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

TL;DR

This study analyzes four decades of HST images of HH~2 to measure proper motions, emission line fluxes, and variability, revealing slight deceleration and significant changes in knot brightness and line ratios.

Contribution

It provides new proper motion measurements and variability analysis of HH~2 knots over 20 years, with insights into their dynamic evolution.

Findings

Proper motions agree with earlier measurements, with slight deceleration.

Knot H has increased in brightness while maintaining line ratios.

Knot A has faded and shows increased [S~II]/H$eta$ ratio.

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.