. I will challenge these oversimplifications by presenting results from a novel 1D multifluid hydrodynamic model of photoevaporation from a water-rich atmosphere, which shows oxygen escape should no longer be described by a simple on/off switch but instead requires careful modelling.

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Fri 17 May 11:30: Title to be confirmed

Mon, 29/01/2024 - 09:56
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Fri 01 Mar 11:30: Title to be confirmed

Fri, 26/01/2024 - 23:12
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Fri 16 Feb 11:30: TBD

Fri, 26/01/2024 - 23:08
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Fri 15 Mar 11:30: Title to be confirmed

Fri, 26/01/2024 - 23:08
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Fri 22 Mar 11:30: Title to be confirmed

Fri, 26/01/2024 - 23:04
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Wed 31 Jan 13:15: Photometric redshifts for high-z quasars in the era of LSST

Fri, 26/01/2024 - 15:14
Photometric redshifts for high-z quasars in the era of LSST

High-redshift (z>5) quasars have an important role in galaxy formation and evolution, including the growth of supermassive black holes, the coevolution of black holes and galaxies, and changes in the ionization state of the Universe. However, conventional methods for identifying high redshift quasars face challenges like high false-positive rates and require resource-intensive spectroscopic follow-up. With the upcoming data influx from optical deep wide area. imaging surveys like Vera Rubin Observatory Legacy Survey of Space and Time (LSST), the need for more efficient methodologies becomes crucial. This presentation introduces a new approach to identifying and characterising high-redshift quasars via optical and infrared Spectral Energy Distribution (SED) fitting, with an emphasis on reliable photometric redshifts. Our SED fitting method characterises quasar candidates by redshift, host galaxy contribution, intrinsic reddening, and luminosity. Additionally, we present a new empirical model for intergalactic hydrogen absorption. By comparing fits between quasar, galaxy, and star models, we are able to reduce rates of foreground contamination. The methodology presented shows promise in identifying quasar candidates and validating known objects, presenting an efficient avenue for discoveries and insights into the early Universe.

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Tue 20 Feb 13:00: TBC

Fri, 26/01/2024 - 11:20
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Fri 08 Mar 11:30: Title to be confirmed

Fri, 26/01/2024 - 09:41
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Wed 31 Jan 13:40: Supermassive black hole binaries and their spin evolution in gas-rich circumbinary discs

Thu, 25/01/2024 - 16:12
Supermassive black hole binaries and their spin evolution in gas-rich circumbinary discs

Supermassive black holes (SMBHs), which reside in the centres of massive galaxies, grow over cosmic time either via accretion of material or through mergers. For the latter, the SMB Hs must be brought close enough together for gravitational wave (GW) emission to take effect and lead to their coalescence. When galaxies merge, their central SMB Hs sink to the centre of the new halos as dynamical friction shrinks their orbits, where they can eventually form binary SMB Hs. Such binaries can be surrounded by gaseous circumbinary discs (CBDs) that impact the binary evolution, potentially providing a channel to shrink the binary orbit via dynamical interactions and accretion, and allow GWs to kick in. I will present 3D hydrodynamic simulations using the moving-mesh code Arepo that model gas-rich CBDs around binary SMBH systems covering a range of binary parameters including mass ratio, eccentricity and inclination. Not only do the simulations able to accurately capture the CBD , but thanks to novel refinement techniques gas streams that penetrate the disc cavity and minidiscs that form around the individual SMB Hs are resolved. These streams and cavities play an important role in torquing the binary and hence in shrinking its orbit. The evolution of the SMBH spin is also an important quantity to track for many reasons including its impact on the direction and power of radio jets, as well as for predicting the recoil velocities of newly merged BHs, which can leave an imprint on the resulting GW signal and impact the event rate in future LISA observations. As such, the simulations employ a sub-grid Shakura-Sunyaev accretion disc model that allows us to track not only the SMBH mass evolution but also the spin evolution that arises due to the accretion of material from the CBD . Therefore, we can track key timescales including binary inspiral times and spin alignment timescales.

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Thu 01 Feb 16:00: The Cosmic Baryon Cycle

Thu, 25/01/2024 - 11:11
The Cosmic Baryon Cycle

These are incredibly exciting times for extra-galactic astrophysics; above all for studies of galaxy formation and growth of structure. New observatories and advanced simulations are revolutionising our understanding of the cycling of matter into, through, and out of galaxies. In this talk I will provide an overview of the normal matter in collapsed structures and their chemical make-up. I will review the physical properties of the multi-phase gas at the interface between the Intergalactic Medium and the galaxies themselves, the so-called Circum-Galactic Medium (CGM). Together, these results provide an increasingly accurate description of the baryon cycle which plays many crucial roles in transforming the bare pristine Universe left after the Big Bang into the rich and diverse Universe in which we live today.

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Mon 29 Jan 13:00: Positivity bounds on effective field theories

Thu, 25/01/2024 - 00:27
Positivity bounds on effective field theories

Unitarity and analyticity are some of the most fundamental properties of S-matrix/quantum field theory. I will explain why these simple properties can impose strong constraints on the Wilson coefficients/coupling constants of a Lorentz invariant effective field theory. The central tool to use is the dispersion relations, and stronger bounds can be extracted if the crossing symmetry and unitarity conditions are fully utilized. I will discuss a couple of applications of these bounds in cosmology and particle physics.

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Tue 30 Jan 13:00: Planetary debris at white dwarfs

Wed, 24/01/2024 - 14:12
Planetary debris at white dwarfs

White dwarfs often accrete material from their remnant planetary systems, polluting their pristine hydrogen or helium atmospheres with metals. I will talk about how we can use this phenomenon to study exoplanetary compositions: photospheric metal lines can reveal the bulk compositions of the building blocks of rocky planets, while infrared emission gives us our only glimpse of the circumstellar debris before it arrives at the stellar surface. Results from dozens of systems tell us that accretion of chondritic material is common, though not universal, while the debris disks feeding accretion are dynamically active environments. The advent of JWST is opening new windows onto those disks, while large spectroscopic surveys are delivering an order-of-magnitude increase in the number of systems to study, and I will present ongoing work on both fronts.

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Fri 26 Jan 13:00: Spin-eccentricity interplay in merging binary black holes

Tue, 23/01/2024 - 14:17
Spin-eccentricity interplay in merging binary black holes

Orbital eccentricity and spin precession are precious observables to infer the formation history of binary black holes with gravitational-wave data. We present a post-Newtonian, multi-timescale analysis of the binary dynamics able to capture both precession and eccentricity over long inspirals. We show that the evolution of an eccentric binary can be reduced that of effective source on quasi-circular orbits, coupled to a post-Newtonian prescription for the secular evolution of the eccentricity. Our findings unveil an interplay between precession and eccentricity: the spins of eccentric binaries precess on shorter timescales and their nutation amplitude is altered compared to black holes on quasi-circular orbits, consequently affecting the so-called spin morphology. Even if binaries circularize by the time they enter the sensitivity window of our detectors, their spin orientations retain some memory of the past evolution on eccentric orbits, thus providing a new link between gravitational-wave detection and astrophysical formation. At the same time, we point out that residual eccentricity should be considered a source of systematics when reconstructing the past history of black-hole binaries using the spin orientations.

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Fri 26 Jan 13:00: Spin-eccentricity interplay in merging binary black holes

Tue, 23/01/2024 - 14:17
Spin-eccentricity interplay in merging binary black holes

Orbital eccentricity and spin precession are precious observables to infer the formation history of binary black holes with gravitational-wave data. We present a post-Newtonian, multi-timescale analysis of the binary dynamics able to capture both precession and eccentricity over long inspirals. We show that the evolution of an eccentric binary can be reduced that of effective source on quasi-circular orbits, coupled to a post-Newtonian prescription for the secular evolution of the eccentricity. Our findings unveil an interplay between precession and eccentricity: the spins of eccentric binaries precess on shorter timescales and their nutation amplitude is altered compared to black holes on quasi-circular orbits, consequently affecting the so-called spin morphology. Even if binaries circularize by the time they enter the sensitivity window of our detectors, their spin orientations retain some memory of the past evolution on eccentric orbits, thus providing a new link between gravitational-wave detection and astrophysical formation. At the same time, we point out that residual eccentricity should be considered a source of systematics when reconstructing the past history of black-hole binaries using the spin orientations.

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Wed 24 Jan 13:15: Gravitational Instability in Irradiated Protoplanetary Discs

Mon, 22/01/2024 - 11:13
Gravitational Instability in Irradiated Protoplanetary Discs

Young protoplanetary discs are expected to be massive and self-gravitating. The resulting gravitational instability (GI) causes large spiral structures in the disc and is a potential formation mechanism of giant planets via direct collapse of overdense regions of gas. The onset of GI is sensitive to thermal processes in the disc. It is, therefore, influenced by irradiation, such as that from the central star, which is expected to suppress the instability. However, this is dependent on the exact implementation of heating. In this talk I will present the results of 2D hydrodynamic simulations comparing the outcomes of two different heating prescriptions and show that a disc heated per unit area remains gravitationally unstable even under high levels of irradiation.

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Wed 24 Jan 13:15: The black hole mass-metallicity relation and insights into galaxy quenching

Mon, 22/01/2024 - 11:12
The black hole mass-metallicity relation and insights into galaxy quenching

Understanding the quenching of star formation in galaxies remains a central question within extragalactic astrophysics. In this talk I will explore the intricate interplay between galaxy quenching, black hole mass, and ‘starvation’. Previous research has established a strong correlation between quiescence and black hole mass, while other studies have highlighted the role of ‘starvation’—the halting of gas inflows—as a driving force in quenching, based on rapid gas depletion through star formation and chemical enrichment. This investigation bridges the gap between these findings, shedding light on the underlying mechanisms. Leveraging an extensive dataset of galaxies, this study uses random forest regression and partial correlation coefficients to uncover the fundamental relationships governing stellar metallicity with respect to other galaxy characteristics. I will show that for actively star-forming galaxies, stellar metallicity exhibits a strong dependence on stellar mass. However, intriguingly, for passive galaxies, a dramatically different narrative emerges, where the primary driver of stellar metallicity is the black hole mass. This means the integrated impact of black hole feedback emerges as the crucial determinant, cutting off gas inflows and inducing a state of ‘starvation’, ultimately leading to the observed rapid rise in stellar metallicity. This, in turn, paves the way for the transition of a galaxy from star-forming to quiescent. This finding not only deepens our understanding of galaxy evolution but also offers key insights into the intricate mechanisms shaping the fate of galaxies across cosmic epochs.

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Wed 24 Jan 14:00: On-shell techniques for the standard-model EFTs

Fri, 19/01/2024 - 10:07
On-shell techniques for the standard-model EFTs

Bypassing fields, operators and Lagrangians to target directly physical amplitudes advantageously avoids gauge and field-redefinition redundancies. Unitarity and analyticity allow for the construction of on-shell amplitudes, recursively in leg and loop numbers. After decades of developments focusing mostly on renormalisable theories, the associated on-shell amplitude techniques have recently been more extensively applied to effective field theories, including those which parameterise hypothetical heavy physics appearing beyond the standard model. The enumeration of independent operators can be substituted for that of contact-term amplitudes. In a given scattering, the possible kinematic structures can be fully characterised. Unitarity and analyticity allow for the derivation of positivity constraints, for the extraction of anomalous dimensions and of matching coefficients. This talk will cover some of these developments.

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Fri 02 Feb 11:30: IoA/KICC Open Day

Thu, 18/01/2024 - 21:49
IoA/KICC Open Day

Abstract not available

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