Particle Acceleration and Depolarization in the Protostellar jet knots HH 80 and HH 81

TL;DR

This study uses VLA polarimetry to search for linearly polarized synchrotron emission in the outer HH 80-81 jet knots, testing magnetic-field geometry and particle acceleration in protostellar jets. They detect no polarization toward HH 80/81, setting tight upper limits that contrast with polarized emission in inner knots, suggesting weaker intrinsic acceleration and strong depolarization in the terminal lobes. The interpretation attributes the non-detection to a combination of reduced acceleration efficiency and significant Faraday and beam depolarization; RM fluctuations likely wash out polarized signals. The results highlight a dichotomy between inner and outer jet regions and underscore the need for future high-resolution, broadband polarimetry, such as with the SKA, to map magnetic fields along protostellar jets.

Abstract

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 $3σ$ upper limit of $30~μ$Jy on the polarization intensity, corresponding to fractional polarization limits of $Π_{\max}\approx0.02$ 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 $χ_\mathrm{s}$ 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 $σ_{\rm RM}\sim400~\mathrm{rad~m^{-2}}$ 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.

Figures (2)

• Figure 1: (a) Linearly polarized emission observed toward the inner jet lobes of I18162. The white contours overlaid on the image are the Stokes I contours ($(6,9,12,15,30,120),\sigma$, where $\sigma = 10~\mu\mathrm{Jy/beam}$.), which are identical to the black contours in panel (b). (b) Total intensity radio map of the HH 80-81 region at the VLA C band, where the beam size is $9".5 \times 6".0$. (c) Linearly polarized map towards HH 80 and HH 81, with Stokes I contours. The cyan ellipse at the bottom-left corner of the middle panel shows the beam size. The colour bar is shown on the top of the image.
• Figure 2: A plot of the shock velocity ($v_{s}$) as a function of the jet radius ($r_{\rm jet}$), with $\chi_\mathrm{s}$ values shown using a colour scale for comparison between the inner knots and HH 80-81. The $\chi_\mathrm{s}$ values derived for each knot are indicated by grey points, and the corresponding colour bar is displayed on the right-hand side of the plot.