High-sensitivity radio study of the non-thermal stellar bow shock EB27.

We present a deep radio-polarimetric observation of the stellar bow shock EB27 associated to the massive star BD+43 3654. This is the only stellar bow shock confirmed to have non-thermal radio emission. We used the Jansky Very Large Array in S band (2 - 4GHz) to test whether this synchrotron emission is polarised. The unprecedented sensitivity achieved allowed us to map even the fainter regions of the bow shock, revealing that the more diffuse emission is steeper and the bow shock brighter than previously reported. No linear polarisation is detected in the bow shock above 0.5%, although we detected polarised emission from two southern sources, probably extragalactic in nature. We modeled the intensity and morphology of the radio emission to better constrain the magnetic field and injected power in relativistic electrons. Finally, we derived a set of more precise parameters for the system EB27-BD+43 3654 using Gaia Early Data Release 3, including the spatial velocity. The new trajectory, back in time, intersects the core of the Cyg OB2 association.