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We review the phenomenology of classical Cepheids (CCs), Anomalous Cepheids (ACs) and type II Cepheids (TIICs) in the Milky Way (MW) and in the Magellanic Clouds (MCs). We also examine the Hertzsprung progression in different stellar systems by using the shape of I-band light curves (Fourier parameters) and observables based on the difference in magnitude and in phase between the bump and the minimum in luminosity. The distribution of Cepheids in optical and in optical–near infrared (NIR) color–magnitude diagrams is investigated to constrain the topology of the instability strip. The use of Cepheids as tracers of young (CCs), intermediate (ACs) and old (TIICs) stellar populations are brought forward by the comparison between observations (MCs) and cluster isochrones covering a broad range in stellar ages and in chemical compositions. The different diagnostics adopted to estimate individual distances (period–luminosity, period–Wesenheit, period–luminosity–color relations) are reviewed together with pros and cons in the use of fundamental and overtones, optical and NIR photometric bands, and reddening free pseudo magnitudes (Wesenheit). We also discuss the use of CCs as stellar tracers and the radial gradients among the different groups of elements (iron, α, neutron-capture) together with their age-dependence. Finally, we briefly outline the role that near-future space and ground-based facilities will play in the astrophysical and cosmological use of Cepheids.

We present mosaic images of the Small Magellanic Cloud (SMC) observed with the Spitzer IRAC 3.6 μ m and 4.5 μ m bands over two epochs, 2017 August 25–2017 September 13 and 2017 November 24–2018 February 12. The survey region comprises ∼30 deg 2 covering the SMC and the Bridge to the Large Magellanic Cloud. The region is covered by 52 ∼ 1.°1 × 1.°1 tiles, with each tile including images in each band for both separate and combined epochs. The mosaics are made in individual tangent projections in J2000 coordinates. The angular pixel size is 0.″6 with a resolution (FWHM) of ∼2.″0. We describe processing to correct or mitigate residual artifacts and remove background discontinuities. The mosaic images are publicly available at the Infrared Science Archive.

The NGC 346 young stellar system and associated N66 giant H ii region in the Small Magellanic Cloud are the nearest example of a massive star-forming event in a low metallicity ( Z ≈ 0.2 Z ⊙ ) galaxy. With an age of ≲3 Myr this system provides a unique opportunity to study relationships between massive stars and their associated H ii region. Using archival data, we derive a total H α luminosity of L (H α ) = 4.1 × 10 38 erg s −1 corresponding to an H-photoionization rate of 3 × 10 50 s −1 . A comparison with a predicted stellar ionization rate derived from the more than 50 known O-stars in NGC 346, including massive stars recently classified from Hubble Space Telescope far-ultraviolet (FUV) spectra, indicates an approximate ionization balance. Spectra obtained with SALT suggest the ionization structure of N66 could be consistent with some leakage of ionizing photons. Due to the low metallicity, the FUV luminosity from NGC 346 is not confined to the interstellar cloud associated with N66. Ionization extends through much of the spatial extent of the N66 cloud complex, and most of the cloud mass is not ionized. The stellar mass estimated from nebular L (H α ) appears to be lower than masses derived from the census of resolved stars which may indicate a disconnect between the formation of high and low mass stars in this region. We briefly discuss implications of the properties of N66 for studies of star formation and stellar feedback in low metallicity environments.

The abundance of interstellar 7 Li in the low-metallicity gas of the Small Magellanic Cloud, a nearby galaxy with a quarter the Sun’s metallicity, is nearly equal to the Big Bang nucleosynthesis predictions. The search for local cosmological lithium-7 The predicted primordial abundance of the lithium-7 isotope in the primordial Universe is four times greater than that measured in the atmospheres of Galactic halo stars, but it is hard to trace this isotope in the Milky Way because it has most probably been burnt up. This paper reports the detection of interstellar lithium beyond the Milky Way, in the low-metallicity gas of the nearby Small Magellanic Cloud galaxy. Present-day lithium-7 abundance in this galaxy is nearly equal to the predictions of the standard theory of Big Bang nucleosynthesis — although the data can also be reconciled with non-standard models. The primordial abundances of light elements produced in the standard theory of Big Bang nucleosynthesis (BBN) depend only on the cosmic ratio of baryons to photons, a quantity inferred from observations of the microwave background 1 . The predicted 2 , 3 , 4 primordial 7 Li abundance is four times that measured in the atmospheres of Galactic halo stars 5 , 6 , 7 . This discrepancy could be caused by modification of surface lithium abundances during the stars’ lifetimes 8 or by physics beyond the Standard Model that affects early nucleosynthesis 9 , 10 . The lithium abundance of low-metallicity gas provides an alternative constraint on the primordial abundance and cosmic evolution of lithium 11 that is not susceptible to the in situ modifications that may affect stellar atmospheres. Here we report observations of interstellar 7 Li in the low-metallicity gas of the Small Magellanic Cloud, a nearby galaxy with a quarter the Sun’s metallicity. The present-day 7 Li abundance of the Small Magellanic Cloud is nearly equal to the BBN predictions, severely constraining the amount of possible subsequent enrichment of the gas by stellar and cosmic-ray nucleosynthesis. Our measurements can be reconciled with standard BBN with an extremely fine-tuned depletion of stellar Li with metallicity. They are also consistent with non-standard BBN.

The intense diffuse radiation of galactic systems in the far ultraviolet (FUV) range is primarily due to dust scattering, with properties such as albedo, scattering phase function, and optical depth playing a crucial role. Utilizing observed data from the Far Ultraviolet Spectroscopic Explorer (FUSE) telescope, we investigated dust-scattered diffuse FUV emission from the 30 Doradus nebula in the Large Magellanic Cloud (LMC), offering insight into interstellar dust properties essential for understanding starburst galaxies. Employing a simple spherical shell model in the Monte Carlo based radiative transfer tool SKIRT , we simulated the dust-scattered FUV emissions, and compared them with the observed data. We predicted an excess of FUV photons being scattered near the star cluster R136 (formerly RMC 136) and ascertained a robust linear correlation between observations and model. Additionally, our best-fit model predicted the hydrogen column density near R136 to be approximately ~ 4.36 × 10 21 cm −2 , corresponding to a color excess of E ( B − V ) = 0.2, with the contribution of diffuse scattered FUV emissions estimated at 13% of the total radiation. This model can be further extended to better understand dust properties of similar starburst environments in the local universe and beyond.

We report 11 new radio continuum measurements of established planetary nebulae (PNe) in the Small Magellanic Cloud (SMC) that we observed at 5.5 and 9 GHz with the Australia Telescope Compact Array (ATCA). These new radio detections are PNe with catalogued names: SMP SMC 2, SMP SMC 3, SMP SMC 5, SMP SMC 8, SMP SMC 13, SMP SMC 14, SMP SMC 19, MGPN SMC 8, SMP SMC 22, SMP SMC 26 and SMP SMC 27. We supplement our data with available high-resolution radio observations from MeerKAT and construct the spectral energy distribution (SED) in the radio regime for each PN. We determine the angular diameters of four of the eleven PNe from radio flux density alone using SED modelling, which are compared to the corresponding Hubble Space Telescope (HST) optical diameters. Our results are in good agreement with the optically-derived angular diameters from independent HST observations. We plot our new diameter estimates against a larger sample of Galactic PNe and compare diameters obtained via the SED method to those found in the literature. Our sample diameters, when compared to the Galactic PNe, suggest that the angular diameter measurement methods are comparable independent of the distance.

Direct observations of the products of binary interactions are sparse, yet they provide important insights on the outcome of the interaction and the physics at play. Young and intermediate-age star clusters are the ideal tool to search for, and characterize such interaction products and allow for a detailed comparison to theoretical predictions. We here report on integral field spectroscopy obtained with MUSE for several such clusters in the Magellanic Clouds.

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.

We present a machine learning method to estimate the physical parameters of classical pulsating stars such as RR Lyrae and Cepheid variables based on an automated comparison of their theoretical and observed light curve parameters at multiple wavelengths. We train artificial neural networks (ANNs) on theoretical pulsation models to predict the fundamental parameters (mass, radius, luminosity, and effective temperature) of Cepheid and RR Lyrae stars based on their period and light-curve parameters. The fundamental parameters of these stars can be estimated up to 60 percent more accurately when the light-curve parameters are taken into consideration. This method was applied to the observations of hundreds of Cepheids and thousands of RR Lyrae in the Magellanic Clouds to produce catalogs of estimated masses, radii, luminosities, and other parameters of these stars.

We probe how common extremely rapid rotation is among massive stars in the early universe by measuring the OBe star fraction in nearby metal-poor dwarf galaxies. We apply a new method that uses broad-band photometry to measure the galaxy-wide OBe star fractions in the Magellanic Clouds and three more distant, more metal-poor dwarf galaxies. We find OBe star fractions of ∼20% in the Large Magallanic Cloud (0.5Zȯ), and ∼30% in the Small Magellanic Cloud (0.2Zȯ) as well as in the so-far unexplored metallicity range 0.1 Z/Zȯ < 0.2 occupied by the other three dwarf galaxies. Our results imply that extremely rapid rotation is common among massive stars in metal-poor environments such as the early universe.