Cool Stars 14 - Submitted Abstract # 106 This version created on 05 October 2006 X-ray and optical emission in protostellar jets: model predictions and comparison with observations Rosaria Bonito, Universita' di Palermo - INAF Osservatorio Astronomico di Palermo Fabio Favata, Astrophysics Div. - Research and Science Support Dept. of ESA Malcolm Fridlund, Astrophysics Div. - Research and Science Support Dept. of ESA Rene Liseau, Stockholm Observatory, AlbaNova University Center Giuseppina Micela, INAF Osservatorio Astronomico di Palermo Salvatore Orlando, INAF Osservatorio Astronomico di Palermo Giovanni Peres, Universita' di Palermo - INAF Osservatorio Astronomico di Palermo Robert Rosner, Center for Astrophysical Thermonuclear Flashes, University of Chicago Salvatore Sciortino, INAF Osservatorio Astronomico di Palermo We investigate the mechanisms causing the X-ray emission recently detected from Herbig - Haro (HH) objects by analyzing variability in the X-ray band, as detected with Chandra for HH 154, the nearest HH object from which X-ray emission has been detected. A detailed comparison between our 2005 deep Chandra observation with previous 2001 Chandra data shows for the first time the proper motion and the complex morphology of the X-ray source associated with a protostellar jet. The source consists of both an unresolved, point-like component, without detectable proper motion over 4 yr, and an elongated component in agreement with a shock moving away from the parent star at about 460 km/s. We have also studied HST observations in H-alpha and [S II] made at three epochs (1996, 1998, 2005) deriving the morphology and the evolution of the larger knotty structure within the jet. The X-ray emission is co-located with optical knots having true velocity of about 500 km/s according to HST optical spectroscopy and imaging. In order to explain the mechanisms governing the relevant X-ray emission, we developed a hydrodynamic numerical model (including thermal conduction and radiative losses) of a continuous supersonic protostellar jet. The model predicts X-ray emission due to the shock forming at the head of the jet; also the X-ray source should move at about 500 km/s, consistent with the velocity of the observed extended X-ray component. ----------------------------------