Insight into the occurrence of particle acceleration through the investigation of Wolf-Rayet stars using uGMRT observations

Massive stars produce strong stellar winds that consist of continuous
outflows of material at speeds of thousands of km/s. These winds convey large
amounts of kinetic power, especially in the case of Wolf-Rayet (WR) stars. When
these winds interact with nearby material, they will likely produce shocks.
Among other processes, particle acceleration is expected to occur. This is
particularly well established in the case of massive binary systems, where the
stellar winds collide, allowing these systems to be identified thanks to the
detection of synchrotron radio emission, produced by a population of
relativistic particles accelerated in the shocks. Our goal is to investigate
the occurrence of particle acceleration among massive stars in their
pre-supernova evolution phases. To this end, we observed a subset of five WR
stars in the radio domain using the upgraded Giant Metrewave Radio Telescope
(uGMRT), located in India. The observations were carried out in bands 4
(550-950 MHz) and 5 (1050-1450 MHz) for all the targets. We detected radio
emission for only WR 110 in bands 4 and 5. Its thermal spectrum displays a
consistent index of +0.74 down to uGMRT bands. The four other targets were not
detected and we derived 3$\sigma$ upper limits. Our upper limits in Band 4 are
the first provided for these targets below 1 GHz. None of the targets was
identified as a synchrotron radio emitter in these radio bands. If some
synchrotron emission is produced in these systems, the non-detection with uGMRT
can be most likely attributed to strong free-free absorption (FFA). This is
especially relevant for WR 98a, which is catalogued as a particle accelerator
based on previous measurements at higher radio frequencies. We discuss how the
prominence of FFA constitutes a severe obstacle to identifying particle
accelerators in the radio domain.