Mon 12 Feb 14:00: The life of erupting flux ropes
Unstable magnetic flux ropes represent the primary driver of eruptive solar flares and thus space weather. We summarize the recent developments of the 3D extensions of the Standard model of solar flares, including formation of the flux ropes, identification of the erupting mechanism(s), and processes during the eruption proper. New reconnection geometries are reviewed, both from theoretical as well as observational standpoint, as these lead to re-building of the erupting flux rope once the eruption is underway.
- Speaker: Jaroslav Dudik (Czech Academy of Sciences)
- Monday 12 February 2024, 14:00-15:00
- Venue: MR14 DAMTP and online.
- Series: DAMTP Astrophysics Seminars; organiser: Roger Dufresne.
Thu 08 Feb 11:30: Problems with (our) galaxy formation simulations and some new angles
Stellar feedback is a crucial component in controlling the baryon cycle in galaxies. However, it is not very clear how this can be done without assuming exotic models of stellar feedback. In this talk I will first discuss why we have not been very successful in producing realistic galaxies in our simulations, and present some attempts to solve this problem. I will also discuss how Lyman alpha profiles can potentially be used to place some constraints on cosmological simulations with cosmic ray feedback. This talk is intended to be thought-provoking rather than a collection of success stories.
- Speaker: Taysun Kimm (Yonsei)
- Thursday 08 February 2024, 11:30-12:30
- Venue: Ryle seminar room + online.
- Series: Galaxies Discussion Group; organiser: Sandro Tacchella.
Thu 08 Feb 11:30: Title to be confirmed
Abstract not available
- Speaker: Taysun Kimm (Yonsei)
- Thursday 08 February 2024, 11:30-12:30
- Venue: Ryle seminar room + online.
- Series: Galaxies Discussion Group; organiser: Sandro Tacchella.
Fri 28 Jun 11:30: Title to be confirmed
Abstract not available
- Speaker: Xuejian (Jacob) Shen (MIT)
- Friday 28 June 2024, 11:30-12:30
- Venue: Ryle seminar room + online.
- Series: Galaxies Discussion Group; organiser: Sandro Tacchella.
Mon 18 Mar 13:00: Title to be confirmed
Abstract not available
- Speaker: Roger de Belsunce (LBNL)
- Monday 18 March 2024, 13:00-14:00
- Venue: CMS, Pav. B, CTC Common Room (B1.19) [Potter Room].
- Series: Cosmology Lunch; organiser: Fiona McCarthy.
Mon 19 Feb 14:00: Title to be confirmed
TBD
- Speaker: Juliana Kwan (DAMTP)
- Monday 19 February 2024, 14:00-15:00
- Venue: MR14 DAMTP and online.
- Series: DAMTP Astrophysics Seminars; organiser: Loren E. Held.
Fri 19 Apr 11:30: Title to be confirmed
Abstract not available
- Speaker: Trevor Mendel (ANU)
- Friday 19 April 2024, 11:30-12:30
- Venue: Ryle seminar room + online.
- Series: Galaxies Discussion Group; organiser: Sandro Tacchella.
Fri 12 Apr 11:30: Title to be confirmed
Abstract not available
- Speaker: Emily Wisnioski (ANU)
- Friday 12 April 2024, 11:30-12:30
- Venue: Ryle seminar room + online.
- Series: Galaxies Discussion Group; organiser: Sandro Tacchella.
Mon 05 Feb 14:00: Dynamo action, magnetorotational instability, Alfvén waves: Theory and experiments on astrophysical magnetohydrodynamics
The homogeneous dynamo effect in moving electrically conducting fluids, such as liquid metals or plasmas, is responsible for magnetic-field generation in planets, stars and galaxies. Magnetic fields, in turn, can promote cosmic structure formation by destabilizing, via the magnetorotational instability (MRI), rotational flows in accretion disks that otherwise would be hydrodynamically stable.
For a long time, those topics have been the subject of purely theoretical and numerical research. This situation changed in 1999 when the threshold of magnetic-field self-excitation was crossed in two large-scale liquid-sodium experiments in Riga and Karlsruhe. Later, the VKS dynamo experiment in Cadarache successfully reproduced field reversals and excursions that are of great geophysical interest. Various types of the MRI were studied in liquid metal experiments at the Princeton Plasma Physics Laboratory and at Helmholtz-Zentrum Dresden-Rossendorf (HZDR). A liquid-rubidium experiment at the Dresden High Magnetic Field Laboratory (HLD) reached the “magic point” of coinciding Alfvén and sound speeds, which is thought to play a key role for the heating of the solar corona.
After a short introduction to the basic equations of magnetohydrodynamics, the lecture gives an overview about previous and future liquid metal experiments on dynamo action, Alfvén waves, and magnetically triggered flow instabilities such as the MRI . Special focus lies on a precession driven dynamo experiment that is presently being constructed in frame of the DRESDYN project at HZDR . Closely related to this, some emphasis is placed on the potential role of various astronomical forcings in triggering reversals of the geodynamo or even synchronizing the solar dynamo.
- Speaker: Frank Stefani (Helmholtz-Zentrum Dresden)
- Monday 05 February 2024, 14:00-15:00
- Venue: MR14 DAMTP and online.
- Series: DAMTP Astrophysics Seminars; organiser: Roger Dufresne.
Fri 02 Feb 13:00: Towards reconciling Cosmology, GR and QFT through non-perturbative Stochastic Inflation
In the context of inflation, we show how to account for quantum modes in general and numerical relativity on scales bigger than the Hubble radius, from where they behave classically and can grow non-perturbatively.
We provide a formulation of Stochastic Inflation in full general relativity that goes beyond the slow-roll and separate universe approximations. Starting from the initial conditions problem in numerical relativity, we show how gauge invariant Langevin source terms can be obtained for the complete set of Einstein equations in their ADM formulation by providing a recipe for coarse-graining the spacetime in any small gauge. These stochastic source terms are defined in terms of the only dynamical scalar degree of freedom in single-field inflation and all depend simply on the first two time derivatives of the coarse-graining window function, on the gauge-invariant mode functions that satisfy the Mukhanov-Sasaki evolution equation, and on the slow-roll parameters.
We validate the efficacy of these Langevin dynamics directly using an example in uniform field gauge, obtaining the stochastic e-fold number without the need for a first-passage-time analysis. As well as investigating the most commonly used gauges in cosmological perturbation theory, we also derive stochastic source terms for the coarse-grained first-order BSSN formulation of Einstein’s equations, which enables a well-posed implementation for 3+1 numerical relativity simulations.
Based on https://arxiv.org/abs/2401.08530v1
- Speaker: Yoann Launay, DAMTP, University of Cambridge
- Friday 02 February 2024, 13:00-14:00
- Venue: Potter room/Zoom.
- Series: DAMTP Friday GR Seminar; organiser: Xi Tong.
Tue 06 Feb 13:00: Photoevaporation from Exoplanet Atmospheres: Understanding the Role of Stellar Winds and Considering Water-rich Atmospheres Room changed
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.
Room changed
- Speaker: Laura Harbach (Imperial)
- Tuesday 06 February 2024, 13:00-14:00
- Venue: Hoyle Committee Room + ONLINE - Details to be sent by email.
- Series: Exoplanet Seminars; organiser: Dr Emily Sandford.
Tue 06 Feb 13:00: 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.
- Speaker: Laura Harbach (Imperial)
- Tuesday 06 February 2024, 13:00-14:00
- Venue: Ryle seminar room + ONLINE - Details to be sent by email.
- Series: Exoplanet Seminars; organiser: Dr Emily Sandford.
Fri 17 May 11:30: Title to be confirmed
Abstract not available
- Speaker: Harry Bevins (KICC)
- Friday 17 May 2024, 11:30-12:30
- Venue: Ryle seminar room + online.
- Series: Galaxies Discussion Group; organiser: Sandro Tacchella.
Fri 01 Mar 11:30: Title to be confirmed
Abstract not available
- Speaker: Xihan Ji (Cavendish)
- Friday 01 March 2024, 11:30-12:30
- Venue: Ryle seminar room + online.
- Series: Galaxies Discussion Group; organiser: Sandro Tacchella.
Fri 16 Feb 11:30: TBD
Abstract not available
- Speaker: Alessandro Trinca (Rome)
- Friday 16 February 2024, 11:30-12:30
- Venue: Ryle seminar room + online.
- Series: Galaxies Discussion Group; organiser: Sandro Tacchella.
Fri 15 Mar 11:30: Title to be confirmed
Abstract not available
- Speaker: Boyuan Liu (IoA)
- Friday 15 March 2024, 11:30-12:30
- Venue: Ryle seminar room + online.
- Series: Galaxies Discussion Group; organiser: Sandro Tacchella.
Fri 22 Mar 11:30: Title to be confirmed
Abstract not available
- Speaker: Martin Bourne (IoA)
- Friday 22 March 2024, 11:30-12:30
- Venue: Ryle seminar room + online.
- Series: Galaxies Discussion Group; organiser: Sandro Tacchella.
Wed 31 Jan 13:15: 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.
- Speaker: Katherine Kauma
- Wednesday 31 January 2024, 13:15-13:40
- Venue: The Hoyle Lecture Theatre + Zoom .
- Series: Institute of Astronomy Seminars; organiser: Hannah Uebler.
Tue 20 Feb 13:00: TBC
Abstract not available
- Speaker: Amy Tuson (Cavendish)
- Tuesday 20 February 2024, 13:00-14:00
- Venue: Ryle seminar room + ONLINE - Details to be sent by email.
- Series: Exoplanet Seminars; organiser: Dr Emily Sandford.
Fri 08 Mar 11:30: Title to be confirmed
Abstract not available
- Speaker: Ryan Hickox (Dartmouth)
- Friday 08 March 2024, 11:30-12:30
- Venue: Ryle seminar room + online.
- Series: Galaxies Discussion Group; organiser: Sandro Tacchella.
