Many astrophysical phenomena can only be studied in detail for objects in our galaxy, the Milly Way, but we know much more about the structure of thousands of nearby galaxies than we do about our own Galaxy. Accurate distance measurements in the Milky Way underpin our ability to understand a wide range of astrophysical phenomena and this requires observations from both the northern and southern hemisphere. Our ability to measure accurate parallaxes to southern masers has been hampered a range of factors, in particular the absence of a dedicated, homogeneous VLBI array in the south. We have recently made significant advances in astrometric calibration techniques which allow us to achieve trigonometric parallax accuracies of around 10 micro-arcseconds (μas) for 6.7 GHz methanol masers with a hetrogeneous array of 4 antennas. We outline the details of this new “multiview” technique and present the first trigonometric parallax measurements that utilise this approach.

We report VLBI monitoring observations of the 22 GHz H2O masers toward the Mira variable BX Cam. Data from 37 epochs spanning ∼3 stellar pulsation periods were obtained between May 2018 and June 2021 with a time interval of 3–4 weeks. In particular, the VERA dual-beam system was used to measure the kinematics and parallaxes of the H2O maser features. The obtained parallax, 1.79±0.08 mas, is consistent with Gaia EDR3 and previous VLBI measurements. The position of the central star was estimated relied on Gaia EDR3 data and the center position of the 43 GHz SiO maser ring imaged with KVN. Analysis of the 3D maser kinematics revealed an expanding circumstellar envelope with a velocity of 13±4 km s−1 and significant spatial and velocity asymmetries. The H2O maser animation achieved by our dense monitoring program manifests the propagation of shock waves in the circumstellar envelope of BX Cam.

We used the unprecedented resolution of ∼25 μas of the VLBI array formed with the RadioAstron satellite to study the structures of H2O maser spots in the star forming region W49N. We found that anisotropic diffractive scattering of the ISM dominates the images of the maser spot, but does not completely blur them. The refractive scattering floor is about 0.001 in visibility at a baseline of 8 Gλ.

Stellar SiO masers are found in the atmospheres of asymptotic giant branch (AGB) stars with several maser transitions observed around 43 and 86 GHz. At least 28 SiO maser stars have been detected within ∼2 pc projected distance from Sgr A* by the Very Large Array (VLA) and Atacama Millimeter/submillimeter Array (ALMA). A subset of these masers have been studied for several decades and form the basis of the radio reference frame that anchors the reference frame for infrared stars in the Galactic Center (GC). We present new observations of the GC masers from VLA and ALMA. These new data combined with extant maser astrometry provide 3D positions, velocities, and acceleration limits. The proper motions and Doppler velocities are measured with unprecedented precision for these masers. We further demonstrate how these measurements may be used to trace the stellar and dark matter mass distributions within a few pc of Sgr A*.

The chemo-dynamics of the stellar populations in the Galactic Bulge inform and constrain the Milky Way’s formation and evolution. The metal-poor population is particularly important in light of cosmological simulations, which predict that some of the oldest stars in the Galaxy now reside in its center. The metal-poor bulge appears to consist of multiple stellar populations that require chemo-dynamical analyses to disentangle. In this paper, I describe the detailed chemo-dynamical study of the metal-poor stars in the inner Galaxy, named The COMBS Survey which uses VLT/FLAMES spectra of 350 metal-poor stars. I discuss the results and the implications for early Milky Way formation and chemical evolution. In addition, I preview results from an ongoing survey of carbon-enhanced metal-poor stars, which are thought to be solely enriched by the first generation of stars.

VERA has been regularly conducting astrometry of Galactic maser sources for ∼ 20 years, producing more than 100 measurements of parallaxes and proper motions of star-forming regions as well as AGB stars. By combining the observational results obtained by VLBA BeSSeL, EVN, and LBA, maser astrometry provides a unique opportunity to explore the fundamental structure of the Galaxy. Here we present the view of the Galaxy revealed by the maser astrometry, and also discuss the importance of maser astrometry in the era of GAIA by comparing the results obtained by VLBI and GAIA. We also present our view of “proper motions toward the future” of the relevant field, expected in the next decade based on global collaborations.

This work presents the study of multiphase relations of classical Cepheids in the Magellanic Clouds for short periods (log P < 1) and long periods (log P > 1). From the analysis, it has been found that the multiphase relations obtained using the models as well as observations are highly dynamic with pulsational phase. The multiphase relations for short and long periods are found to display contrasting behaviour for both LMC and SMC. It has been observed that the multiphase relations obtained using the models agree better with the observations in the PC plane in most phases in comparison to the PL plane. Multiphase relations obtained using the models display a clear distinction among different convection sets in most phases. Comparison of models and observations in the multiphase plane is one way to test the models with the observations and to constrain the theory of stellar pulsation.

The mass-loss mechanism in asymptotic giant branch (AGB) stars is not yet fully understood. We present 20-milliarcsecond resolution ALMA imaging of the well-studied AGB star W Hya in multiple molecular lines at 250–269 GHz, including masers from SiO, H2O, and SO2. The images show complex plumes, arcs, and clumps over the stellar disk and in the atmosphere extending to several stellar radii. We detected prominent emission components over the stellar disk—instead of pure absorption as expected—in some Si17O, 30SiO, H2O, and SO2 lines. The surface emission seen in the Si17O and vibrationally excited H2O lines is particularly strong, indicating maser actions. The masers seen over the stellar disk indicate radial amplification.

IRAS 20126+4104 is an extensively studied high-mass star-forming region with many astrophysical maser lines from methanol and water molecules. The brightest and highly variable is the 6.7 GHz methanol maser transition. We present follow-up studies on this target including the monitoring with the Irbene radio telescope with high-cadence data and European VLBI Network imaging extending the VLBI monitoring to 19 years. We also plan to study the target in the future based on its variability, both in the radio domain (EVN observations are planned for June 2023) and in infra-red with the 1 meter Kagoshima University Telescope operated by Amanogawa Galaxy Astronomy Research Center (AGARC).

We monitored 22 GHz water masers in NGC 2071-IR using the Hartebeesthoek 26-m telescope and identified a significant flare (up to 4722 Jy) originating from the 14.4 km s-1 feature associated with the protostellar core NGC 2071-IRS1. To determine if the maser flare resulted from an accretion burst, we analyzed related signatures such as simultaneous flaring of other maser species and an increase in infrared luminosity. Near-infrared (Ks-band) observations conducted on 28 December 2019 during the flare, using the Kanata/HONIR telescope, exhibited a 0.2 magnitude increase in comparison to the 2MASS magnitude obtained from observations conducted on 10 October 1999. However, our findings indicate that the flare was attributed to mechanisms other than an accretion burst.

In this invited review I discuss the calibration and applications of the period–luminosity relation of classical Cepheid and RR Lyrae stars. After a brief introduction, starting with results from Hipparcos and discussing some post-Hipparcos era developments, I focus on recent results using Gaia Data Release 3 data. I present an overview of the most recent period–luminosity relations, a discussion and some new results on Cepheids in open clusters. I also discuss the effect of reddening and that the use of Wesenheit indices is actually an oversimplification to dealing with the problem of reddening.

Pulsating variable δ Scuti stars are intermediate-mass stars with masses in the range of 1–3 δ and spectral types between A2 and F2. They can be found at the intersection of the Cepheid instability strip with the main sequence. They can be used as astrophysical laboratories to test theories of stellar evolution and pulsation. In this contribution, we investigate the observed period–colour and amplitude–colour (PCAC) relations at maximum/mean/minimum light of Galactic bulge and Large Magellanic Cloud δ Scuti stars for the first time and test the hydrogen ionization front (HIF)-photosphere interaction theory using the mesa-rsp code. The PCAC relations, as a function of pulsation phase, are crucial probes of the structure of the outer stellar envelope and provide insight into the physics of stellar pulsation and evolution. The observed behaviour of the δ Scuti PCAC relations is consistent with the theory of the interaction between the HIF and the stellar photosphere.

Old stellar populations in our Galaxy provide fossil records of the metal enrichment in the first few billion years of the cosmic history. Growing elemental abundance data of individual stars combined with stellar ages and kinematics allow us to make constraints on characteristic properties of the metal-enrichment sources in the early Universe, such as the first stars. In order to interpret observed chemical abundances in the oldest stellar populations in terms of metal-enrichment sources, stellar and supernova yield models are crucial. In this article, we review how we can interpret observed chemical abundances in old stars in terms of the nature of metal enrichment sources. We discuss the limitations and the prospects of empirically constraining supernova yield models based on a large sample of extremely metal-poor stars. At the same time, we emphasize the importance of hunting old stars beyond the Solar neighborhood, which can be achieved with the next-generation multi-object spectrographs at large telescopes.

LLAMA (Large Latin America Millimeter/submillimeter Array) is a new radio observatory that is being constructed in a collaboration between Argentina and Brazil. It will consist of a 12 meters diameter antenna that is being installed in Alto Chorrillos at 4850 m of altitude, in the Salta province of Argentina. Alto Chorrillos is a high-quality astronomical site similar to Chajnantor (Chile), where ALMA observatory operates. When completed, LLAMA will allow line, continuum and linear polarization observations between 35 and 700 GHz, approximately. For the first light, LLAMA will be equipped with ALMA-like receivers at bands 5 (163 - 211 GHz), 6 (211 - 275 GHz) and 9 (602 - 720 GHz). LLAMA is being planned to be a versatile astronomical facility that will serve the scientific community for the exploration of scientific topics as diverse as the molecular evolution of the Universe, black holes and their accretion disks, astrophysical jets, stellar formation and evolution, the structure of our galaxy and the Sun, planetary atmospheres and extragalactic astronomy. In this work, I will present the LLAMA project and the perspectives for this new astronomical facility in the context of the investigation of galactic and extragalactic masers.

We have made a lot of progress in the study of the MW. In spite of this, much of our Galaxy remains unknown, and amazing breakthroughs await to be made in the exploration of the far side of the Galaxy. Focussing on the Galactic extinction horizon problem with current surveys like the Two Micron All-Sky Survey (2MASS) and the Vista Variables in the Via Lactea Survey (VVV) and its extension VVVX, the extinction horizon is a fundamental difficulty, and it is my intention here to reveal how profound is our ignorance, and also to try to suggest ways for improvement with future near-IR Galactic surveys.

We compare published solutions for the rotation angle W(t) which describes the location of the prime meridian of Mars with respect to the ICRF equator in IAU recommendations. If the model for W includes a very long period term, we transform it into a quadratic polynomial with updated epoch value and rate, resulting in a difference in the mean epoch value up to 200 km. The mean and true epoch rotation angles are about 800 mas (13 m) apart in J2000 and should not be confused with each other in order to accurately locate the prime meridian. We identify two groups of radio-science solutions for W, which can be distinguished by the prime meridian location they used as a priori that differ from each other by about 100 m at J2000.

We started high-cadence monitoring observations of 6.7 GHz methanol masers from Dec. 2012 using Hitachi 32-m radio telescope (Yonekura et al.(2016). Observations have been conducted basically every day. On average, 13 hours of observations have been made per day, amounting to 4000–5000 hours per year. The cadence varies by sources: one observation in 1–50 days. In addition to already known 29 sources (Tanabe et al. 2023 and references therein), we have newly identified ∼20 sources with periodic flux variability. We have also detected 5 sources with sudden flux rises in 2019–2022, including G358.93–0.03 which was confirmed to be associated with the accretion burst.

While the direct detection of the dark-matter particle remains very challenging, the physical properties of dark matter could potentially be constrained indirectly – by comparing the population characteristics of substructures in real galaxies with predictions from the phenomenological dark-matter models, such as cold, warm or self-interacting dark matter. Whereas these models are practically indistinguishable with respect to the expected abundance and statistical properties of massive galactic substructures, the critical difference lies in the low-mass regime ≲ 108M⊙. Galaxy-galaxy strong gravitational lensing provides a unique opportunity to search for gravitational signatures of such low-mass substructures in galaxies acting as a strong gravitational lens on a more distant background galaxy, serendipitously located along the same line of sight. In [Bayer et∼al.(2023)Bayer, Chatterjee, Koopmans, Vegetti, McKean, Treu, Fassnacht, and Glazebrook, Bayer et∼al.(2023)Bayer, Koopmans, McKean, Vegetti, Treu, Fassnacht, and Glazebrook, Bayer et∼al.(2023)Bayer, Vernardos, and Koopmans], we have recently introduced a novel approach to investigate the collective perturbative effect of low-mass substructures in the inner regions of massive elliptical lens galaxies and observationally constrain their power spectrum (on 1-10 kpc scales) based on the Fourier analysis of the associated surface-brightness anomalies in the lensed images (i.e., Einstein rings and gravitational arcs) observed with the Hubble Space Telescope. Here, we present a concise summary of the methodology, the encountered modelling challenges and the inferred observational constraints on the sub-galactic matter power spectrum. The future comparison of these results with predictions from hydrodynamical simulations might either verify the cold-dark-matter paradigm or require its substantial revision. Moreover, we demonstrate that the grid-based smooth lens modelling might model away surface-brightness anomalies caused by the presence of substructures in the lens galaxy and incorrectly interpret them as surface-brightness structure in the lensed background galaxy itself. If not properly understood and accounted for, such signal suppression might lead to severely biased constraints on the properties of substructures in galactic haloes.

Galaxy-scale strong lensing is a powerful tool in Astrophysics and Cosmology, enabling studies of massive galaxies’ internal structure, their formation and evolution, stellar initial mass function, and cosmological parameters. In this conference proceeding, we highlight key findings from the past decade in astrophysical applications of strong lensing at the galaxy scale. We then briefly summarize the present status of discovery and analyses of new samples from recent or ongoing surveys. Finally, we offer insights into anticipated developments in the upcoming era of big data shaping the future of this field, thanks to the Rubin, Euclid, and Roman observatories.