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1. New Mass Estimates for Massive Binary Systems: A Probabilistic Approach Using Polarimetric Radiative Transfer

   https://doi.org/10.3847/1538-4357/ac589e  

   Fullard, Andrew G.; O’Brien, John T.; Kerzendorf, Wolfgang E.; Shrestha, Manisha; Hoffman, Jennifer L.; Ignace, Richard; van der Smagt, Patrick (May 2022, The Astrophysical Journal)

   Abstract Understanding the evolution of massive binary stars requires accurate estimates of their masses. This understanding is critically important because massive star evolution can potentially lead to gravitational-wave sources such as binary black holes or neutron stars. For Wolf–Rayet (WR) stars with optically thick stellar winds, their masses can only be determined with accurate inclination angle estimates from binary systems which have spectroscopic M sin i measurements. Orbitally phased polarization signals can encode the inclination angle of binary systems, where the WR winds act as scattering regions. We investigated four Wolf–Rayet + O star binary systems, WR 42, WR 79, WR 127, and WR 153, with publicly available phased polarization data to estimate their masses. To avoid the biases present in analytic models of polarization while retaining computational expediency, we used a Monte Carlo radiative-transfer model accurately emulated by a neural network. We used the emulated model to investigate the posterior distribution of the parameters of our four systems. Our mass estimates calculated from the estimated inclination angles put strong constraints on existing mass estimates for three of the systems, and disagree with the existing mass estimates for WR 153. We recommend a concerted effort to obtain polarization observations that can be used to estimate the masses of WR binary systems and increase our understanding of their evolutionary paths. 

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2. 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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3. Visual Orbits of Wolf–Rayet Stars. II. The Orbit of the Nitrogen-rich Wolf–Rayet Binary WR 138 Measured with the CHARA Array

   https://doi.org/10.3847/1538-4357/ad9024  

   Holdsworth, Amanda; Richardson, Noel; Schaefer, Gail_H; Eldridge, Jan_J; Hill, Grant_M; Spejcher, Becca; Mackey, Jonathan; Moffat, Anthony_F_J; Navarete, Felipe; Monnier, John_D; et al (December 2024, The Astrophysical Journal)

   Abstract Classical Wolf–Rayet (WR) stars are descendants of massive OB-type stars that have lost their hydrogen-rich envelopes and are in the final stages of stellar evolution, possibly exploding as Type Ib/c supernovae. It is understood that the mechanisms driving this mass loss are either strong stellar winds and or binary interactions, so intense studies of these binaries including their evolution can tell us about the importance of the two pathways in WR formation. WR 138 (HD 193077) has a period of just over 4 yr and was previously reported to be resolved through interferometry. We report on new interferometric data combined with spectroscopic radial velocities in order to provide a three-dimensional orbit of the system. The precision on our parameters tend to be about an order of magnitude better than previous spectroscopic techniques. These measurements provide masses of the stars, namely,M_WR= 13.93 ± 1.49M_⊙andM_O= 26.28 ± 1.71M_⊙. The derived orbital parallax agrees with the parallax from Gaia, namely, with a distance of 2.13 kpc. We compare the system’s orbit to models from BPASS, showing that the system likely may have been formed with little interaction but could have formed through some binary interactions either following or at the start of a red supergiant phase but with the most likely scenario occurring as the red supergiant phase starts for a ∼40M_⊙star. 

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4. Aluminium-26 from Massive Binary Stars. III. Binary Stars up to Core Collapse and Their Impact on the Early Solar System

   https://doi.org/10.3847/1538-4357/acd7ea  

   Brinkman, Hannah_E; Doherty, Carolyn; Pignatari, Marco; Pols, Onno; Lugaro, Maria (July 2023, The Astrophysical Journal)

   Abstract Many of the short-lived radioactive nuclei that were present in the early solar system can be produced in massive stars. In the first paper in this series, we focused on the production of²⁶Al in massive binaries. In our second paper, we considered rotating single stars; two more short-lived radioactive nuclei,³⁶Cl and⁴¹Ca; and the comparison to the early solar system data. In this work, we update our previous conclusions by further considering the impact of binary interactions. We used the MESA stellar evolution code with an extended nuclear network to compute massive (10–80M_⊙), binary stars at various initial periods and solar metallicity (Z= 0.014), up to the onset of core collapse. The early solar system abundances of²⁶Al and⁴¹Ca can be matched self-consistently by models with initial masses ≥25M_⊙, while models with initial primary masses ≥35M_⊙can also match³⁶Cl. Almost none of the models provide positive net yields for¹⁹F, while for²²Ne the net yields are positive from 30M_⊙and higher. This leads to an increase by a factor of approximately 4 in the amount of²²Ne produced by a stellar population of binary stars, relative to single stars. In addition, besides the impact on the stellar yields, our 10M_⊙primary star undergoing Case A mass transfer ends its life as a white dwarf instead of as a core-collapse supernova. This demonstrates that binary interactions can also strongly impact the evolution of stars close to the supernova boundary. 

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5. Grids of stellar models with rotation VII: models from 0.8 to 300 M⊙ at supersolar metallicity ( Z  = 0.020)

   https://doi.org/10.1093/mnras/stac230  

   Yusof, Norhasliza; Hirschi, Raphael; Eggenberger, Patrick; Ekström, Sylvia; Georgy, Cyril; Sibony, Yves; Crowther, Paul A.; Meynet, Georges; Kassim, Hasan Abu; Harun, Wan Aishah Wan; et al (January 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We present a grid of stellar models at supersolar metallicity (Z = 0.020) extending the previous grids of Geneva models at solar and sub-solar metallicities. A metallicity of Z = 0.020 was chosen to match that of the inner Galactic disc. A modest increase of 43 per cent (= 0.02/0.014) in metallicity compared to solar models means that the models evolve similarly to solar models but with slightly larger mass-loss. Mass-loss limits the final total masses of the supersolar models to 35 M⊙ even for stars with initial masses much larger than 100 M⊙. Mass-loss is strong enough in stars above 20 M⊙ for rotating stars (25 M⊙ for non-rotating stars) to remove the entire hydrogen-rich envelope. Our models thus predict SNII below 20 M⊙ for rotating stars (25 M⊙ for non-rotating stars) and SNIb (possibly SNIc) above that. We computed both isochrones and synthetic clusters to compare our supersolar models to the Westerlund 1 (Wd1) massive young cluster. A synthetic cluster combining rotating and non-rotating models with an age spread between log10(age/yr) = 6.7 and 7.0 is able to reproduce qualitatively the observed populations of WR, RSG, and YSG stars in Wd1, in particular their simultaneous presence at $$\log _{10}(L/\mathit {\mathrm{ L}}_{\odot })$$ = 5–5.5. The quantitative agreement is imperfect and we discuss the likely causes: synthetic cluster parameters, binary interactions, mass-loss and their related uncertainties. In particular, mass-loss in the cool part of the HRD plays a key role. 

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