Collisionally excited filaments in HST H$\alpha$ and H$\beta$ images of HH~1/2

TL;DR

This paper presents H$ extalpha$ and H$eta$ imaging of HH~1/2, revealing collisional excitation regions behind shock fronts, providing direct evidence of collisional processes in Herbig-Haro objects.

Contribution

First direct detection of collisional excitation/ionization regions of hydrogen behind Herbig-Haro shock fronts using H$ extalpha$ and H$eta$ images.

Findings

High H$ extalpha$/H$eta$ ratios along shock edges indicating collisional excitation.

Ridges with ratios 4 to 6 consistent with collisional excitation at temperatures 1.5 to 10 x 10^4 K.

Confirmation of collisional processes in the shock regions of HH objects.

Abstract

We present new H$\alpha$ and H$\beta$ images of the HH~1/2 system, and we find that the H$\alpha$/H$\beta$ ratio has high values in ridges along the leading edges of the HH~1 bow shock and of the brighter condensations of HH~2. These ridges have H$\alpha$/H$\beta=4\to 6$, which is consistent with collisional excitation from the $n=1$ to the $n=3$ and 4 levels of hydrogen in a gas of temperatures $T=1.5\to 10\times 10^4$~K. This is therefore the first direct proof that the collisional excitation/ionization region of hydrogen right behind Herbig-Haro shock fronts is detected.