Proper motions of the inner condensations in the HH 80-81 thermal radio jet

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 compl ex. 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 estima ted for the condensations amounts to similar to 60 mas yr(-1), equivalent t o similar to 500 km s(-1) at the distance of the complex. The brightness de cay can be fitted by a power law of the time elapsed since ejection, yieldi ng power law exponents of about -2 and -3 for the northern and southern con densations, respectively. We also discuss our observations in the context o f simple biconical jet models, suggesting that both condensations are consi stent with relatively weak density enhancements in the otherwise steady jet flow. Their fade in brightness seems to be in agreement with the density d ecay expected in a freely expanding jet.