A targeted radio survey of infrared-selected bow shock candidates

Context. Bow shocks around massive stars have primarily been detected in IR emission, but radio detections are becoming more frequent with the commissioning of sensitive and large field-of-view interferometers. Radio data are used to probe both thermal and non-thermal emission in order to constrain the relativistic electron population and potentially particle-acceleration processes. Aims. We undertook a radio survey for bow shocks accessible from the Northern Hemisphere based on IR catalogues of candidates using the Very Large Array (VLA) and the 100 m Effelsberg Telescope. Our aim was to yield new detections and better characterise the multi-wavelength emission. Methods. We used Gaia DR3 to re-calculate spatial motion of the driving stars with respect to the surrounding stellar population. We studied the radio emission from bow shocks using emission maps and spectral-index measurements and compared our results with data from catalogues and multi-wavelength emission. Results. Of the 24 targets observed with the VLA in the 4–12 GHz band, six were clearly detected (including two previously reported) and five were possibly detected. A subset of these were also observed and detected with Effelsberg at 4–8 GHz. The VLA-derived spectral index maps indicate non-thermal emission for most sources, but the statistical uncertainties are large for most sources and all Effelsberg observations indicate thermal emission. Assuming thermal emission, we obtained upper limits on the electron density within the shocked layer. We also obtained upper limits on radio emission from the bow shock of ζ Oph at a similar flux level as predictions from magnetohydrodynamic simulations. Conclusions. Our survey marks a significant addition to the approximately ten previously known radio-emitting bow shocks in the literature and demonstrates that deep targeted radio surveys can have a good success rate in detecting IR-selected bow shocks. Followup observations of these targets at lower and higher frequencies are encouraged to determine whether any are non-thermal emitters such as the bow shocks of BD+43° 3654, BD+60° 2522, and LS 2355.