Richard de Grijs

Context . The central regions of galaxies pose challenges for observers due to their high density of stars, gas, and dust. The Large Magellanic Cloud (LMC) hosts a dense stellar bar, whereas the Small Magellanic Cloud (SMC) hosts a dense core. Both of these dense centres have eluded even the most sensitive large-scale wide-field surveys of individual stars from the ground and from space. Aims . This work describes deep imaging observations obtained with the Visual and Infrared Survey Telescope for Astronomy (VISTA) of the inner regions of the Clouds. They were obtained to resolve stars fainter than the red clump, which due to crowding were missed in previous observations, and to quantify features associated with the star formation, kinematics, and structure of the galaxies. These studies will allow us to establish how the dynamical interactions between the LMC and the SMC, and the interaction between the Clouds and the Milky Way, have influenced the most gravitationally bound regions. Methods . Multiple near-infrared images were obtained in the J and K _s filters during excellent weather conditions (e.g. with a seeing of ∼0.7 arcsec). The data were reduced using the VISTA Data Flow System, which provides single-filter and band-merged calibrated images and catalogues, with associated confidence maps, for individual observations and combined observations for deep tiles. Results . Approximately one million and 920 thousand sources, with only minor quality issues (i.e. by excluding sources with a problematic photometric calibration or location on the detectors) and about 40 epochs per filter, are detected in the central 1.77 deg ² region of the LMC and SMC, respectively. They provide a more contiguous spatial area coverage than sources in the VISTA survey of the Magellanic Clouds system (VMC), which covers the same regions by overlapping two VISTA tiles per galaxy. Two thirds of these sources are likely stars, whereas the other one third could be stellar blends or background galaxies. The catalogues, which are made available with this work, contain parameters to select unique good-quality detections for which we provide examples of ongoing scientific applications. Conclusions . These data represent valuable complementary information to deep imaging observations at other wavelengths and provide additional epochs for sources in common with the VMC survey, for proper motion and photometric variability studies extending the time baseline to ∼11 yr, as well as targets for spectroscopic observations.

Common-envelope (CE) ejection determines which close binaries survive to become cataclysmic variables (CVs), but its efficiency, α _CE , cannot be measured cleanly in field systems, where the progenitor mass, metallicity, and elapsed time are known only through the models being tested. The dwarf nova NS Per avoids this circularity. It is an astrometric member of the open cluster NGC 1528, whose catalog age of t _cl = 209 Myr is independent of the binary, and its accreting state guarantees that the post-CE inspiral is complete. From 33 absorption-line radial velocities of the donor obtained with the MDM 2.4 m telescope, we derive P _orb = 6.29500 ± 0.00050 hr, and we compare this period with synthetic binary populations computed over a range of ejection efficiencies. In the population that reproduces NS Per, the white dwarf forms after ∼95 Myr, and only ∼19 Myr of detached inspiral separate envelope ejection from the onset of mass transfer, so the observed period still records the orbit at ejection. Within this population-synthesis framework, the cluster age implies α _CE = 0.26 (0.20–0.33, 68% credible; 90% upper limit 0.35) and excludes α _CE ≥ 0.70: efficient ejection leaves the white dwarf less massive than its donor, which precludes the stable mass transfer observed in a dwarf nova. The posterior predicts M _WD = 0.94 (0.85–1.02) M _⊙ . NS Per is the first accreting binary whose CE efficiency is calibrated against an externally supplied age, and the inefficient ejection it requires agrees with the low α _CE favored by post-CE binaries, double white dwarfs, and the white-dwarf mass distribution of CVs.

We present a comprehensive data-driven spectroscopic analysis of 357,415 red giant stars using Gaia DR3 Radial Velocity Spectrometer (RVS) spectra (8460–8700 Å; R ≈ 11, 500), aimed at deriving precise stellar parameters and elemental abundances (collectively referred to as stellar labels). We employ The Cannon, a generative model based on 2747 giants in common with GALAH DR4, adopting GALAH labels (R ≈ 28, 000) for training. The resulting model predicts eleven stellar labels for RVS giants: effective temperature (Teff), surface gravity (log g), projected rotational velocity (vsin i), and abundances of [Fe/H], [Ca/Fe], [Si/Fe], [Ni/Fe], [Ti/Fe], as well as the neutron-capture elements [Zr/Fe], [Ce/Fe], and [Nd/Fe]. Building on these results, we develop a probabilistic framework to chemically identify debris from the Gaia–Sausage–Enceladus (GSE) accretion event. A logistic regression classifier, optimised via Markov Chain Monte Carlo sampling and trained on a small reference sample of GSE members and comparison stars, identifies stars with high GSE membership probabilities based solely on their chemical abundances, with the resulting candidates exhibiting distinctive abundance-ratio patterns, including [Ca/Ti], [Ti/Ce], and [Nd/Zr]. Applying independent kinematic constraints yields a robust sample of GSE candidates, demonstrating that the characteristic chemical signatures remain consistent after applying these constraints. This work demonstrates the power of data-driven analysis techniques to extract detailed chemical information from medium-resolution spectra and establishes a framework for tracing Galactic accretion events using chemical abundances.

Context. The Small Magellanic Cloud (SMC) exhibits significant kinematic disequilibrium due to interactions with the Large Magellanic Cloud (LMC). Aims. Here, we investigate the two-dimensional stellar kinematics of the SMC to understand the dynamical effects of these interactions by exploiting the increased time baseline of 6−11 years from the VISTA Survey of the Magellanic Clouds (VMC) data release 7. Methods. We derive proper motions with a threefold improvement in precision compared to previous studies based on VMC data. We used a geometric framework accounting for perspective effects from line-of-sight motion to model the systemic motion across the SMC and construct a residual proper motion map. We further introduce an anisotropic linear velocity gradient model to quantify the stretching of the galaxy. Results. For the first time across all stellar populations, the residual proper motion map reveals expansion along the south-east and north-west directions, consistent with LMC-induced tidal forces, detectable even in the central regions. The gradient-corrected residuals show predominantly radial motions towards the SMC centre with no evidence of rotation. Velocity maps for different stellar populations, without assuming a rotating-disk model, reveal a coherent northward motion away from the centre exclusively in older red giant branch stars, interpreted as a kinematic signature of a past (> 2 Gyr ago) interaction. Conclusions. This study highlights the inadequacy of simple rotating-disk models in capturing the internal kinematics of the galaxy.

Context . Recent studies have revealed that the Milky Way’s stellar halo is a composite of stellar populations of different origins, including multiple accretion events. To better understand how the Milky Way and other spiral galaxies were formed, it is necessary to thoroughly characterize the chemical and kinematic properties of these structures. Aims . We search for kinematic structures of the stellar halo to find any substructures within them (when indeed present) and characterize the chemodynamical properties of the identified groups with the GALAH DR4 and Gaia surveys. Methods . We applied wavelet transforms in the space defined by a square root of radial action ( J _r ) and azimuthal action ( L _z ) to search for kinematic overdensities. Then, we selected stars in the detected structures and investigated their elemental abundances to determine their origin. Additionally, we checked for any contamination from other stellar populations within the detected groups with the unsupervised machine learning algorithm t-Distributed Stochastic Neighbor Embedding (t-SNE), for which we performed chemical tagging in a high-dimensional parameter space using 15 elemental abundances as input. Results . We recovered five kinematic structures in the action space with the wavelet transform. These groups are the Galactic disk, Splash, Gaia -Sausage-Enceladus (GSE), Thamnos1, and Thamnos2. We found that GSE has two peaks with the wavelet transform. One of these peaks is located at J _r ≃ 25 kpc km s ⁻¹ and is a result of contamination from disk stars. The other peak corresponds to the ‘cleanest’ GSE population and is located above J _r ≃ 40 kpc km s ⁻¹ . We also detected three peaks in Thamnos. We linked two of them to Thamnos1, while the peak with the stars on the most retrograde orbits was linked to Thamnos2. The t-SNE algorithm confirmed these findings. We also analyzed individual elemental abundances of each group and found that Thamnos2 has a higher [ α /Fe] ratio than the other groups and that iron-peak elements are more abundant in the Splash than in the halo groups, while the halo structures retain a higher r -process signature than the splashed disk. Conclusions . A multiply peaked substructure we observe in action space in GSE and Thamnos suggests that the splashed disk extends beyond the borders of prograde orbits. Each of the four halo groups studied in this paper have unique chemodynamical properties that confirm their extra-galactic origin.

Speculation about Martian canals was a recurring feature of late nineteenth-century popular astronomy. This paper examines how colonial newspapers used humour to negotiate the epistemic uncertainty and interpretive excess associated with canal theory. Drawing on over one thousand metropolitan and regional Australian newspapers published between 1877 and 1899, we identify five overlapping modes of humour: imported metropolitan wit; satire of modern engineering culture; humour grounded in observational uncertainty; scale-based exaggeration and colonial self-comparison; and overt sceptical parody. These modes tracked shifting relationships between observation, interpretation and authority, allowing newspapers to entertain speculative ideas while marking the limits of scientific credibility. At the same time, humorous treatments positioned Australian readers within a global culture of science and modernity. Comparisons with projects such as the Suez and Panama Canals, and with European and American astronomers, aligned colonial audiences with metropolitan discourse, even as local experience with land, water and scale shaped the tone of satire. We demonstrate that Australian newspapers did not passively transmit overseas ideas but actively reworked them through humour, balancing fascination with restraint. More broadly, we show how humour operated as a shared transnational strategy for managing scientific uncertainty at the cultural margins of empire.

White dwarfs (WDs), the evolutionary endpoints of most stars, can form through both single-star and binary channels. While single-star evolutionary models enable reliable WD age estimates, binary evolution introduces interactions that can accelerate WD formation and result in a variety of exotic WDs, which may exhibit strong magnetic fields, rapid rotation, or even serve as potential gravitational wave sources. Such systems offer valuable insights into magnetic field generation, angular momentum evolution, and compact object physics. Star clusters, with their approximately coeval populations, allow precise age determination of member WDs. If a WD’s total age derived from single-star evolution exceeds that of its host cluster, it likely indicates a binary origin. In this study, we use Gaia 5D astrometry to identify 439 WD candidates in 117 open clusters, with 244 likely formed via binary evolution. We discuss the possibility of dynamical ejection for WDs meeting only 2D (proper motion space) membership criteria. Spectroscopic observations further reveal a subset with strong magnetic fields and rapid rotation, supporting their binary evolutionary origin.