The rapid formation of supermassive black holes (SMBHs) at high redshifts is still a puzzle. One hypothesis is that intermediate-mass black holes (IMBHs) serve as seeds for their formation, which could arise from hierarchical mergers in dense star clusters. There are two possible pathways for IMBH formation: 1) very massive stars may form in young star clusters, such as Pop3 clusters, and evolve into IMBHs within a few million years; 2) multiple stellar-mass black holes can merge into IMBHs in dense nuclear star clusters. Detailed insights into these scenarios can be obtained through high-resolution star-by-star simulations of dense star clusters. Furthermore, upcoming observations of faint quasars, nuclear star clusters, and Pop3 stars with the James Webb Space Telescope (JWST) will offer valuable data to constrain theoretical models and deepen our understanding of the rapid formation of SMBHs.

One of the largest sources of systematics in time-delay cosmography arises from Mass Sheet Transformation (MST). The degeneracy associated with this transformation is often broken by an assumed profile shape, such as a power-law. A hierarchical strategy has been developed which constrains the global profile shape on a population level, constrained collectively by the kinematics measurements of the lenses. This framework allows one to include non-time-delay lenses to provide constraints to the global profile, improving the H0 constraints. This work tests the hierarchical framework using analytical profiles, and additionally tests the capacity to combine two populations which come from the same profiles but probe different radii due to a change in source redshift. We find that the hierarchical framework is able to compensate for this effect, and the addition of non-time-delay lenses improves the H0 constraint, even though these lenses have different Einstein radii than their time-delay counterparts.

When low- and intermediate-mass stars pass through the Asymptotic Giant Branch (AGB) they experience dramatic changes in their circumstellar shell (CSE) influenced by their mass loss, the possible presence of a (closeby) companion and the magnetic field. Masers, well spread in this environment, provide a powerful tool to reveal the CSE changes occurring when the stars undergo a transitional phase on the AGB. These can be indirect, via for instance the modification of the pumping conditions or a direct consequence of e.g. a companion and/or of the magnetic field. Evidences of such changes have been observed towards Miras, materialized by strong - both in intensity and degree of polarisation - (OH) flaring events and towards stars believed to be transitioning from the Mira to the OH/IR phase, showing an unusual high degree of polarisation. How OH maser emission can be used as a signpost of transitional phases along the AGB is explored.

The evolution of granulation is an important mechanism of the light variations of red supergiants (RSGs). Based on pure and complete samples of RSGs in the Magellanic Clouds, the mechanisms and characteristics of the granulation of RSGs are investigated based on time-series data. As predicted by the basic physical process of granulation and previous works, there are tight relations between granulation and stellar parameters of RSGs (i.e., the scaling relations). The scaling relations of RSGs provide a new method to infer stellar parameters by using the characteristic timescale and amplitude of granulations. Some faint sources deviate from the scaling relations, which may be due to the difference in the properties of the granulation of the RSGs before and after the blue loop or contamination by Mira variables. However, both of these possibilities suggest that the scaling relations of granulation is different among different types of stars.

The protostellar environment where young stars form has physical conditions suitable to excite a number of molecular maser lines that have traditionally provided an unique probe of star formation kinematics, at the highest angular resolution of radio very long baseline interferometry (VLBI) observations. In the following, we will discuss a number of recent results on our understanding of the gas dynamics traced by masers in the vicinity of young forming stars. These findings provide direct clues on how our community can substantially contribute to the field of star formation in the next decade.

Water fountains (WFs) are thought to represent an early stage in the morphological evolution of circumstellar envelopes surrounding low- and intermediate-mass evolved stars. These objects are considered to transition from spherical to asymmetric shapes. Despite their potential importance in this transformation process of evolved stars, there are only a few known examples. To identify new WF candidates, we used databases of circumstellar OH (1612 MHz) and H2O (22.235 GHz) maser sources, and compared the velocity ranges of the two maser lines. Finally, 41 sources were found to have a velocity range for the H2O maser line that exceeded that of the OH maser line. Excluding known planetary nebulae and after reviewing the maser spectra in the original literature, we found for 11 sources the exceedance as significant, qualifying them as new WF candidates.

In this work, the secular evolution of exoplanetary systems is investigated, when the variability of the masses of celestial bodies is the leading factor of dynamical evolution. The masses of the parent star and the planets change due to the particles leaving the bodies and falling on them. At the same time, bodies masses are assumed to change isotropically at different rates. The law of mass change is considered to be known and given function of time. The relative motions of the planets are investigated by the methods of the canonical perturbation theory in the absence of resonances. It is assumed that the orbits of the planets do not intersect. Evolutionary equations in analogues of Poincaré variables (Λi, λi, ξi, ηi, pi, qi) are obtained and used to study the K2-3 exoplanetary system. All analytical and numerical calculations are performed with the aid of the Wolfram Mathematica.

Multi-transition SiO maser emission has been detected in over 10 thousand evolved stars across the plane of the Milky Way by the Bulge Asymmetries and Dynamical Evolution (BAaDE) survey. In addition to the large source catalog of the survey, the frequency coverage is also unprecedented: the J=1-0 (43 GHz) data cover seven separate transitions of SiO, and the J=2-1 (86 GHz) data cover ten SiO transitions. In contrast, most other SiO maser data only probe the SiO v=1 and v=2 at 43 GHz and/or the v=1 at 86 GHz. Our extended range allows for the derivation of SiO line ratios for a huge population of evolved stars, including those derived from rare transitions associated with 29SiO and 30SiO isotopologues. We examine how these ratios are affected by the specific combinations of transitions that are detected in a single source. Furthermore, we present a class of ‘isotopologue dominated’ sources where the 29SiO transitions are the brightest in the 43 GHz spectrum. Finally, using Optical Gravitational Lensing Experiment (OGLE) light curves of our maser stars, changes in line ratios as a function of stellar phase are discussed.

Solar-like stars evolve through the Asymptotic Giant Branch (AGB) phase. This phase is characterized by increased radii, high luminosities, and significant mass loss. In order to understand the survival of companions during this phase, and explain the presence of planets orbiting white dwarfs, it is essential to examine the orbital evolution of these systems. Several physical mechanisms come into play for AGB stars, including stellar mass loss and tidal interactions between the star and its companion. Assessing mass-loss rates and accretion to the companion requires complex radiation-hydro-chemical simulations. Furthermore, comprehending the full history of tidal dissipation in low-mass stars during their late evolutionary stages, which strongly depends on their internal structure, requires dedicated analytical and numerical studies.

We present mean horizontal branch absolute magnitudes and iron abundances for a sample of 39 globular clusters. These quantities were calculated in an unprecedented homogeneous fashion based on Fourier decomposition of ligt curves of RR Lyrae cluster members. Zero points for the luminosity calibrations are discussed. Our photometrically derived metallicities and distances compare very well with spectroscopic determinations of [Fe/H] and accurate distances obtained using Gaia and Hubble Space Telescope data. The need to distinguish between the results for RRab and RRc stars for a correct evaluation of the MV–[Fe/H] relation is discussed. For RRab stars, the relation is non-linear, and the horizontal branch structure plays a significant role. For RRc stars, the relation remains linear and tight, and the slope is very shallow. Hence, the RRc stars seem better indicators of the parental cluster distances. Systematic time-series CCD imaging performed over the last 20 years enabled to discover and classify 330 variables in our sample of globular clusters.

In gravitational imaging, the mass model for the main lensing galaxy is one of the main sources of systematic uncertainty. We use subhalo detection models with increasing levels of angular complexity in the lens mass model to analyse 100 HST mock observations. We find that perturbations of just 1% are enough to cause a 20% false positive subhalo detection rate, with order 3 multipoles having the strongest effect. The area in an observation where a substructure can be detected drops by a factor of 10 if multipoles up to 3 per cent amplitude are included in the lens model. The mass of the smallest detectable substructure however is not affected. We find a detection limit of M>108.2 M⊙ at 5σ in all models. In order for strong lensing searches for dark matter objects to remain reliable in the future, angular structure beyond the elliptical power-law must be included.

Maser polarization changes during a pulsation in the CSE of an AGB star are related in a complicated way to the magnetic field structure. 43 GHz SiO maser transitions are useful for polarization study because of their relatively simple Zeeman splitting structure and their location. This work uses 3D maser simulation to investigate the effect of the magnetic field on maser polarization with different directions. The results show that linear polarization depends on the magnetic direction while circular polarization is less significant. The EVPA changes through π/2 at an angle of around 50 degrees, approximately the Van Vleck angle. The EVPA rotation result from 3D maser simulation is consistent with results from 1D simulations, and may explain the 90 degree change of the EVPA within a single cloud in the observational cases of TX Cam and R Cas.

Supermassive stars have been proposed as the solution to a number of longstanding problems in globular cluster formation. The hypothetical stars have been suggested as potential polluters responsible for the observed chemical peculiarities within those clusters. In recent hydrodynamic simulations, we have demonstrated that accretion discs around such stars are stable even with large stellar accretion and flyby rates and produce H2O kilomasers. We propose that the W1 kilomaser, associated with a super star cluster in the starburst galaxy NGC 253, may arise in an accretion disc around a supermassive star with a mass of around 4000 Mʘ.

Understanding properties of galaxies in the epoch of reionization (EoR) is a frontier in the modern astronomy. ALMA observations have demonstrated that i) some [O iii] 88 μm emitters have matured stellar populations at z>6, implying early star formation activity at z>10, and that ii) high-z star-forming galaxies typically have very high [O iii] 88 μm-to-[C ii] 158 μm luminosity ratios ranging from 3 to 12 or higher, indicating interstellar media of high-z galaxies could be highly ionized. We discuss initial results of a medium-sized JWST GO1 program that targets a sample of 12 z=6–8 ALMA [O iii] 88 μm emitters with NIRCam and NIRSPec IFU modes (GO-1840). Our JWST GO1 program, in conjunction with ALMA data, will characterize the stellar, nebular, and dust properties of these [O iii] 88 μm emitters and place this galaxies in the context of reionization.