Particle Acceleration and Depolarization in the Protostellar jet knots HH 80 and HH 81
Linearly polarized emission is a powerful tracer of magnetic field geometry and particle acceleration in protostellar jets. We present a polarimetric study of the HH objects HH 80 and HH 81 from where non-thermal emission has been confirmed through spectral index measurements at low frequencies. We carried out observations of HH 80 and HH 81 with the Karl G. Jansky Very Large Array in 4-6 GHz. Unlike the inner jet knots, no linear polarization is detected towards the knots HH 80 and HH 81. We place a $3\sigma$ upper limit of $30~\mu$Jy on the polarization intensity, corresponding to fractional polarization limits of $\Pi_{\max}\approx0.02$ and $0.01$ for HH 80 and HH 81, respectively. To interpret this non-detection, we assess the conditions for synchrotron polarization and the impact of depolarization mechanisms. The shock cooling parameter $\chi_\mathrm{s}$ is lower in these outermost HH objects than in the inner knots, indicating that the reverse shocks in HH 80-81 are less efficient at accelerating relativistic electrons compared with the inner knots. Moreover, Faraday depolarization appears severe: the dispersion in the estimated rotation measure $\sigma_{\rm RM}\sim400~\mathrm{rad~m^{-2}}$ is comparable to or larger than observed RM values themselves. This is consistent with strong fluctuations and turbulence. Together with beam depolarization, these effects can suppress the observable fractional polarization flux densities below the detectable thresholds. We conclude that reduced acceleration efficiency (when compared to inner knots) and strong depolarization account for the absence of polarized emission towards HH 80 and HH 81.