We present a sample of 19,583 ultracool dwarf candidates brighter than z ≤23 selected from the Dark Energy Survey DR2 coadd data matched to VHS DR6, VIKING DR5 and AllWISE covering ∼ 4,800 deg². The ultracool candidates were first pre-selected based on their (i-z), (z-Y), and (Y-J) colours. They were further classified using a method that compares their optical, near-infrared and mid-infrared colours against templates of M, L and T dwarfs. 14,099 objects are presented as new L and T candidates and the remaining objects are from the literature, including 5,342 candidates from our previous work. Using this new and deeper sample of ultracool dwarf candidates we also present: 20 new candidate members to nearby young moving groups (YMG) and associations, variable candidate sources and four new wide binary systems composed of two ultracool dwarfs. Finally, we also show the spectra of twelve new ultracool dwarfs discovered by our group and presented here for the first time. These spectroscopically confirmed objects are a sanity check of our selection of ultracool dwarfs and photometric classification method.

We investigate the distribution of the lithium abundances, A(Li), of metal-poor dwarf and subgiant stars within the limits 5500 K < T_eff < 6700 K, −6.0 < [Fe/H] < −1.5, and log g ≳ 3.5 (a superset of parameters first adopted by Spite and Spite), using literature data for some 200 stars. We address the problem of the several methods that yield T_eff differences up to 350 K, and hence uncertainties of 0.3 dex in [Fe/H] and A(Li), by anchoring T_eff to the Infrared Flux Method. We seek to understand the behaviour of A(Li) as a function of [Fe/H] – small dispersion at highest [Fe/H], ‘meltdown’ at intermediate values (i.e. large spread in Li below the Spite Plateau), and extreme variations at lowest [Fe/H]. Decreasing A(Li) is accompanied by increasing dispersion. Insofar as [Fe/H] increases as the universe ages, the behavior of A(Li) reflects chaotic star formation involving destruction of primordial Li, which settles to the classic Spite Plateau, with A(Li) ∼ 2.3, by the time the Galactic halo reaches [Fe/H] ∼ −3.0. We consider three phases: (1) first star formation in C-rich environments ([C/Fe] > 2.3), with depleted Li; (2) silicates-dominated star formation and destruction of primordial Li during pre-main-sequence evolution; and (3) materials from these two phases co-existing and coalescing to form C-rich stars with A(Li) below the Spite Plateau, leading to a toy model with the potential to explain the ‘meltdown’. We comment on the results of Mucciarelli et al. on the Lower RGB, and the suggestion of Aguado et al. favouring a lower primordial lithium abundance than generally accepted.

In the second paper of this series, we developed a new distance determination method using the median J magnitude of carbon-rich asymptotic giant branch stars (CS) as standard candles and the Magellanic Clouds as the fundamental calibrators. The J-band CS luminosity function was modeled using a modified Lorentzian distribution whose parameters were used to determined whether the LMC or SMC was the most suitable calibrator. In this third paper of the series, we expand our sample of galaxies and introduce a more robust method to determine the parameters of the Lorentzian model. The new fitting method uses an un-binned maximum likelihood estimator to determine the parameters of the Lorentzian model resulting in parameter errors that are significantly smaller compared to the second paper. We test our method in NGC 6822, IC 1613, NGC 3109 and WLM. We also estimate the distances to the same sample of galaxies via the tip of the red giant branch (TRGB) detection method. Our results from the CS measurements agree well with those obtained from the TRGB.

Launch of the Gaia space observatory started a new era in astrometry when the accuracy of star coordinates increased by thousands of times. Significant improvement of accuracy was also expected for the coordinates of the Solar system bodies. Gaia DR3 provided us with the data which could be used to test our expectations. In this work, we refine the orbits of a number of outer planetary salellites using both ground-based and Gaia observations. From thirteen outer satellites observed by Gaia, we chose six to obtain their orbits. Some specific moments in using observations of outer satellites made by Gaia are demonstrated. These pecularities stem from scanning motion of Gaia, in particular from the fact that the accuracy of observations is significantly different along and across the scanning direction. As expected, Gaia observations proved to be more precise than those made from Earth, which results in more accurate satellite ephemerides. We estimate accuracy of the ephemerides of considered satellites for the interval between 1996 and 2030. As astrometric positions published in Gaia DR3 were not corrected for the relativistic light deflection by the Sun, we took into account this effect, which slightly diminished the rms residuals. In addition, relativistic light deflection by the giant planets was estimated, which, as it turned out, can be neglected with the given accuracy of Gaia observations.

Tracking the motions of transient jets launched by low-mass X-ray binaries (LMXBs) is critical for determining the moment of jet ejection, and identifying any corresponding signatures in the accretion flow. However, these jets are often highly variable and can travel across the resolution element of an image within a single observation, violating a fundamental assumption of aperture synthesis. We present a novel approach in which we directly fit a single time-dependent model to the full set of interferometer visibilities, where we explicitly parameterise the motion and flux density variability of the emission components, to minimise the number of free parameters in the fit, while leveraging information from the full observation. This technique allows us to detect and characterize faint, fast-moving sources, for which the standard time binning technique is inadequate. We validate our technique with synthetic observations, before applying it to three Very Long Baseline Array (VLBA) observations of the black hole candidate LMXB MAXI J1803−298 during its 2021 outburst. We measured the proper motion of a discrete jet component to be 1.37 ± 0.14 mas hr−1, and thus we infer an ejection date of MJD $59348.08_{-0.06}^{+0.05}$, which occurs just after the peak of a radio flare observed by the Australia Telescope Compact Array (ATCA) and the Atacama Large Millimeter/Sub-Millimeter Array (ALMA), while MAXI J1803−298 was in the intermediate state. Further development of these new VLBI analysis techniques will lead to more precise measurements of jet ejection dates, which, combined with dense, simultaneous multi-wavelength monitoring, will allow for clearer identification of jet ejection signatures in the accretion flow.

ε Eridani is the closest star to our Sun known to host a debris disc. Prior observations in the (sub-)millimetre regime have potentially detected clumpy structure in the disc and attributed this to interactions with an (as yet) undetected planet. However, the prior observations were unable to distinguish between structure in the disc and background confusion. Here we present the first ALMA image of the entire disc, which has a resolution of 1.6″×1.2″. We clearly detect the star, the main belt and two point sources. The resolution and sensitivity of this data allow us to clearly distinguish background galaxies (that show up as point sources) from the disc emission. We show that the two point sources are consistent with background galaxies. After taking account of these, we find that resolved residuals are still present in the main belt, including two clumps with a >3σ significance – one to the east of the star and the other to the northwest. We perform n-body simulations to demonstrate that a migrating planet can form structures similar to those observed by trapping planetesimals in resonances. We find that the observed features can be reproduced by a migrating planet trapping planetesimals in the 2:1 mean motion resonance and the symmetry of the most prominent clumps means that the planet should have a position angle of either ∼10^○ or ∼190^○. Observations over multiple epochs are necessary to test whether the observed features rotate around the star.

The water vapour in the atmosphere is the main factor affecting ground-based astronomical observations at IR and submillimetre/millimetre wavelengths. Precipitable water vapour (PWV) forecasts over astronomical sites are usually provided at the zenith. However, better observational scheduling could be carried out if PWV forecasts were provided along lines of sight from an observing position. In this study, we implemented a methodology to provide PWV forecasts along all lines of sight at the APEX observatory from a regional numerical weather model. Results show that the best observing conditions are located between -10^o̠; and 150^o̠; of azimuth, from the zenith to 50^o̠; elevation and approximately between 00 and 10 LT (4 and 14 UTC). The Chajnantor region shows a large homogeneity over the sky during each observing time, with half the time showing standard deviation (SD) values below 0.26 mm and less than 15% of the time showing SD values larger than 0.5 mm, when considering all azimuth and elevation angles from the zenith to 15^o̠; elevation. The PWV along lines of sight over this region may differ from the PWV calculated at the zenith. Thus, PWV forecasts along lines of sight should be better used in everyday operations at observatories rather than regular PWV visualizations at the zenith. This will allow better observational planning in specific directions, depending on the objects that want to be observed. In addition, site testing studies can be benefited from using this numerical methodology when searching the best sites for IR and submillimetre/millimetre observations.

In the paper we develop multi-class classification of Fermi-LAT gamma-ray sources using machine learning with hierarchical determination of classes. One of the main challenges in the multi-class classification of the Fermi-LAT sources is that the size of some of the classes is relatively small, for example with less than 10 associated sources belonging to a class. In the paper we propose an hierarchical structure for the determination of the classes. This enables us to have control over the size of classes and to compare the performance of the classification for different numbers of classes. In particular, the class probabilities in the two-class case can be computed either directly by the two-class classification or by summing probabilities of children classes in multi-class classification. We find that the classifications with few large classes have comparable performance with classifications with many smaller classes. Thus, on the one hand, the few-class classification can be recovered by summing probabilities of classification with more classes while, on the other hand, the classification with many classes gives a more detailed information about the physical nature of the sources. As a result of this work, we construct three probabilistic catalogs, which are available online. This work opens up a possibility to perform population studies of sources including unassociated sources and to narrow down searches for possible counterparts of unassociated sources, such as active galactic nuclei, pulsars, or millisecond pulsars.

The wave instability acts in astrophysical plasmas to redistribute energy and momentum in the absence of frequent collisions. There are many different types of waves, and it is important to quantify the wave energy density and growth rate for understanding what type of wave instabilities are possible in different plasma regimes. There are many situations throughout the universe where plasmas contain a significant fraction of relativistic particles. Theoretical estimates for the wave energy density and growth rate are constrained to either field-aligned propagation angles, or non-relativistic considerations. Based on linear theory, we derive the analytic expressions for the energy density and growth rate of an arbitrary resonant wave with an arbitrary propagation angle in relativistic plasmas. For this derivation, we calculate the Hermitian and anti-Hermitian parts of the relativistic-plasma dielectric tensor. We demonstrate that our analytic expression for the wave energy density presents an explicit energy increase of resonant waves in the wavenumber range where the analytic expression for the growth rate is positive (i.e., where a wave instability is driven). For this demonstration, we numerically analyse the loss-cone driven instability, as a specific example, in which the whistler-mode waves scatter relativistic electrons into the loss cone in the radiation belt. Our analytic results further develop the basis for linear theory to better understand the wave instability, and have the potential to combine with quasi-linear theory, which allows to study the time evolution of not only the particle momentum distribution function but also resonant wave properties through an instability.

Using very deep, high spectral resolution data from the SAMI Integral Field Spectrograph we study the stellar population properties of a sample of dwarf galaxies in the Fornax Cluster, down to a stellar mass of 10⁷ M_⊙, which has never been done outside the Local Group. We use full spectral fitting to obtain stellar population parameters. Adding massive galaxies from the ATLAS^3D project, which we re-analysed, and the satellite galaxies of the Milky Way, we obtained a galaxy sample that covers the stellar mass range 10⁴ to 10¹²M_⊙. Using this large range we find that the mass – metallicity relation is not linear. We also find that the [α/Fe]-stellar mass relation of the full sample shows a U-shape, with a minimum in [α/Fe] for masses between 10⁹ − 10¹⁰M_⊙. The relation between [α/Fe] and stellar mass can be understood in the following way: When the faintest galaxies enter the cluster environment, a rapid burst of star formation is induced, after which the gas content is blown away by various quenching mechanisms. This fast star formation causes high [α/Fe] values, like in the Galactic halo. More massive galaxies will manage to keep their gas longer and form several bursts of star formation, with lower [α/Fe] as a result. For massive galaxies, stellar populations are regulated by internal processes, leading to [α/Fe] increasing with mass. We confirm this model by showing that [α/Fe] correlates with clustercentric distance in three nearby clusters, and also in the halo of the Milky Way.