Proper Motions of the Inner Condensations in the HH 80–81 Thermal Radio Jet

Abstract

We report multi-epoch interferometric radio observations of the IRAS source 18162-2048 that drives the thermal radio jet in the HH 80-81/GGD 27 complex. Our main goal was to follow the proper motion and flux density decay of the two inner young jet condensations N4 and S4, discovered by Marti et al., on their way out from the driving source.The tangential velocity estimated for the condensations amounts to ~60 mas yr-1, equivalent to ~500 km s-1 at the distance of the complex. The brightness decay can be fitted by a power law of the time elapsed since ejection, yielding power law exponents of about -2 and -3 for the northern and southern condensations, respectively. We also discuss our observations in the context of simple biconical jet models, suggesting that both condensations are consistent with relatively weak density enhancements in the otherwise steady jet flow. Their fade in brightness seems to be in agreement with the density decay expected in a freely expanding jet.