Proper Motion of the Irradiated Jet HH 399 in the Trifid Nebula

HH 399 is one of the first Herbig Haro flows recognized to be irradiated bythe UV radiation of the massive O7.5 star in the Trifid nebula. We present theproper motion of the first irradiated jet based on two epochs of HSTobservations of HH 399 separated nearly by five years using H$\alpha$ and [SII]line filters. High proper motion with continuous velocities between 200$\pm$55and 528$\pm24$ \kms are detected in both lines along the 18$''$ extent of thejet axis. The irradiated fully-ionized jet consists of numerous knots along thejet but also shows the evidence for a number of isolated blob-like structuresrunning immediately outside the jet with lower transverse velocities. Thetransverse velocities combined with radial velocity measurements indicate thatthe jet axis lies away from the plane of the sky by only few degrees. We arguethat the jet is fully ionized based on [SII]/H$\alpha$ line ratio as well asradio continuum emission detected from the full extent of the jet at 3.6cmwavelength. The stellar mass-loss rate producing HH 399 is estimated to be$\approx 2\times10^{-6} \msol $yr$^{-1}$.Comment: 14 pages, 6 figures, ApJ (in press