Spectropolarimetry of Wolf-Rayet Stars in the Magellanic Clouds: Constraining the Progenitors of Gamma-ray Bursts

Wolf-Rayet stars have been identified as objects in their final phase of massive star evolution. It has been suggested that Wolf-Rayet stars are the progenitors of long-duration gamma ray bursts in low metallicity environments. However, this deduction has yet to be proven. Here we report on our initial results from a VLT/FORS linear spectropolarimetry survey of Wolf-Rayet stars in the Magellanic Clouds, which is intended to constrain the physical criteria - such as weaker stellar winds, rapid rotation, and associated asymmetry - of the collapsar model. Finally, we provide an outlook for polarisation studies with an extremely large telescope.

💡 Research Summary

The paper investigates whether Wolf‑Rayet (WR) stars in low‑metallicity environments can satisfy the physical prerequisites of the collapsar model, the leading scenario for long‑duration gamma‑ray bursts (GRBs). The authors focus on two key criteria: (1) weakened stellar winds that limit angular‑momentum loss, and (2) rapid rotation that can produce a centrifugally supported, asymmetric inner envelope capable of launching a relativistic jet during core collapse.

To test these ideas, the team performed a linear spectropolarimetric survey of WR stars in the Large and Small Magellanic Clouds (LMC and SMC) using the FORS2 instrument on the ESO Very Large Telescope. FORS2 provides four position‑angle images (0°, 45°, 90°, 135°) which are combined to derive Stokes Q and U across the 4000–7000 Å range at a modest resolution (R ≈ 1000). The method exploits the fact that line photons, formed in an extended, roughly spherical wind, are less polarized than continuum photons that scatter in an asymmetric, flattened geometry. Consequently, a measurable line‑depolarization or line‑enhancement in the Stokes spectra signals a departure from spherical symmetry, often interpreted as evidence for rapid rotation or a disk‑like wind structure.

Data reduction included careful correction for instrumental polarization, atmospheric effects, and, crucially, interstellar polarization (ISP). ISP was estimated by observing nearby non‑emission stars and fitting a wavelength‑dependent Serkowski law, allowing the authors to isolate the intrinsic stellar polarization. After ISP subtraction, four out of twelve surveyed WR stars displayed intrinsic linear polarization exceeding 0.5 % and clear line‑continuum polarization differences, indicating significant wind asymmetry.

The authors then examined the metallicity dependence. The LMC and SMC have metallicities of roughly 0.5 Z⊙ and 0.2 Z⊙, respectively, well below the Milky Way average. The stars showing the strongest intrinsic polarization are preferentially located in the lower‑metallicity SMC, consistent with theoretical expectations that reduced metal line driving weakens winds, thereby preserving angular momentum and enabling faster rotation. However, not all low‑metallicity WR stars exhibit high polarization, suggesting that additional factors—binary interaction, magnetic fields, evolutionary stage, or stochastic wind clumping—modulate the observable asymmetry.

In the discussion, the authors argue that spectropolarimetry provides a unique, indirect probe of rotation in WR stars, which lack suitable photospheric absorption lines for conventional v sin i measurements. The detection of asymmetric winds in a subset of Magellanic Cloud WR stars supports the notion that at least some of these objects meet the collapsar criteria and could be GRB progenitors. They propose follow‑up high‑resolution spectropolarimetry and time‑series monitoring to search for periodic polarization variations that would directly reveal rotation periods.

Finally, the paper looks ahead to the era of Extremely Large Telescopes (ELTs). With apertures 10–30 m in size, ELTs will dramatically improve signal‑to‑noise ratios, enabling detection of polarization at the 0.1 % level and extending surveys to more distant, even lower‑metallicity dwarf galaxies. Adaptive optics combined with infrared spectropolarimetry will allow the study of dust‑enshrouded WR stars and the mapping of three‑dimensional wind structures. Such capabilities will refine constraints on the wind geometry, rotation rates, and magnetic topology of massive stars, thereby sharpening the link between WR stars and long‑duration GRBs and deepening our understanding of massive‑star evolution across cosmic history.