Runaway O-star Bow Shocks as Particle Accelerators? The Case of AE Aur revisited

We present results of our Chandra/ACIS observations of the field centered on the fast, runaway O star AE Aur and its bowshock. Previous XMM-Newton observations revealed an X-ray " blob" near the IR arc tracing the bowshock, possibly a nonthermal source consistent with models of Inverse Compton scatteringof dust IR photons by electrons accelerated at the shock. The new, subarcsecond resolution Chandra data, while confirming the presence of the XMM-Newton source, clearly indicate that the latter is neither extended nor coincident with the IR arc and strongly suggest it is a background AGN. Motivated by results published for the bowshock of BD+43 3654, we extended our study to the radio domain, by analyzing archival EVLA data. We find no radio emission from the AE Aur bowshock either. The corresponding upper limits for the absorbed (unabsorbed) X-ray flux of 5.9(7.8)x10^-15 erg/cm^2/s (3 sigma) and, in the radio range, of 2 mJy (1.4 GHz), and 0.4 mJy (5.0 GHz), are used to put constraints on model predictions for particle accelerationwithin the bowshock. In the "classical" framework of Diffusive Shock Acceleration, we find that the predicted X-ray and radio emission by the bowshock is at least two orders of magnitude below the current upper limits, consistent with the systematic non-detections of up to 60 stellar bowshocks. The only exception so far remains that of BD+43 3654, probably the result of its very large mass-loss rate among runaway O stars.