Alexander James Mustill

Context. Understanding the large-scale dynamics of molecular clouds (MCs) is crucial to constraining the processes that govern star formation and the structure and evolution of the Galaxy. While gas tracers have traditionally been used to map MC kinematics, stellar tracers such as young stellar objects (YSOs) and open clusters (OCs) provide a complementary approach that enables direct comparisons between the stellar and gaseous components. Aims. We aim to compare the kinematics of young (≤30 Myr) OCs with YSOs in the same star-forming region, the latter being assumed to trace the motions of the parent cloud gas. Subsequently, we intend to reconstruct the three-dimensional (3D) motions of the main MC complexes within 2.5 kpc of the Sun using both YSOs and young OCs as tracers. Methods. Using Gaia Data Release 3 (DR3) astrometry and complementary spectroscopic surveys providing radial velocities (RVs), we compiled a unified sample of24 146 young stars, comprising YSOs and OC members. We applied robust clustering in proper-motion space to identify co-moving YSOs and derived cloud-averaged motions via Monte Carlo sampling. These motions were compared with the kinematics inferred from OCs younger than 30 Myr. Finally, we performed orbital integrations in a realistic Galactic potential to trace the past evolution of the clouds and to quantify their expansion and rotation. Results. We derive homogeneous 3D kinematics for 15 star-forming regions within 2.5 kpc. YSOs and OCs exhibit strongly consistent kinematics within each cloud, with a median spatial velocity offset of ≲2 km s ⁻¹ , allowing us to combine them into a unified tracer sample. The resulting bulk cloud motions show a median peculiar velocity of ≃9 km s ⁻¹ with respect to the Galactic rotation. We used our catalogue to trace back the Solar System’s voyage through the Orion cloud and the common origin of Lupus, Ophiuchus, and Corona Australis in the Sco-Cen region. Internally, we detected significant expansion in Orion and Ophiuchus (at a 5 σ level) and coherent rotation in at least six complexes, likely reflecting local processes such as stellar feedback and angular-momentum redistribution. Conclusions. Young OCs and YSOs exhibit broadly consistent kinematics, and therefore OCs can also be used to trace the bulk motion of the star-forming regions.

A growing number of white dwarfs (WDs) have been observed to exhibit broadband photometric variability indicative of transiting material from the remnants of their planetary systems. The transits typically arise from comet-like bodies, asymmetric disks, or other non-solid structures and exhibit temporally varying morphology. The long-term behavior of these systems is poorly understood, but the systems observed so far exhibit diverse behavior. We present Camera for Quasars in Early Universe (CQUEAN) and ProEM i -band photometry of the known transiting debris around WD ZTF J0328-1219, from 2023 November to 2024 January. Variability is observed at a qualitatively similar depth to that at the discovery epoch. However, the duration of our observations was too short to draw a statistically significant, quantitative comparison.

The radii of small exoplanets form two populations, super-Earths and sub-Neptunes, separated by a gap known as the radius valley. This feature could be produced by the removal of atmospheres by stellar or internal heating or by the lack of an initial envelope. We used transit photometry and radial velocity measurements to detect and characterize four exoplanets orbiting LHS 1903, a red dwarf star in the Milky Way’s thick disk. These four planets have orbital periods ranging from 2.2 to 29.3 days and span the radius valley within a single planetary system. The derived densities indicate that LHS 1903 b is rocky, whereas LHS 1903 c and LHS 1903 d have extended atmospheres. The most distant planet from the host star, LHS 1903 e, has no gaseous envelope, indicating that it formed from gas-depleted material.

3I/ATLAS is the third interstellar object discovered to date, following 1I/‘Oumuamua and 2I/Borisov. Its unusually high excess velocity and active cometary nature make it a key probe of the Galactic population of icy planetesimals. Understanding its origin requires its past trajectory through the Galaxy to be traced and the possible role of stellar encounters to be assessed, both as a potential origin and a perturber to its orbit. We integrated the orbit of 3I/ATLAS backward in time for 10 Myr, together with a sample of Gaia DR3 stars with high-quality astrometry and radial velocities, to identify close passages within 2 pc. We identify 93 nominal encounters, 62 of which are significant at the 2 σ level. However, none of these encounters produced any meaningful perturbation. The strongest perturber Gaia DR3 6863591389529611264 at 0.30 pc and with a relative velocity of 35 km s ⁻¹ , imparted only a velocity change of ∣Δ v ∣ ≃ 5 × 10 ⁻⁴ km s ⁻¹ to the orbit of 3I/ATLAS. Our results indicate that no stellar flybys within the past 10 Myr and 500 pc contained in Gaia DR3 can account for the present trajectory of 3I/ATLAS or be associated with its origin. We further show that 3I/ATLAS is kinematically consistent with a thin-disk population, despite its large peculiar velocity.

Context . Beta Pictoris is an A-type star that hosts a complex planetary system with two massive gas giants and a prominent debris disc. Variable absorption lines in its stellar spectrum have been interpreted as signatures of exocomets – comet-like bodies transiting the star. Stellar flybys can gravitationally perturb objects in the outer comet reservoir, altering their orbits and potentially injecting them into the inner system, thereby triggering exocomet showers. Aims . We assessed the contribution of stellar flybys to the observed exocomet activity by reconstructing the stellar encounter history of β Pictoris in the past and future. Methods . We used Gaia DR3 data, supplemented with radial velocities from complementary spectroscopic surveys, to compile a catalogue of stars currently within 80 pc of β Pictoris. Their orbits were integrated backwards and forwards in time in an axisymmetric Galactic potential (via the GALA package) to identify encounters within 2 pc of the system. Results . We identified 99 416 stars currently within 80 pc of β Pictoris with resolved kinematics. Among these, 49 stars (including the eight components of five binaries) encounter β Pictoris within 2 pc between –1.5 Myr and +2 Myr. For four of the binaries, the centre-of-mass trajectories also pass within 2 pc. We estimated the sample to be more than 60% complete within 0.5 Myr of today. Conclusions . Despite β Pictoris being the eponym of its famous moving group, none of the identified encounters involved its moving group members; all are unrelated field stars. We found no encounter capable of shaping the observed disc structures, although stellar flybys may contribute to the long-term evolution of an Oort Cloud-like structure. Our catalogue constitutes the most complete reconstruction of the β Pictoris encounter history to date and provides a robust foundation for future dynamical simulations.

Small bodies exist in distinct populations within their planetary systems. These reservoir populations hold a range of compositions, which to first order are dependent on formation location relative to their star. We provide a general overview of the nature of the reservoirs that source exocomets, from the influence of the stellar environment through planetesimal formation to comparisons with Solar System populations. Once transitioned from a young protoplanetary disc to a debris disc, a star can expect to be rained with exocomets. While exocomets are predominantly detected to date at A-type stars, planetesimals plausibly exist across a range of stellar masses, based on exoplanet abundance, debris disc occurrence and white dwarf infall.