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arXiv:2411.04793v1 Announce Type: new Abstract: The Legacy Survey of Space and Time (LSST) at Vera C. Rubin Observatory is planned to begin in the Fall of 2025. The LSST survey cadence has been designed via a community-driven process regulated by the Survey Cadence Optimization Committee (SCOC), which recommended up to 3% of the observing time to carry out Target of Opportunity (ToO) observations. Experts from the scientific community, Rubin Observatory personnel, and members of the SCOC were brought together to deliver a recommendation for the implementation of the ToO program during a workshop held in March 2024. Four main science cases were identified: gravitational wave multi-messenger astronomy, high energy neutrinos, Galactic supernovae, and small potentially hazardous asteroids possible impactors. Additional science cases were identified and briefly addressed in the documents, including lensed or poorly localized gamma-ray bursts and twilight discoveries. Trigger prioritization, automated response, and detailed strategies were discussed for each science case. This document represents the outcome of the Rubin ToO 2024 workshop, with additional contributions from members of the Rubin Science Collaborations. The implementation of the selection criteria and strategies presented in this document has been endorsed in the SCOC Phase 3 Recommendations document (PSTN-056). Although the ToO program is still to be finalized, this document serves as a baseline plan for ToO observations with the Rubin Observatory.

To be confirmed

Abstract not available

• Speaker: Professor Tilman Plehn - Heidelberg University
• Wednesday 12 March 2025, 16:00-17:00
• Venue: MR3.
• Series: Theoretical Physics Colloquium; organiser: Amanda Stagg.

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Cold Oceans in the Solar System

There are thought to be many ice-covered oceans in the Solar System, ranging from Earth’s Arctic Ocean to the liquid-water ocean of Europa. In this talk, I will discuss how an understanding of Earth-based polar research may be exploited to explore ice-ocean interactions on ice-covered moons in the Solar System.

• Speaker: Nicole Shibley / DAMTP, Earth Sciences
• Wednesday 06 November 2024, 13:40-14:05
• Venue: The Hoyle Lecture Theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: Xander Byrne.

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Black Hole Cartography

As a perturbed black hole relaxes into its Kerr state following a merger, it rings down by producing quasinormal modes (QNMs). These are oscillations at specific complex frequencies and with angular structures determined by first-order perturbation theory. Spatial information can be extracted from numerical simulations by fitting a feature of known time dependence; a program we call black hole cartography. We demonstrate that QNMs can be reconstructed using our method by recovering the predictions of first-order perturbation theory. Furthermore, we apply the technique to quadratic QNMs (appearing at second-order) and demonstrate that we can determine their angular structures. This is a step towards an improved understanding of non-linearities in the ringdown, which will be important for observations with future gravitational wave detectors.

• Speaker: Richard Dyer / IoA
• Wednesday 06 November 2024, 13:15-13:40
• Venue: The Hoyle Lecture Theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: Xander Byrne.

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arXiv:2410.22036v1 Announce Type: new Abstract: We present an exploration of the Milky Way's structural parameters using an all-sky sample of RC giants to map the stellar density from the inner to the outer parts of the Galactic disc. These evolved giants are considered to be standard candles due to their low intrinsic variance in their absolute luminosities, allowing us to estimate their distances with reasonable confidence. We exploit all-sky photometry from the AllWISE mid-infrared survey and the Gaia survey, along with astrometry from Gaia Data Release 3 and recent 3D extinction maps, to develop a probabilistic scheme in order to select with high confidence \rc{}-like stars. Our curated catalogue contains about 10 million sources, for which we estimate photometric distances based on the WISE $W1$ photometry. We then derive the selection function for our sample, which is the combined selection function of sources with both \gaia{} and \allwise{} photometry. Using the distances and accounting for the full selection function of our observables, we are able to fit a two-disc, multi-parameter model to constrain the scale height (\hz{}), scale-length (\rd{}), flaring, and the relative mass ratios of the two disc components. We illustrate and verify our methodology using mock catalogues of \rc{} stars. We find that the \rc{} population is best described by a flared thin disc with scale length \rd{}=$3.56\pm0.32$ kpc and scale height at the Sun of \hzsun{}=$0.17\pm0.01$ kpc, and a shorter and thicker disc with \rd{}=$2.59\pm0.11$ kpc, \hzsun{}=$0.45\pm0.11$ kpc, with no flare. The thicker disc constitutes 64\% of the \rc{} stellar mass beyond 3 kpc, while the thin disk shows evidence of being warped beyond 9 kpc from the Galactic center. The residuals between the predicted number density of RC stars from our axisymmetric model and the measured counts show possible evidence of a two-armed spiral perturbation in the disc of the Milky Way.

Unveiling the Role of Magnetic Field in Generating Quasi-Periodic Oscillations

Almost all accreting black hole and neutron star X-ray binary systems (XRBs) exhibit prominent brightness variations on a few characteristic time-scales and their harmonics. These quasi-periodic oscillations (QPOs) are thought to be associated with the precession of a warped accretion disc, but the physical mechanism that generates the precessing warp remains uncertain. Relativistic frame dragging (Lense-Thirring precession) is one promising candidate, but a misaligned magnetic field is an alternative, especially for neutron star XRBs. Here, we report the discovery of 5 accreting white dwarf systems (AWDs) that display strong optical QPOs with characteristic frequencies and harmonic structures that suggest they are the counterpart of the QPOs seen in XRBs. Since AWDs are firmly in the classical (non-relativistic) regime, Lense-Thirring precession cannot account for these QPOs. By contrast, a weak magnetic field associated with the white dwarf can drive disc warping and precession in these systems, similar to what has been proposed for neutron star XRBs. Our observations confirm that magnetically-driven warping is a viable mechanism for generating QPOs in disc-accreting astrophysical systems, certainly in AWDs. And furthermore, I will discuss the possible new application of the model to explain mHz QPOs in Ultraluminous X-ray Sources (ULXs). Additionally, they establish a new way to estimate magnetic field strengths, even in relatively weak-field systems where other methods are not available.

• Speaker: Martina Veresvarska / Durham University
• Wednesday 30 October 2024, 13:40-14:05
• Venue: The Hoyle Lecture Theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: Xander Byrne.

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Bar chrono-kinematics and its link to the Galactic disc formation

The galactic bar is a common structure in the Universe; approximately two-thirds of spiral galaxies host a bar at their centers. Due to its non-axisymmetric nature, the galactic bars influence not only the dynamics of the surrounding stars but also reshape the galactic disk and halo. Therefore, understanding galactic bars is crucial for telling the story of galaxy evolution in general. Our Milky Way provides a unique opportunity to study the kinematics and dynamics of the Galactic bar in detail, as we can resolve individual stars in phase space. By complementing the kinematics of stars with ages, we have chances to decode the history of the inner Galaxy. We present the chrono-kinematic signature of the inner Galaxy traced by Mira variable stars. Observing the evolution of bar morphology and kinematics with age allows us to explore the heating history of the inner Milky Way. We establish a connection between the bar chrono-kinematic evolution and the formation of the Galactic disk, providing a novel and independent constraint on the epoch of the Milky Way disk formation.

• Speaker: Hanyuan Zhang / IoA
• Wednesday 30 October 2024, 13:15-13:40
• Venue: The Hoyle Lecture Theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: Xander Byrne.

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Fred Simmons: Topic TBA

In person.

• Speaker: Fred Simmons (Cambridge Divinity)
• Thursday 13 March 2025, 11:00-12:00
• Venue: Thirkill Room, Old Court, Clare College.
• Series: LCLU Coffee Meetings; organiser: Paul B. Rimmer.

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Clancy Jiang, topic TBA

In person

• Speaker: Clancy Jiang (Cambridge Earth Science)
• Thursday 27 March 2025, 11:00-12:00
• Venue: Thirkill Room, Old Court, Clare College.
• Series: LCLU Coffee Meetings; organiser: Paul B. Rimmer.

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Rick Anslow & Tereza Constantinou on Icy Moons

In person

• Speaker: Rick Anslow & Tereza Constantinou (Cambridge IoA)
• Thursday 10 April 2025, 11:00-12:00
• Venue: Thirkill Room, Old Court, Clare College.
• Series: LCLU Coffee Meetings; organiser: Paul B. Rimmer.

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LCLU Coffee

No topic. Come along for the coffee, cookies and conversation.

• Speaker: None
• Thursday 24 April 2025, 11:00-12:00
• Venue: Thirkill Room, Old Court, Clare College.
• Series: LCLU Coffee Meetings; organiser: Paul B. Rimmer.

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Nicole Shibley, topic TBA

In person

• Speaker: Nicole Shibley (Cambridge, DAMTP)
• Thursday 08 May 2025, 11:00-12:00
• Venue: Thirkill Room, Old Court, Clare College.
• Series: LCLU Coffee Meetings; organiser: Paul B. Rimmer.

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David Russell on Compartmentalization at Life's Origins

In person

• Speaker: David Russell (Cambridge Biochemistry)
• Thursday 22 May 2025, 11:00-12:00
• Venue: Thirkill Room, Old Court, Clare College.
• Series: LCLU Coffee Meetings; organiser: Paul B. Rimmer.

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arXiv:2410.13747v1 Announce Type: new Abstract: We apply the probabilistic hierarchical SN Ia SED model BayeSN to analyse SALT-based simulations of SNe Ia to probe consistency between the two models. This paper is the first cross-comparison of dust inference methods using SALT and BayeSN, of great importance given the history of conflicting conclusions regarding the distributions of host galaxy dust properties between the two. Overall we find that BayeSN is able to accurately recover our simulated SALT inputs, establishing excellent consistency between the two models. When inferring dust parameters with simulated samples including non-Ia contamination, we find that our choice of photometric classifier causes a bias in the inferred dust distribution; this arises because SNe Ia heavily impacted by dust are misclassified as contaminants and excluded. We then apply BayeSN to a sample of SNe from DES5YR to jointly infer host galaxy dust distributions and intrinsic differences on either side of a `mass step' at $10^{10}$ M$\odot$. We find evidence in favour of an intrinsic contribution to the mass step and a considerably smaller difference in $R_V$ distributions than most SALT-based analyses, at most $\Delta\mu_{R_V}=0.72\pm0.26$. We also build on recent results in favour of an environmental-dependence on the secondary maximum of SNe Ia in $i$-band. Twenty days post-peak, we find a offset in intrinsic $i$-band light curve between each mass bin at a significance in excess of $3\sigma$.

arXiv:2311.03199v3 Announce Type: replace Abstract: A detailed map of the distribution of dust at high Galactic latitudes is essential for future cosmic microwave background (CMB) polarization experiments because the dust, while diffuse, remains a significant foreground in these regions. We develop a Bayesian model to identify a region of the Hertzsprung-Russell (HR) diagram suited to constrain the single-star extinction accurately at high Galactic latitudes. Using photometry from Gaia, 2MASS and ALLWISE, and parallax from Gaia, we employ nested sampling to fit the model to the data and analyse the posterior over stellar parameters for both synthetic and real data. Charting low variations in extinction is complex due to systematic errors and degeneracies between extinction and other stellar parameters. The systematic errors can be minimised by restricting our data to a region of the HR diagram where the stellar models are most accurate. Moreover, the degeneracies can be reduced by including astrophysical priors and spectroscopic constraints. We show that accounting for the measurement error of the data and the assumed inaccuracies of the stellar models are critical in accurately recovering small variations in extinction. We compare the posterior distribution for individual stars with spectroscopic stellar parameter estimates from LAMOST and Gaia ESO and illustrate the importance of estimating extinction and effective temperature as a joint solution.

Constraining star-formation efficiency in the early Universe using JWST and the cosmic 21-cm signal

The cosmic 21-cm signal encodes a vast array of information about the early Universe, shedding light on the Dark Ages, the earliest stars and galaxies during the Cosmic Dawn, and on the tomography of the Epoch of Reionization. Many attempts are being made to capture this unique signal with increasingly good sensitivity, through interferometers like HERA and NenuFAR, and monopole antennas like EDGES , SARAS and REACH . In parallel, the ambitious attempts at peering into deep space through the eyes of the JWST has already revealed a great deal about these early luminous sources. Large surveys of the ultraviolet luminosity function (UVLFs) at z > 10 show that there are many more massive, bright galaxies, beyond those predicted by our simple assumptions of star-formation. Furthermore, observations also reveal a surprisingly early formation of supermassive black holes and Active Galactic Nuclei, providing an intriguing prospect for novel theories. By combining the latest limits on the 21-cm power spectrum and global signal with recent JWST observations, we provide the first synergetic constraints on star-formation efficiency of the early Universe. We also open up the possibility of investigating the role of stellar variability and impact of Population III stars in the early Universe.

• Speaker: Jiten Dhandha / IoA
• Wednesday 23 October 2024, 13:15-13:40
• Venue: The Hoyle Lecture Theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: Xander Byrne.

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Gravitational instability in planet forming discs – a kinematical perspective

Gravitational instability (GI) is believed to be one of the main drivers of angular momentum transport in young protoplanetary discs. These discs are often massive, and the influence of their self-gravity plays a crucial role in their evolution. In this talk, I will explore the connection between protoplanetary disc kinematics and gravitational instability, highlighting how GI leaves distinct kinematic signatures. Properly interpreting these perturbations provides valuable insights into the structure and evolution of protoplanetary discs. I will present two case studies — Elias 2-27 and AB Aur — demonstrating how our kinematic models accurately capture the characteristics of these GI discs.

• Speaker: Cristiano Longarini / IoA
• Wednesday 16 October 2024, 13:15-13:40
• Venue: The Hoyle Lecture Theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: Xander Byrne.

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Dynamics in gas is different

Gas-rich 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 gas-rich 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, IoA / DAMTP
• Wednesday 09 October 2024, 13:40-14:05
• Venue: The Hoyle Lecture Theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: Xander Byrne.

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Spectral classification of white dwarfs by dimensionality reduction

As a suite of large-sky spectroscopic surveys comes online, automated spectral classification techniques are needed more than ever. For white dwarfs—the evolutionary endpoint of the vast majority of stars—spectral classification is vital for understanding their properties, yet still almost exclusively done by eye. Upcoming surveys will return of order 10^5 white dwarf spectra, highlighting the need for automated tools that are fast, but do not miss rare or unique objects, as supervised machine learning models often do. We present the use of dimensionality reduction, an unsupervised method, on white dwarf spectra from the DESI EDR . I will outline the theory behind dimensionality reduction, as well as results showing its effectiveness in classifying white dwarf spectra. I will also discuss two extensions of the technique: the highlighting of spectral regions, and its use in a pseudo-supervised manner.

• Speaker: Xander Byrne, IoA
• Wednesday 09 October 2024, 13:15-13:40
• Venue: The Hoyle Lecture Theatre + Zoom .
• Series: Institute of Astronomy Seminars; organiser: Xander Byrne.

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arXiv:2410.00956v1 Announce Type: new Abstract: Measurements of galaxy clustering are affected by RSD. Peculiar velocities, gravitational lensing, and other light-cone projection effects modify the observed redshifts, fluxes, and sky positions of distant light sources. We determine which of these effects leave a detectable imprint on several 2-point clustering statistics extracted from the EWSS on large scales. We generate 140 mock galaxy catalogues with the survey geometry and selection function of the EWSS and make use of the LIGER method to account for a variable number of relativistic RSD to linear order in the cosmological perturbations. We estimate different 2-point clustering statistics from the mocks and use the likelihood-ratio test to calculate the statistical significance with which the EWSS could reject the null hypothesis that certain relativistic projection effects can be neglected in the theoretical models. We find that the combined effects of lensing magnification and convergence imprint characteristic signatures on several clustering observables. Their S/N ranges between 2.5 and 6 (depending on the adopted summary statistic) for the highest-redshift galaxies in the EWSS. The corresponding feature due to the peculiar velocity of the Sun is measured with a S/N of order one or two. The $P_{\ell}(k)$ from the catalogues that include all relativistic effects reject the null hypothesis that RSD are only generated by the variation of the peculiar velocity along the line of sight with a significance of 2.9 standard deviations. As a byproduct of our study, we demonstrate that the mixing-matrix formalism to model finite-volume effects in the $P_{\ell}(k)$ can be robustly applied to surveys made of several disconnected patches. Our results indicate that relativistic RSD, the contribution from weak gravitational lensing in particular, cannot be disregarded when modelling 2-point clustering statistics extracted from the EWSS.