Carla S. Buemi

B[e] supergiants represent a rare class of luminous, evolved massive stars surrounded by dusty circumstellar disks. Since their intense UV fields were long thought to sterilize their surroundings, molecular detections beyond carbon monoxide have remained elusive, leaving their chemical reservoirs largely unexplored. Whether these environments can sustain a complex molecular chemistry is a fundamental question with significant astrochemical implications. Here we report the detection of chemically rich molecular gas surrounding the B[e] supergiant HD 87643, using Atacama Large Millimeter/submillimeter Array observations. Our data reveal the presence of the sulfur oxides SO and SO ₂ and other sulfur-bearing species, marking the first detection of these molecules in an early-type evolved massive star. We find a high fractional abundance of SO ₂ relative to H ₂ , which our chemical modeling can reproduce in timescales as short as ∼10 ⁴ yr in an oxygen-rich environment. These results indicate that the detected molecules trace a short-lived, rapidly evolving phase of out-of-equilibrium chemistry. Furthermore, we measure an anomalously low ³² SO/ ³³ SO, which we attribute to mass-independent fractionation driven by intense photochemistry. This mechanism mirrors processes proposed to explain the ³³ S excesses in the atmosphere of the Archaean Earth. Our findings suggest that B[e] supergiants could serve as unique laboratories for studying sulfur chemistry under extreme radiation conditions, opening potential avenues to investigate the fractionation processes that shaped the isotopic signatures found in the early geological record.

Context. Magnetic BA-type stars typically host dipole-like magnetospheres. When detected as nonthermal radio sources, their luminosities are correlated with the stellar magnetic field and rotation speed. Rotation is crucial because the mechanism undergirding the relativistic electron production is powered by centrifugal breakouts (CBOs). Small-scale breakouts occur wherever magnetic tension does not balance out the centrifugal force, resulting in magnetic reconnection and particle acceleration. Aims. To investigate how physical conditions at the site of the CBOs affect the efficiency of the nonthermal electron acceleration mechanism, we broadly explore the parameter space governing radio emission by increasing the available sample of radio-loud magnetic stars. Methods. High-sensitivity VLA observations of 32 early-type magnetic stars were performed in the hope of identifying new centrifugally supported magnetospheres (CMs) and associated CBOs. We calculated the radio spectra produced by the gyrosynchrotron emission mechanism using a 3D modeling of a dipole-shaped corotating magnetosphere. We evaluated combinations of stellar parameters and equatorial thermal plasma densities. The number of relativistic electrons was constrained by the need to produce the emission level predicted by the well-known empirical scaling relationship for the radio emission from magnetic BA stars. Results. About half of the observed early-type magnetic stars were detected, with radio luminosities that show an excellent agreement with the expected values, reinforcing the robust nature of the scaling relationship for CBO-powered radio emission. Comparing the competing centrifugal and magnetic effects on plasma locked in a rigidly rotating magnetosphere, we located the site of CBOs and inferred the local plasma density. We then estimated the efficiency of the CBO-powered acceleration mechanism needed to produce enough nonthermal electrons to support the expected radio emission level. Conclusions. Given a constant acceleration efficiency, relativistic electrons represent a fixed fraction of the local thermal plasma. Thus, we find that dense magnetospheres host many more energetic particles than less dense ones; consequently, with the other parameters remaining similar, they are shown to be intrinsically brighter radio sources.

Context. Recently, a powerful magnetic field was discovered in the hot helium star classified as a quasi-Wolf-Rayet (qWR) star (∼2 M_⊙), in the HD 45166 system. Upon its explosion as a core-collapse supernova, it is expected to produce a strongly magnetic neutron star – a magnetar. Among the key parameters that govern pre-supernova evolution is the amount of mass lost via stellar wind. However, the magnetic nature of this helium star is expected to affect its stellar wind, which makes the estimation of the wind parameters uncertain. Aims. We report the first observations of HD 45166 in X-rays with the XMM-Newton telescope and in radio with the VLA interferometer array. By placing the observation results in a theoretical framework, we aim to provide a reliable estimate of the wind strength of the magnetic qWR star. Methods. We explain the X-ray properties in the framework of the magnetically confined wind shock scenario, and we apply the semianalytic model of a dynamical magnetosphere (DM) to reproduce the X-ray emission. We compute the thermal radio emission of the wind and its absorption effect on possible gyro-synchrotron emission from the underlying dipolar magnetosphere, sampled in 3D, by integrating the radiative transfer equation. Results. We did not detect radio emissions, which enabled us to set sensitive upper limits on the radio luminosity. The magnetic qWR star is a slow rotator, and comparison with models reveals that the possible acceleration mechanisms that occur within its DM are not as efficient as in fast-rotating magnetic ApBp-type stars. In contrast, the system is detected in X-rays with log(L_X/L_bol)∼−5.6. Using suitable models, we constrain the mass lost from this magnetic quasi-Wolf-Rayet star as Ṁ ≈ 3 × 10⁻¹⁰ M_⊙ yr⁻¹. Conclusions. This novel empirical estimate of the mass-loss rate in a ∼2 M_⊙ helium star confirms that it maintains super-Chandrasekhar mass until collapse and can produce a magnetar as its final evolutionary product.

We present a catalogue of extended radio sources from the SARAO MeerKAT Galactic Plane Survey (SMGPS). Compiled from 56 survey tiles and covering approximately 500 deg² across the first, third, and fourth Galactic quadrants, the catalogue includes 16 534 extended and diffuse sources with areas larger than 5 synthesised beams. Of them, 3891 (24% of the total) are confidently associated with known Galactic radio-emitting objects in the literature, such as HII regions, supernova remnants, planetary nebulae, luminous blue variables, and Wolf-Rayet stars. A significant fraction of the remaining sources, 5462 (33%), are candidate extragalactic sources, while 7181 (43%) remain unclassified. Isolated radio filaments are excluded from the catalogue. The diversity of extended sources underscores MeerKAT’s contribution to the completeness of censuses of Galactic radio emitters, and its potential for new scientific discoveries. For the catalogued sources, we derived basic positional and morphological parameters, as well as flux density estimates, using standard aperture photometry. This paper describes the methods followed to generate the catalogue from the original SMGPS tiles, detailing the source extraction, characterisation, and crossmatching procedures. Additionally, we analyse the statistical properties of the catalogued populations.

Context. The integrated radio spectrum of supernova remnants (SNRs) and the spatial variation of the spectral indices across these extended sources are powerful tools for studying the shocks and particle acceleration processes occurring in different SNR regions. Characterization of these processes requires sensitive flux density measurements and high-resolution images, which are not always available due to observing difficulties. Aims. We want to show the potentiality of the high-resolution SARAO MeerKAT legacy Galactic Plane Survey (SMGPS) images regarding the morphological and spectral characterization of 29 known galactic SNRs. Methods. We used the SMGPS data at 1.284 GHz coupled with data from the GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey (0.072–0.231 GHz) to characterize the integrated spectrum of each source and search for spatial spectral variation through analysis of sensitive spectral index maps. Results. We were able to redefine the exact morphology of four SNRs (G024.7–00.6, G051.4+00.7, G348.7+0.3, and G351.9+00.1), distinguishing them from unrelated sources or identifying new emission regions associated with them and never observed before. In many other cases, we identified in the SMGPS images several H II regions overlaid with the remnants, and we were able to estimate their spectral contribution through inspection of the spatial variation of the spectral indices across the remnants. The integrated spectral indices show a more uniform distribution with respect to what is obtained by considering the values reported in the literature. Conclusions. We show that new sensitive and high-resolution data are crucial to firmly constraining both the integrated and spatially resolved spectrum of SNRs, especially for the less studied objects of the southern hemisphere. The comparison of our SMGPS-GLEAM spectral index maps with IR, molecular, and γ-ray images allowed us to investigate the nature of the peculiar remnant regions.

We present the serendipitous discovery of a new radio-continuum ring-like object nicknamed Kýklos (J1802–3353), with MeerKAT UHF and L-band observations. The radio ring, which resembles the recently discovered odd radio circles (ORCs), has a diameter of ∼80″ and is located just ∼6° from the Galactic plane. However, Kýklos exhibits an atypical thermal radio-continuum spectrum (α = −0.1 ± 0.3), which led us to explore different possible formation scenarios. We concluded that a circumstellar shell around an evolved massive star, possibly a Wolf-Rayet, is the most convincing explanation with the present data.

Context. Luminous blue variable (LBV) stars represent a short-lived stage in the late evolution of the most massive stars. Highly unstable, LBVs exhibit dense stellar winds and episodic eruptions that produce complex circumstellar nebulae, the study of which is crucial for properly constraining the impact of these sources at a Galactic scale from a structural, dynamical, and chemical perspective. Aims. We aim to investigate the molecular environment of AFGL 2298, an obscured Galactic LBV that hosts a highly structured circumstellar environment with hints of multiple mass-loss events in the last few 10⁴ a. Methods. We present spectral line observations of AFGL 2298 at 1 and 3 mm performed with the IRAM 30 m radio telescope. Results. We report the detection of several carbon- and nitrogen-bearing species (CO, ¹³CO, C¹⁸O, C¹⁷O, HCO⁺, HCN, HNC, H¹³CO⁺, CN, N₂H⁺, and C₂H) in the surroundings of AFGL 2298. We identified three velocity components that clearly stand out from the Galactic background. The morphology, kinematics, masses, and isotopic ratios, together with a comparative study of the fractional abundances, lead us to suggest that two of these components (36 and 70 km s⁻¹) have a stellar origin. The other component (46 km s⁻¹) most likely traces swept-up interstellar material, and probably also harbours a photon-dominated region. Conclusions. We provide the first inventory of the circumstellar molecular gas around AFGL 2298. Our results are compatible with the hypothesis of former mass-loss events produced before the one that created the infrared nebula. The chemistry of this LBV suggests the presence of ejected stellar material, and also swept up gas. These findings will help us to better understand the mass-loss history of this class of evolved massive stars, which is important given that they heavily influence the overall chemical evolution of the Galaxy.

In exoplanetary systems, the interaction between the central star and the planet can trigger Auroral Radio Emission (ARE), due to the Electron Cyclotron Maser mechanism. The high brightness temperature of this emission makes it visible at large distances, opening new opportunities to study exoplanets and to search for favourable conditions for the development of extra-terrestrial life, as magnetic fields act as a shield that protects life against external particles and influences the evolution of the planetary atmospheres. In the last few years, we started an observational campaign to observe a sample of nearby M-type stars known to host exoplanets with the aim to detect ARE. We observed YZ Ceti with the upgraded Giant Metrewave Radio Telescope (uGMRT) in band 4 (550-900 MHz) nine times over a period of five months. We detected radio emission four times, two of which with high degree of circular polarization. With statistical considerations we exclude the possibility of flares due to stellar magnetic activity. Instead, when folding the detections to the orbital phase of the closest planet YZ Cet b, they are at positions where we would expect ARE due to star-planet interaction (SPI) in sub-Alfvenic regime. With a degree of confidence higher than 4.37 sigma, YZ Cet is the first extrasolar systems with confirmed SPI at radio wavelengths. Modelling the ARE, we estimate a magnetic field for the star of about 2.4 kG and we find that the planet must have a magnetosphere. The lower limit for the polar magnetic field of the planet is 0.4 G.

We present ALMA band 6 observations of the luminous blue variable η Car obtained within the ALMAGAL program. We report SiO J = 5 → 4, SiS J = 12 → 11, and SiN N = 5 → 4 emission in the equatorial region of the Homunculus nebula, constituting the first detection of silicon- and sulfur-bearing molecules in the outskirts of a highly evolved, early-type massive star. The SiO, SiS, and SiN trace a clumpy equatorial ring that surrounds the central binary at a projected distance of ∼2″, delineating the inner rims of the butterfly-shaped dusty region. The formation of silicon-bearing compounds is presumably related to the continuous recycling of dust due to the variable wind regime of η Car, which destroys grains and releases silicon back to the gas phase. We discuss possible formation routes for the observed species, contextualizing them within the current molecular inventory of η Car. We find that the SiO and SiS fractional abundances in localized clumps of the ring, 6.7 × 10⁻⁹ and 1.2 × 10⁻⁸, respectively, are exceptionally lower than those measured in C- and O-rich AGB stars and cool supergiants, while the higher SiN abundance, 3.6 × 10⁻⁸, evidences the nitrogen-rich chemistry of the ejecta. These abundances must be regarded as strict upper limits, since the distribution of H₂ in the Homunculus is unknown. In any case, these findings shed new light on the peculiar molecular ecosystem of η Car and establish its surroundings as a new laboratory to investigate the life cycle of silicate dust in extreme astrophysical conditions.