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Title to be confirmed

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• Speaker: Anibal Sierra (UCL)
• Tuesday 03 June 2025, 13:00-14:00
• Venue: Ryle seminar room + ONLINE - Details to be sent by email.
• Series: Exoplanet Seminars; organiser: Dr Dolev Bashi.

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• Speaker: Lev Tal-Or (Ariel)
• Tuesday 20 May 2025, 13:00-14:00
• Venue: Ryle seminar room + ONLINE - Details to be sent by email.
• Series: Exoplanet Seminars; organiser: Dr Dolev Bashi.

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• Speaker: Olja Panic (Leeds)
• Tuesday 13 May 2025, 13:00-14:00
• Venue: Ryle seminar room + ONLINE - Details to be sent by email.
• Series: Exoplanet Seminars; organiser: Dr Dolev Bashi.

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• Speaker: Craig Walton (ETH)
• Tuesday 18 March 2025, 13:00-14:00
• Venue: Ryle seminar room + ONLINE - Details to be sent by email.
• Series: Exoplanet Seminars; organiser: Dr Dolev Bashi.

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arXiv:2410.13935v2 Announce Type: replace-cross Abstract: Quasinormal modes (QNMs) are usually characterized by their time dependence; oscillations at specific frequencies predicted by black hole (BH) perturbation theory. QNMs are routinely identified in the ringdown of numerical relativity waveforms, are widely used in waveform modeling, and underpin key tests of general relativity and of the nature of compact objects; a program sometimes called BH spectroscopy. Perturbation theory also predicts a specific spatial shape for each QNM perturbation. For the Kerr metric, these are the ($s=-2$) spheroidal harmonics. Spatial information can be extracted from numerical relativity by fitting a feature with known time dependence to all of the spherical harmonic modes, allowing the shape of the feature to be reconstructed; a program initiated here and that we call BH cartography. Accurate spatial reconstruction requires fitting to many spherical harmonics and is demonstrated using highly accurate Cauchy-characteristic numerical relativity waveforms. The loudest QNMs are mapped, and their reconstructed shapes are found to match the spheroidal harmonic predictions. The cartographic procedure is also applied to the quadratic QNMs -- nonlinear features in the ringdown -- and their reconstructed shapes are compared with expectations based on second-order perturbation theory. BH cartography allows us to determine the viewing angles that maximize the amplitude of the quadratic QNMs, an important guide for future searches, and is expected to lead to an improved understanding of nonlinearities in BH ringdown.

Title to be confirmed

Abstract not available

• Speaker: Hendrik Hildebrandt (Ruhr University Bochum)
• Monday 16 June 2025, 13:00-14:00
• Venue: CMS, Pav. B, CTC Common Room (B1.19) [Potter Room].
• Series: Cosmology Lunch; organiser: Louis Legrand.

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• Speaker: Arnaud de Mattia (IRFU, CEA, Université Paris-Saclay)
• Monday 12 May 2025, 13:00-14:00
• Venue: CMS, Pav. B, CTC Common Room (B1.19) [Potter Room].
• Series: Cosmology Lunch; organiser: Louis Legrand.

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An international team of astrophysicists has imaged a large number of exocomet belts around nearby stars, and the tiny pebbles within them. The crystal-clear images show light being emitted from these millimetre-sized pebbles within the belts that orbit 74 nearby stars of a wide variety of ages – from those that are just...

arXiv:2501.08372v1 Announce Type: new Abstract: Data from the Euclid space telescope will enable cosmic shear measurements with very small statistical errors, requiring corresponding systematic error control level. A common approach to correct for shear biases involves calibrating shape measurement methods using image simulations with known input shear. Given their high resolution, Hubble Space Telescope (HST) galaxies can, in principle, be utilised to emulate Euclid observations. In this work, we employ a GalSim-based testing environment to investigate whether uncertainties in the HST point spread function (PSF) model or in data processing techniques introduce significant biases in weak-lensing (WL) shear calibration. We used single S\'ersic galaxy models to simulate both HST and Euclid observations. We then `Euclidised' our HST simulations and compared the results with the directly simulated Euclid-like images. For this comparison, we utilised a moment-based shape measurement algorithm and galaxy model fits. Through the Euclidisation procedure, we effectively reduced the residual multiplicative biases in shear measurements to sub-percent levels. This achievement was made possible by employing either the native pixel scales of the instruments, utilising the Lanczos15 interpolation kernel, correcting for noise correlations, and ensuring consistent galaxy signal-to-noise ratios between simulation branches. However, the Euclidisation procedure requires further analysis on the impact of the correlated noise, to estimate calibration bias. Additionally, we conducted an in-depth analysis of the accuracy of TinyTim HST PSF models using star fields observed in the F606W and F814W filters. We observe that F606W images exhibit a broader scatter in the recovered best-fit focus, compared to those in the F814W filter.

Title to be confirmed

Abstract not available

• Speaker: Kim V. Berghaus (Caltech)
• Monday 24 March 2025, 13:00-14:00
• Venue: CMS, Pav. B, CTC Common Room (B1.19) [Potter Room].
• Series: Cosmology Lunch; organiser: Thomas Colas.

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Reconstructing the History of the Milky Way Galaxy Using Stars

Astronomy of the Milky Way Galaxy has entered a transformative era. The Gaia mission and an ensemble of ground-based spectroscopic surveys are delivering element abundances and velocities for millions of stars. These data provide both an opportunity to deepen our understanding of galaxy formation and to test the “limits of knowledge.” There have been several surprises that have come out of the large stellar surveys and data-driven methodologies built to analyse them. We have learned that up to 1 in 100 stars in the disk are “abundance doppelgangers” – chemically identical but unrelated – limiting the prospect of reconstructing the disk’s star cluster building blocks. Furthermore, for stars in the disk, most of the element abundances measured for most of the stars can be predicted to a precision of better than 10 percent given only two key abundances. However, this is not the case for stars in the stellar halo. These findings frame how we can most effectively work with the data to turn photons into a quantified description of Galactic history and provide strong constraints on the star formation and mixing processes that have set the Galactic environment.

• Speaker: Melissa Ness, Research School of Astronomy and Astrophysics (RSAA), Australian National University, Canberra, Australia
• Thursday 13 March 2025, 16:00-17:00
• Venue: Hoyle Lecture Theatre, IoA (tea at 3:30 pm).
• Series: The Eddington Lectures ; organiser: .

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Reconstructing the History of the Milky Way Galaxy Using Stars

Astronomy of the Milky Way Galaxy has entered a transformative era. The Gaia mission and an ensemble of ground-based spectroscopic surveys are delivering element abundances and velocities for millions of stars. These data provide both an opportunity to deepen our understanding of galaxy formation and to test the “limits of knowledge.” There have been several surprises that have come out of the large stellar surveys and data-driven methodologies built to analyse them. We have learned that up to 1 in 100 stars in the disk are “abundance doppelgangers” – chemically identical but unrelated – limiting the prospect of reconstructing the disk’s star cluster building blocks. Furthermore, for stars in the disk, most of the element abundances measured for most of the stars can be predicted to a precision of better than 10 percent given only two key abundances. However, this is not the case for stars in the stellar halo. These findings frame how we can most effectively work with the data to turn photons into a quantified description of Galactic history and provide strong constraints on the star formation and mixing processes that have set the Galactic environment.

• Speaker: Melissa Ness, Research School of Astronomy and Astrophysics (RSAA), Australian National University, Canberra, Australia
• Thursday 13 March 2025, 16:00-17:00
• Venue: Hoyle Lecture Theatre, IoA (tea at 3:30 pm).
• Series: The Eddington Lectures ; organiser: .

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arXiv:2409.04200v3 Announce Type: replace Abstract: The Zwicky Transient Facility SN Ia Data Release 2 (ZTF SN Ia DR2) contains more than 3,000 Type Ia supernovae (SNe Ia), providing the largest homogeneous low-redshift sample of SNe Ia. Having at least one spectrum per event, this data collection is ideal for large-scale statistical studies of the photometric, spectroscopic and host-galaxy properties of SNe Ia, particularly of the rarer 'peculiar' sub-classes. In this paper we first present the method we developed to spectroscopically classify the SNe in the sample, and the techniques we used to model their multi-band light curves and explore their photometric properties. We then show a method to distinguish between the peculiar sub-types and the normal SNe Ia. We also explore the properties of their host galaxies and estimate their relative rates, focusing on the peculiar sub-types and their connection to the cosmologically useful SNe Ia. Finally, we discuss the implications of our study with respect to the progenitor systems of the peculiar SN Ia events.

arXiv:2409.04200v3 Announce Type: replace Abstract: The Zwicky Transient Facility SN Ia Data Release 2 (ZTF SN Ia DR2) contains more than 3,000 Type Ia supernovae (SNe Ia), providing the largest homogeneous low-redshift sample of SNe Ia. Having at least one spectrum per event, this data collection is ideal for large-scale statistical studies of the photometric, spectroscopic and host-galaxy properties of SNe Ia, particularly of the rarer 'peculiar' sub-classes. In this paper we first present the method we developed to spectroscopically classify the SNe in the sample, and the techniques we used to model their multi-band light curves and explore their photometric properties. We then show a method to distinguish between the peculiar sub-types and the normal SNe Ia. We also explore the properties of their host galaxies and estimate their relative rates, focusing on the peculiar sub-types and their connection to the cosmologically useful SNe Ia. Finally, we discuss the implications of our study with respect to the progenitor systems of the peculiar SN Ia events.

The European Space Agency’s Milky Way-mapper Gaia has completed the sky-scanning phase of its mission, racking up more than three trillion observations of about two billion stars and other objects over the last decade to revolutionise the view of our home galaxy and cosmic neighbourhood. Launched on 19 December 2013, Gaia’...

It is with great sadness that we announce the passing of our colleague and friend, Sverre Aarseth, on December 28, 2024, at the age of 90. A former student of Fred Hoyle, Sverre was the earliest resident researcher at the Institute of Astronomy and remained a constant and influential presence in the department for many...

Title to be confirmed

Abstract not available

• Speaker: Matt Grayling (IoA)
• Friday 21 March 2025, 11:30-12:30
• Venue: Ryle Seminar Room, KICC + online.
• Series: Galaxies Discussion Group; organiser: Sandro Tacchella.

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Title to be confirmed

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• Speaker: Giulia Cusin (Institut d'Astrophysique de Paris)
• Monday 17 March 2025, 13:00-14:00
• Venue: CMS, Pav. B, CTC Common Room (B1.19) [Potter Room].
• Series: Cosmology Lunch; organiser: Louis Legrand.

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arXiv:2501.06342v1 Announce Type: new Abstract: We present the results of the Distant Giants Survey, a three-year radial velocity (RV) campaign to search for wide-separation giant planets orbiting Sun-like stars known to host an inner transiting planet. We defined a distant giant to have $a$ = 1--10 AU and $M_{p} \sin i = 70-4000$ \mearth~ = 0.2-12.5 \mj, and required transiting planets to have $a<1$ AU and $R_{p} = 1-4$ \rearth. We assembled our sample of 47 stars using a single selection function, and observed each star at monthly intervals to obtain $\approx$30 RV observations per target. The final catalog includes a total of twelve distant companions: four giant planets detected during our survey, two previously known giant planets, and six objects of uncertain disposition identified through RV/astrometric accelerations. Statistically, half of the uncertain objects are planets and the remainder are stars/brown dwarfs. We calculated target-by-target completeness maps to account for missed planets. We found evidence for a moderate enhancement of distant giants (DG) in the presence of close-in small planets (CS), P(DG|CS) = $30^{+14}_{-12}\%$, over the field rate of P(DG) = $16^{+2}_{-2}\%$. No enhancement is disfavored ($p \sim$ 8%). In contrast to a previous study, we found no evidence that stellar metallicity enhances P(DG|CS). We found evidence that distant giant companions are preferentially found in systems with multiple transiting planets and have lower eccentricities than randomly selected giant planets. This points toward dynamically cool formation pathways for the giants that do not disturb the inner systems.

arXiv:2501.06295v1 Announce Type: new Abstract: We present VIRAC2, a catalogue of positions, proper motions, parallaxes and $Z$, $Y$, $J$, $H$, and $K_s$ near-infrared photometric time series of 545 346 537 unique stars. The catalogue is based on a point spread function fitting reduction of nearly a decade of VISTA VVV and VVVX images, which cover $560~{\rm deg}^2$ of the Southern Galactic plane and bulge. The catalogue is complete at the $>90$ per cent level for $11