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

TL;DR

This study uses high-resolution X-ray and radio observations to investigate particle acceleration in the bow shock of the runaway O star AE Aur, finding no detectable nonthermal emission and constraining acceleration models.

Contribution

The paper provides the first high-resolution Chandra X-ray analysis of AE Aur's bow shock and extends the study to radio wavelengths, offering new constraints on particle acceleration in stellar bow shocks.

Findings

No X-ray or radio emission detected from AE Aur's bow shock.

Upper limits constrain particle acceleration models, showing predicted emissions are below current detection thresholds.

Results support the idea that most stellar bow shocks do not produce detectable nonthermal emission, except in special cases like BD+43 3654.

Abstract

We present results of our Chandra/ACIS observations of the field centered on the fast, runaway O star AE Aur and its bow shock. Previous XMM-Newton observations revealed an X-ray "blob" near the IR arc tracing the bow shock, possibly a nonthermal source consistent with models of Inverse Compton scattering of 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 bow shock 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 bow shock 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 acceleration within the bow shock. In the "classical" framework of Diffusive Shock Acceleration, we find that the predicted X-ray and radio emission by the bow shock is at least two orders of magnitude below the current upper limits, consistent with the systematic non-detections of up to 60 stellar bow shocks. 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.