A comparison of the radio and optical time-evolution of HH~1 and 2

TL;DR

This study compares 20 years of radio and H-alpha observations of HH 1 and 2, revealing similar trends and differences in emission properties, suggesting lower temperature emission regions compared to planetary nebulae.

Contribution

It provides the first detailed comparison of radio and optical time-evolution of HH 1 and 2 over two decades, highlighting emission behavior and temperature differences.

Findings

HH 1 faded while HH 2 brightened significantly

Radio and H-alpha emissions show similar trends in both objects

H-alpha to free-free ratio indicates lower temperature emission regions

Abstract

We present a comparison between the time-evolution over the past $\sim 20$ years of the radio continuum and H$\alpha$ emission of HH~1 and 2. We find that the radio continuum and the H$\alpha$ emission of both objects show very similar trends, with HH~1 becoming fainter and HH~2 brightening quite considerably (about a factor of 2). We also find that the $F_{\rm H\alpha}/F_{ff}$ (H$\alpha$ to free-free continuum) ratio of HH~1 and 2 has higher values than the ones typically found in planetary nebulae (PNe) which we interpret as an indication that the H$\alpha$ and free-free emission of HH~1/2 is produced in emitting regions with lower temperatures ($\sim 2000$~K) than the emission of PNe (with $\sim 10^4$~K).