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The First Ionization Potential Effect in Solar/Stellar Coronae and Winds

Since 1963, we have known, or at least suspected, that element abundances in the solar corona and wind are different to those in the photosphere. Elements that are predominantly ionized in the photosphere and chromosphere, e.g. Fe, Si, Mg, are observed to be enhanced in abundance once transported to the corona and wind by a factor of typically 3-4, while elements that are mainly neutral are relatively unaffected. Due to the dependence on the ionization potential, this phenomenon has been dubbed the First Ionization Potential (FIP) Effect. A model capturing the FIP and effect and the various modifications to it seen in different coronal regions and the solar wind invokes ion-neutral separation by the ponderomotive force due to Alfven and magnetosonic waves propagating through the chromosphere. This acts on ions, but not neutrals, and depends on the interaction of the waves with the magnetic geometry of the solar atmosphere.

The launch of the Extreme Ultraviolet Explorer (EUVE) in 1992 afforded us the first observations of abundances in stellar coronae. This, and succeeding missions like Chandra, XMM -Newton and Suzaku, have shown that solar-like stars show a similar FIP effect, which transitions to Inverse FIP (IFIP), i.e. a coronal depletion of Fe, Si, Mg, etc., as the activity level increases.

In this presentation, I will review the theoretical basis of FIP and IFIP fractionation by the ponderomotive force. Fractionation by such means is new to solar physics and astrophysics, but is not new to science. Manipulation of atoms, molecules, biological samples, etc. by the forces due to refraction of photons from lasers, known as “optical tweezers”, has a rather long history in optical sciences, and won Nobel Prizes for Steven Chu (in 1997) and Arthur Ashkin (in 2018). Our model is a precise analog of this work, but with magnetohydrodynamic waves instead of optical photons. This connection leads to a better physical understanding of the mechanisms at work in FIP fractionated plasma.

Work supported by NASA Heliophysics Supporting Research Program (80HQTR20T0076), and by Basic Research Funds of the Office of Naval Research.

• Speaker: Martin Laming (Naval Research Laboratory, Washington DC)
• Monday 04 March 2024, 14:00-15:00
• Venue: MR14 DAMTP and online.
• Series: DAMTP Astrophysics Seminars; organiser: Roger Dufresne.

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arXiv:2312.09721v3 Announce Type: replace Abstract: Context. The determination of accurate photometric redshifts (photo-zs) in large imaging galaxy surveys is key for cosmological studies. One of the most common approaches are machine learning techniques. These methods require a spectroscopic or reference sample to train the algorithms. Attention has to be paid to the quality and properties of these samples since they are key factors in the estimation of reliable photo-zs. Aims. The goal of this work is to calculate the photo-zs for the Y3 DES Deep Fields catalogue using the DNF machine learning algorithm. Moreover, we want to develop techniques to assess the incompleteness of the training sample and metrics to study how incompleteness affects the quality of photometric redshifts. Finally, we are interested in comparing the performance obtained with respect to the EAzY template fitting approach on Y3 DES Deep Fields catalogue. Methods. We have emulated -- at brighter magnitude -- the training incompleteness with a spectroscopic sample whose redshifts are known to have a measurable view of the problem. We have used a principal component analysis to graphically assess incompleteness and to relate it with the performance parameters provided by DNF. Finally, we have applied the results about the incompleteness to the photo-z computation on Y3 DES Deep Fields with DNF and estimated its performance. Results. The photo-zs for the galaxies on DES Deep Fields have been computed with the DNF algorithm and added to the Y3 DES Deep Fields catalogue. They are available at https://des.ncsa.illinois.edu/releases/y3a2/Y3deepfields. Some techniques have been developed to evaluate the performance in the absence of "true" redshift and to assess completeness. We have studied... (Partial abstract)

Fifth forces in and around galaxies

Light scalar fields are often considered as part of explanations for dark energy and dark matter. They also appear in theories in which gravity is modified on cosmological scales. If the scalar field theory has a screening mechanism, the fifth forces that the scalar mediates can be significant in the cosmological vacuum but suppressed on Earth and in the solar system. It has been claimed that current data from galaxy surveys is sufficient to exclude the possibility that these theories could be relevant on astrophysical or cosmological scales. In this talk I will consider how such scalar fields behave in galaxies, and show that they may not be excluded after all.

• Speaker: Clare Burrage (Nottingham)
• Monday 04 March 2024, 13:00-14:00
• Venue: CMS, Pav. B, CTC Common Room (B1.19) [Potter Room].
• Series: Cosmology Lunch; organiser: Dr Dong-Gang Wang.

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How Did Cassiopeia A Explode?

Cassiopeia A is the best-observed core-collapse supernova remnant in our galaxy. Analyses of the 1 Million second Chandra Very Large Project X-ray observation and the data from infrared spectroscopy by Spitzer lead to a “complete” (within the limitations of the data quality) assessment of the elemental composition of the explosion ejecta, comprising both the reverse shocked X-ray emitting plasma and the photoionized unshocked ejecta emitting primarily in the infrared. This is the first time such a detailed census of supernova ejecta has ever been accomplished. More recently, Cassiopeia A has been observed with the James Webb Space Telescope. A first look suggests that these data corroborate and extend our previous analysis. Hard X-ray observations by NUSTAR reveal the mass and location of the radioactive nucleus 44Ti and optical imaging reveals a natal kick imparted to the compact central object (presumed to be a neutron star), anti-correlated with the 44Ti location, as expected. However, X-ray imaging reveals almost “pure” Fe knots on the east limb, presumably the ashes of alpha rich freeze out, which do not correlate so well. All these observables carry information about processes at the core of the supernova and allow us (and others) to speculate about the nature of the explosion, in ways that complement conclusions drawn from the prompt observations of supernovae.

• Speaker: Martin Laming (US Naval Research Laboratory)
• Thursday 14 March 2024, 16:00-17:00
• Venue: Hoyle Lecture Theatre, Institute of Astronomy.
• Series: Institute of Astronomy Colloquia; organiser: eb694.

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Dynamics in gas is different

Gaseous environments are abundant in the Universe and include AGN disks, star-forming regions, protoplanetary disks and regions of late star formation in globular clusters. While the dynamics of binaries in gas-free environments have been studied extensively, gas-rich environments are fertile ground for phenomena that are still largely unexplored, and in this talk, we will discuss some of the unique phenomena of dynamics in gas. Binaries tend statistically to get softer as they encounter other stars, according to Heggie’s law. However, in gaseous environments, this law should be modified as gas-hardening could lead to a significant energy dissipation that could dominate over stellar softening. Here we explore the effect of gas hardening on the softening rate of binaries and its implications. We will also discuss gas-assisted binary formation. In gaseous media, interactions between two initially unbound objects could result in gas-assisted binary formation, induced by a loss of kinetic energy to the ambient gas medium. Here, we derive analytically the criteria for gas-assisted binary capture through gas dynamical friction dissipation. In some environments, these captures could occur more than once per object, leading to multicaptures. We will discuss that and further implications.

• Speaker: Mor Rozner (Technion-Israel Institute of Technology)
• Monday 26 February 2024, 14:00-15:00
• Venue: MR14 DAMTP and online.
• Series: DAMTP Astrophysics Seminars; organiser: Thomas Jannaud.

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Eddington lecture 2024: The Dawn of Galaxy-scale Gravitational Wave Astronomy

For more than 15 years, NANO Grav and other pulsar-timing array collaborations have been carefully monitoring networks of pulsars across the Milky Way. The goal was to find a tell-tale correlation signature amid the data from all those pulsars that would signal the presence of an all-sky background of nanohertz-frequency gravitational waves, washing through the Galaxy. At the end of June 2023, the global pulsar-timing array community finally announced its evidence for this gravitational-wave background, along with a series of studies that interpreted this signal as either originating from a population of supermassive black-hole binary systems, or as relics from cosmological processes in the very early Universe. I will describe the journey up to this point (including the integral role that the IoA played), what led to the ultimate breakthrough, how this affects our knowledge of supermassive black holes and the early Universe, and what lies next for gravitational-wave astronomy at light-year wavelengths.

• Speaker: Dr Stephen Taylor, Vanderbilt University, Nashville, Tennessee, USA
• Thursday 07 March 2024, 16:00-17:00
• Venue: Hoyle Lecture Theatre, Institute of Astronomy.
• Series: Institute of Astronomy Colloquia; organiser: eb694.

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AdS Black Holes and Their Microstates

Investigating the fundamental origins of Bekenstein-Hawking entropy is a profound challenge in theoretical physics. This investigation is especially relevant for anti de Sitter (AdS) black holes, where it is proposed that the entropy can be explained through the microstates of the holographic dual quantum field theory, in line with the AdS/CFT correspondence. In this talk, I will review recent advancements in deriving the Bekenstein-Hawking entropy for various supersymmetric AdS black holes across different dimensions.

• Speaker: Seyed Morteza Hosseini, Imperial College London
• Friday 01 March 2024, 13:00-14:00
• Venue: Potter room/Zoom.
• Series: DAMTP Friday GR Seminar; organiser: Xi Tong.

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arXiv:2402.14917v1 Announce Type: cross Abstract: It is well-known that the Einstein-Hilbert action exhibits a projective invariance in metric-affine gravity, generated by a single vector (just like diffeomorphisms). However, this symmetry offers no protection against formulating inconsistent models, e.g., with ghost and strong coupling problems. In this letter, we observe that non-minimal kinetic terms of Dirac spinors point to a new extended projective (EP) symmetry generated by a pair of vectors. We prove that the most general EP-invariant theory (at most quadratic in field strengths) is naturally free from all pathologies. Its spectrum only features the massless graviton and a single additional scalar field arising from the square of the Holst curvature. The scalar potential is suitable for inflation and our model moreover contains effective 4-Fermi interactions capable of producing fermionic dark matter. Finally, we point out an alternative double-vector symmetry that similarly leads to a healthy theory with a propagating vector field.

Astronomers have spotted a gargantuan protocluster – the primordial beginnings of a galaxy cluster – by searching near a quasar in the early universe

Title to be confirmed

Abstract not available

• Speaker: Evan Kirby (Notre Dame)
• Friday 24 May 2024, 11:30-12:30
• Venue: Ryle seminar room + online.
• Series: Galaxies Discussion Group; organiser: Sandro Tacchella.

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New Space & the CubeSat Revolution

CubeSats stand at the forefront of the New Space revolution, a paradigm shift in space exploration characterised by reduced launch costs and increased accessibility to space. These miniature satellites, defined by their standardised dimensions and modular design, have emerged as a pivotal technology with some implications of research in astronomy. With their standardised dimensions and modular design, these Nanosatellites enable a wide range of experiments that were previously the domain of larger, more costly missions. In my talk, I aim to introduce you to valuable opportunities that can emerge by leading a CubeSat project with special interest in payloads dedicated to astrophysics research. As a cost-effective space instrument, CubeSats unlock observational windows across the ultraviolet, far-infrared, and low-frequency radio spectra, which are inaccessible from Earth’s surface. Beyond their technical capabilities, these satellites enable sustained observations of celestial bodies over extended periods, free from the scheduling constraints of larger telescopes.

• Speaker: Dolev Bashi
• Tuesday 05 March 2024, 11:15-12:00
• Venue: Coffee area, Battcock Centre.
• Series: Hills Coffee Talks; organiser: Dolev Bashi.

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The chemical abundance pattern of a nitrogen-loud AGN in the early Universe

Recent JWST /NIRSpec observations have revealed several interesting galaxies with particularly enhanced nitrogen abundances at redshifts of z > 6. These galaxies are featured by strong nitrogen emission lines in the rest-frame UV, similar to the ’’nitrogen-loud’’ quasars previously observed at lower redshifts. Among these nitrogen-loud galaxies, there is an AGN candidate, GN-z11, at a redshift of z = 10.6, also showing potentially exotic abundances of carbon and iron. The chemical enrichment history of the high-redshift nitrogen-loud galaxies and their connection with nitrogen-loud quasars remain unclear despite that many theories have been proposed. In this talk, I present our recent analyses on a nitrogen-loud Type-1 AGN at z = 5.55 observed within the JWST /NIRSpec IFS GTO Program, GA-NIFS. By combining the ground-based observations and JWST observations, we confirmed the nitrogen loudness of this AGN and further constrained its carbon abundance and iron abundance. We found evidence of a chemically stratified narrow-line region (NLR) in this AGN , where the denser and more highly ionized part of the NLR is more nitrogen enhanced. Our results suggest the peculiar chemical enrichment in this AGN is confined to the inner part of the NLR and potentially traces a specific stage of chemical evolution in a dense environment.

• Speaker: Xihan Ji (Cavendish)
• Friday 01 March 2024, 11:30-12:30
• Venue: Ryle seminar room + online.
• Series: Galaxies Discussion Group; organiser: Sandro Tacchella.

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Positronium has the potential to revolutionise physics but the elusive substance had been too hot to handle.

Nature, Published online: 22 February 2024; doi:10.1038/d41586-024-00555-1

Astrophysicist whose radio-wave observations confirmed the Big Bang origin of the Universe.

Dynamics in gas is different

Gaseous environments are abundant in the Universe and include AGN disks, star-forming regions, protoplanetary disks and regions of late star formation in globular clusters. While the dynamics of binaries in gas-free environments have been studied extensively, gas-rich environments are fertile ground for phenomena that are still largely unexplored, and in this talk, we will discuss some of the unique phenomena of dynamics in gas. Binaries tend statistically to get softer as they encounter other stars, according to Heggie’s law. However, in gaseous environments, this law should be modified as gas-hardening could lead to a significant energy dissipation that could dominate over stellar softening. Here we explore the effect of gas hardening on the softening rate of binaries and its implications. We will also discuss gas-assisted binary formation. In gaseous media, interactions between two initially unbound objects could result in gas-assisted binary formation, induced by a loss of kinetic energy to the ambient gas medium. Here, we derive analytically the criteria for gas-assisted binary capture through gas dynamical friction dissipation. In some environments, these captures could occur more than once per object, leading to multicaptures. We will discuss that and further implications.

• Speaker: Mor Rozner (Technion-Israel Institute of Technology)
• Monday 26 February 2024, 14:00-15:00
• Venue: MR14 DAMTP and online.
• Series: DAMTP Astrophysics Seminars; organiser: Thomas Jannaud.

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New Space & the CubeSat Revolution

CubeSats stand at the forefront of the New Space revolution, a paradigm shift in space exploration characterised by reduced launch costs and increased accessibility to space. These miniature satellites, defined by their standardised dimensions and modular design, have emerged as a pivotal technology with some implications of research in astronomy. With their standardised dimensions and modular design, these small satellites enable a wide range of experiments that were previously the domain of larger, more costly missions. In my talk, I aim to introduce you to valuable opportunities that can emerge by leading a CubeSat project with special interest in payloads dedicated to astrophysics research. As a cost-effective space instrument, CubeSats unlock observational windows across the ultraviolet, far-infrared, and low-frequency radio spectra, which are inaccessible from Earth’s surface. Beyond their technical capabilities, these satellites enable sustained observations of celestial bodies over extended periods, free from the scheduling constraints of larger telescopes.

• Speaker: Dolev Bashi
• Tuesday 05 March 2024, 11:15-12:00
• Venue: Coffee area, Battcock Centre.
• Series: Hills Coffee Talks; organiser: Dolev Bashi.

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Protoplanetary disc: what can we learn by combining theory and observations?

Protoplanetary discs serve as the cradle for planetary formation and evolution. It is then fundamental to study their evolution to gain a comprehensive understanding of exoplanetary system formation. These discs can be studied using two distinct approaches.

On one side, they can be analysed as a set of single sources, allowing for a detailed analysis of the mechanisms behind the diversity of observed morphologies using gas and dust tracers such as rings, gaps and asymmetries.

On the other side, it is crucial to study star-forming regions, understanding which physical processes are governing the global disc evolution.

In this talk, I will firstly describe results from the modelling of single sources, underlining the information we can obtain by comparing multi-wavelengths observations with results from the hydrodynamical models of specific sources (e.g., HD169142 , PDS70, GG Tau A). In particular, I will focus on how simulations can help in constraining the mass and position of the candidate proto-planets that may be responsible for the ALMA and SPHERE observational results, as well as how they can support future observational strategies.

I will then summarize some of the results obtained by testing disc evolution models by comparing them with the Lupus star forming region. In these works, we tested the secular evolution of the observed dust and gas radius of disc populations and their ratio, to test the efficiency of radial drift and the viscous evolution theory.

• Speaker: Claudia Toci (ESO)
• Tuesday 27 February 2024, 13:00-14:00
• Venue: Ryle seminar room + ONLINE - Details to be sent by email.
• Series: Exoplanet Seminars; organiser: Dr Emily Sandford.

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Real-time pipelines for SKA — Progress and challenges

The Square Kilometre Array (SKA) will be the largest interferometric radio telescope to date, with unprecedented resolution and sensitivity to study various phenomena of the universe. In data processing pipelines for radio telescopes, real time calibration, such as beam former and pointing offset calibration, are crucial for obtaining high-quality interferometric data from the observations. As an example, I will discuss the design and ongoing implementation of the pointing offset calibration pipeline within SKA ’s data processing software, describing the steps carried out to integrate the pipeline into telescope execution control and data queue system, as well as outlining challenges and greater implications on the data processing algorithm and software within the radio astronomy community.

• Speaker: Dr. Ying-He Celeste Lü (Cavendish Astrophysics)
• Tuesday 27 February 2024, 11:15-12:00
• Venue: Coffee area, Battcock Centre.
• Series: Hills Coffee Talks; organiser: Charles Walker.

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New Space & the CubeSat Revolution

CubeSats stand at the forefront of the New Space revolution, a paradigm shift in space exploration characterised by reduced launch costs and increased accessibility to space. These miniature satellites, defined by their standardised dimensions and modular design, have emerged as a pivotal technology with some implications of research in astronomy. With their standardised dimensions and modular design, these small satellites enable a wide range of experiments that were previously the domain of larger, more costly missions. In my talk, I aim to introduce you to valuable opportunities that can emerge by leading a CubeSat project with special interest in payloads dedicated to astrophysics research. As a cost-effective space instrument, CubeSats unlock observational windows across the ultraviolet, far-infrared, and low-frequency radio spectra, which are inaccessible from Earth’s surface. Beyond their technical capabilities, these satellites enable sustained observations of celestial bodies over extended periods, free from the scheduling constraints of larger telescopes.

• Speaker: Dolev Bashi
• Tuesday 05 March 2024, 11:15-12:00
• Venue: Coffee area, Battcock Centre.
• Series: Hills Coffee Talks; organiser: Dolev Bashi.

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arXiv:2306.04536v2 Announce Type: replace Abstract: We present the properties of 17 faint Ly$\alpha$ emitting galaxies (LAEs) at $z>5.8$ from the JWST Advanced Deep Extragalactic Survey (JADES) in the Hubble Ultra Deep Field/GOODS-S. These LAEs span a redshift range $z\approx5.8-8.0$ and a UV magnitude range $M_{UV}\approx-17$ to $-20.6$, with the Ly$\alpha$ equivalent width (EW) in the range $\approx 25-350$ \AA. The detection of other rest-optical emission lines in the spectra of these LAEs enables the determination of accurate systemic redshifts and Ly{\alpha} velocity offsets, as well as the physical and chemical composition of their stars and interstellar media. These faint LAEs are consistent with metal-poor systems with high ionization parameters, similar to the general galaxy population at $z>6$. We measured an average ionizing photon production efficiency, log($\xi_\rm{ion}$/erg$^{-1}$ Hz) $\approx25.57$ across our LAEs, which does not evolve strongly with redshift. We report an anti-correlation between the Ly$\alpha$ escape fraction (f_\rm{esc}) and the velocity offset from systemic redshift, consistent with model expectations. We further find that the strength and velocity offset of Ly$\alpha$ are neither correlated with galaxy spectroscopic properties nor with $\xi_\rm{ion}$. We find a decrease in $f_\rm{esc}$(Ly$\alpha$) with redshift, indicative of decreasing sizes of ionized bubbles around LAEs at high redshifts. We used a range of galaxy properties to predict Lyman continuum $f_\rm{esc}$ for our LAEs, finding that the ionizing photon output into the intergalactic medium remains roughly constant across the observed Ly$\alpha$ EW, showing a mild increase at fainter M$_{UV}$ and at higher redshifts. We derived correlations between the ionizing photon output from LAEs and $M_{UV}$, Ly$\alpha$ EW and redshifts, which can be used to constrain the ionizing photon contribution of LAEs at $z > 6$ towards cosmic reionization.