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1. Visual Orbits of Wolf–Rayet Stars. I. The Orbit of the Dust-producing Wolf–Rayet Binary WR 137 Measured with the CHARA Array

   https://doi.org/10.3847/1538-4357/ad8d5c  

   Richardson, Noel_D; Schaefer, Gail_H; Eldridge, Jan_J; Spejcher, Rebecca; Holdsworth, Amanda; Lau, Ryan_M; Monnier, John_D; Moffat, Anthony_F_J; Weigelt, Gerd; Williams, Peredur_M; et al (December 2024, The Astrophysical Journal)

   Abstract Classical Wolf–Rayet (W-R) stars are the descendants of massive OB stars that have lost their hydrogen envelopes and are burning helium in their cores prior to exploding as Type Ib/c supernovae. The mechanisms for losing their hydrogen envelopes are either through binary interactions or through strong stellar winds potentially coupled with episodic mass loss. Among the bright classical W-R stars, the binary system WR 137 (HD 192641; WC7d + O9e) is the subject of this paper. This binary is known to have a 13 yr period and produces dust near periastron. Here we report on interferometry with the Center for High Angular Resolution Astronomy Array collected over a decade of time and providing the first visual orbit for the system. We combine these astrometric measurements with archival radial velocities to measure masses of the stars ofM_WR= 9.5 ± 3.4M_⊙andM_O= 17.3 ± 1.9M_⊙when we use the most recent Gaia distance. These results are then compared to predicted dust distribution using these orbital elements, which match the observed imaging from JWST as discussed recently by Lau et al. Furthermore, we compare the system to the Binary Population And Spectral Synthesis models, finding that the W-R star likely formed through stellar winds and not through binary interactions. However, the companion O star did likely accrete some material from the W-R star’s mass loss to provide the rotation seen today that drives its status as an Oe star. 

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2. 3D models of the circumstellar environments of evolved stars: Formation of multiple spiral structures

   https://doi.org/10.1093/mnras/stac749  

   Aydi, Elias; Mohamed, Shazrene (March 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We present 3D hydrodynamic models of the interaction between the outflows of evolved, pulsating, Asymptotic Giant Branch (AGB) stars and nearby (<3 stellar radii) substellar companions (Mcomp ≲ 40 MJ). Our models show that due to resonances between the orbital period of the companion and the pulsation period of the AGB star, multiple spiral structures can form; the shocks driven by the pulsations are enhanced periodically in different regions as they encounter the denser material created by the substellar companion’s wake. We discuss the properties of these spiral structures and the effect of the companion parameters on them. We also demonstrate that the gravitational potential of the nearby companion enhances the mass-loss from the AGB star. For more massive (Mcomp > 40 MJ) and more distant companions (>4 stellar radii), a single spiral arm forms. We discuss the possibility of observing these structures with the new generations of high-resolution, high-sensitivity instruments, and using them to ‘find’ substellar companions around bright, evolved stars. Our results also highlight possible structures that could form in our Solar system when the Sun turns into an AGB star. 

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3. Spectroscopic and evolutionary analyses of the binary system AzV 14 outline paths toward the WR stage at low metallicity

   https://doi.org/10.1051/0004-6361/202345881  

   Pauli, D.; Oskinova, L. M.; Hamann, W.-R.; Bowman, D. M.; Todt, H.; Shenar, T.; Sander, A. A.; Erba, C.; Gómez-González, V. M.; Kehrig, C.; et al (May 2023, Astronomy & Astrophysics)

   Context. The origin of the observed population of Wolf-Rayet (WR) stars in low-metallicity galaxies, such as the Small Magellanic Cloud (SMC), is not yet understood. Standard, single-star evolutionary models predict that WR stars should stem from very massive O-type star progenitors, but these are very rare. On the other hand, binary evolutionary models predict that WR stars could originate from primary stars in close binaries. Aims. We conduct an analysis of the massive O star, AzV 14, to spectroscopically determine its fundamental and stellar wind parameters, which are then used to investigate evolutionary paths from the O-type to the WR stage with stellar evolutionary models. Methods. Multi-epoch UV and optical spectra of AzV 14 are analyzed using the non-local thermodynamic equilibrium (LTE) stellar atmosphere code PoWR. An optical TESS light curve was extracted and analyzed using the PHOEBE code. The obtained parameters are put into an evolutionary context, using the MESA code. Results. AzV 14 is a close binary system with a period of P  = 3.7058 ± 0.0013 d. The binary consists of two similar main sequence stars with masses of M 1, 2  ≈ 32  M ⊙ . Both stars have weak stellar winds with mass-loss rates of log Ṁ /( M ⊙ yr −1 ) = −7.7 ± 0.2. Binary evolutionary models can explain the empirically derived stellar and orbital parameters, including the position of the AzV 14 components on the Hertzsprung-Russell diagram, revealing its current age of 3.3 Myr. The model predicts that the primary will evolve into a WR star with T eff  ≈ 100 kK, while the secondary, which will accrete significant amounts of mass during the first mass transfer phase, will become a cooler WR star with T eff  ≈ 50 kK. Furthermore, WR stars that descend from binary components that have accreted significant amount of mass are predicted to have increased oxygen abundances compared to other WR stars. This model prediction is supported by a spectroscopic analysis of a WR star in the SMC. Conclusions. Inspired by the binary evolutionary models, we hypothesize that the populations of WR stars in low-metallicity galaxies may have bimodal temperature distributions. Hotter WR stars might originate from primary stars, while cooler WR stars are the evolutionary descendants of the secondary stars if they accreted a significant amount of mass. These results may have wide-ranging implications for our understanding of massive star feedback and binary evolution channels at low metallicity. 

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4. The impact of wind mass loss on nucleosynthesis and yields of very massive stars at low metallicity

   https://doi.org/10.1051/0004-6361/202453472  

   Higgins, Erin R; Vink, Jorick S; Hirschi, Raphael; Laird, Alison M; Sabhahit, Gautham (July 2025, Astronomy & Astrophysics)

   The chemical feedback from stellar winds in low metallicity (Z) environments is key to understanding the evolution of globular clusters and the early Universe. With a disproportionate amount of mass lost from the most massive stars (M > 100 M_⊙) and an excess of such stars expected at the lowest metallicities, their contribution to the enrichment of the early pristine clusters could be significant. In this work, we examine the effect of mass loss at low metallicity on the nucleosynthesis and wind yields of (very) massive stars. We calculated stellar models with initial masses ranging from 30 to 500 M_⊙during core hydrogen and helium burning phases at four metallicities ranging from 20% Z_⊙down to 1% Z_⊙. We provide the ejected masses and net yields for each grid of models. While mass-loss rates decrease withZ, we find that not only are wind yields significant, but the nucleosynthesis is also altered due to the change in central temperatures, and therefore it also plays a role. We find that 80–300 M_⊙models can produce large quantities of Na-rich and O-poor material, which is relevant for the observed Na-O anti-correlation in globular clusters. 

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5. Constraining the assembly time of the stellar haloes of nearby Milky Way-mass galaxies through AGB populations

   https://doi.org/10.1093/mnras/stad2480  

   Harmsen, Benjamin; Bell, Eric_F; D’Souza, Richard; Monachesi, Antonela; de Jong, Roelof_S; Smercina, Adam; Jang, In_Sung; Holwerda, Benne_W (August 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT The star formation histories (SFHs) of galactic stellar haloes offer crucial insights into the merger history of the galaxy and the effects of those mergers on their hosts. Such measurements have revealed that while the Milky Way’s most important merger was 8–10 Gyr ago, M31’s largest merger was more recent, within the last few Gyr. Unfortunately, the required halo SFH measurements are extremely observationally expensive outside of the Local Group. Here, we use asymptotic giant branch (AGB) stars brighter than the tip of the red giant branch (RGB) to constrain stellar halo SFHs. Both stellar population models and archival data sets show that the AGB/RGB ratio constrains the time before which 90 per cent of the stars formed, t90. We find AGB stars in the haloes of three highly inclined roughly Milky Way-mass galaxies with resolved star measurements from the Hubble Space Telescope; this population is most prominent in the stellar haloes of NGC 253 and NGC 891, suggesting that their stellar haloes contain stars born at relatively late times, with inferred t90 ∼ 6 ± 1.5 Gyr. This ratio also varies from region to region, tending towards higher values along the major axis and in tidal streams or shells. By combining our measurements with previous constraints, we find a tentative anticorrelation between halo age and stellar halo mass, a trend that exists in models of galaxy formation but has never been elucidated before, i.e. the largest stellar haloes of Milky Way-mass galaxies were assembled more recently. 

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