Proper motions of the HH1 jet

TL;DR

This paper introduces a new method for measuring proper motions of extended objects, applies it to the HH1 jet using HST images over 20 years, and reveals an acceleration in ejection velocities leading to knot merging.

Contribution

A novel method for determining proper motions of extended objects and its application to the HH1 jet to analyze velocity variability over time.

Findings

Proper motions of HH1 jet knots were measured.

Ejection velocities show an acceleration trend.

Knots are predicted to merge in ~450 years near HH1.

Abstract

We describe a new method for determining proper motions of extended objects, and a pipeline developed for the application of this method. We then apply this method to an analysis of four epochs of [S~II] HST images of the HH~1 jet (covering a period of $\sim 20$~yr). We determine the proper motions of the knots along the jet, and make a reconstruction of the past ejection velocity time-variability (assuming ballistic knot motions). This reconstruction shows an "acceleration" of the ejection velocities of the jet knots, with higher velocities at more recent times. This acceleration will result in an eventual merging of the knots in $\sim 450$~yr and at a distance of $\sim 80"$ from the outflow source, close to the present-day position of HH~1.