The time evolution of HH~2 from four epochs of HST images

Abstract

We have analyzed four epochs of H$\alpha$ and [S~II] HST images of the HH~1/2 outflow (covering a time interval from 1994 to 2014) to determine proper motions and emission line fluxes of the knots of HH~2. We find that our new proper motions agree surprisingly well with the motions measured by Herbig \& Jones (1981), although there is partial evidence for a slight deceleration of the motion of the HH~2 knots from 1945 to 2014. We also measure the time-variability of the H$\alpha$ intensities and the [S~II]/H$\alpha$ line ratios, and find that knots H and A have the largest intensity variabilities (in $1994\to 2014$). Knot H (which now dominates the HH~2 emission) has strengthened substantially, while keeping an approximately constant [S~II]/H$\alpha$ ratio. Knot A has dramatically faded, and at the same time has had a substantial increase in its [S~II]/H$\alpha$ ratio. Possible interpretations of these results are discussed.