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

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).