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1. To binary or not to binary: An analysis of WR 71’s polarized line profiles

   Johnson, R.A. (January 2021, Bulletin of the American Astronomical Society)

   null (Ed.)

   Massive Wolf-Rayet (WR) stars in binary systems may produce supernovae capable of emitting long duration gamma ray bursts. Characterizing the structure of the colliding winds in these systems may help constrain the mass loss and transfer properties and help predict their future evolution. I will present new spectropolarimetric data for the possible WR+O binary system WR 71, collected using RSS at the Southern African Large Telescope. WR 71 is a WN6 whose binary status is unknown, but it displays similar spectropolarimetric variations to the known WR+O binary system V444 Cygni. I investigate the orbital and rotational velocity of WR 71's winds by analyzing its polarized emission line profiles as a function of phase, the first analysis of its kind. I compare the line polarization behavior with predictive models of both colliding wind binaries and single stars with co-rotating interaction regions. Describing the wind structure of WR 71 will help determine the rate of mass loss from the system, an important indicator for LGRB progenitors, and shed light on its binary status. 

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2. 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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3. 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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4. Perfect Circles: A Study of the Scattering Regions of Wolf Rayet Binary Stars

   Yoos, Stella; Fullard, Andrew; Hoffman, Jennifer (May 2020, University of Denver Undergraduate Research Journal)

   Although we have been able to develop an understanding of many aspects of stellar evolution and formation, a few key gaps remain. One is the fate of massive binary star systems composed of Wolf-Rayet (WR) and O-type stars. In these WR + O binaries, the stellar winds surrounding these stars collide, creating a complex interaction region in which light from the stars scatters and becomes polarized. To study these scattering regions, I employ a technique that allows me to map the polarization of the light emitted from these stars and track its variation over the binary orbit. I found that although we have some models for this behavior, they do not fully reproduce the observed data, suggesting these systems are more complex that previously known. The unexplained behaviors give clues to the complexity of these systems and shows how these models can be improved upon in the future. Understanding the structure and evolution of this scattering region could be the key to understanding the lives and eventual deaths of these stars. 

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5. Constraints on the Binarity of the WN3/O3 Class of Wolf–Rayet Stars*

   https://doi.org/10.3847/1538-4357/acc5ed  

   Massey, Philip; Neugent, Kathryn F.; Morrell, Nidia I. (April 2023, The Astrophysical Journal)

   Abstract The WN3/O3 Wolf–Rayet (WR) stars were discovered as part of our survey for WRs in the Magellanic Clouds. The WN3/O3s show the emission lines of a high-excitation WN star and the absorption lines of a hot O-type star, but our prior work has shown that the absorption spectrum is intrinsic to the WR star. Their place in the evolution of massive stars remains unclear. Here we investigate the possibility that they are the products of binary evolution. Although these are not WN3+O3 V binaries, they could still harbor unseen companions. To address this possibility, we have conducted a multiyear radial velocity study of six of the nine known WN3/O3s. Our study finds no evidence of statistically significant radial velocity variations, and allows us to set stringent upper limits on the mass of any hypothetical companion star: for probable orbital inclinations, any companion with a period less than 100 days must have a mass <2M_⊙. For periods less than 10 days, any companion would have to have a mass <1M_⊙. We argue that scenarios where any such companion is a compact object are unlikely. The absorption lines indicate a normal projected rotational velocity, making it unlikely that these stars evolved with the aid of a companion star that has since merged. The modest rotation also suggests that these stars are not the result of homogenous evolution. Thus it is likely that these stars are a normal but short-lived stage in the evolution of massive stars. 

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