Dip. Scienze Fisiche ed Astronomiche, Sez. Astronomia, Università di Palermo, P.zza del Parlamento 1, 90134 Palermo, Italy e-mail: email@example.com
2 INAF – Osservatorio Astronomico di Palermo, P.zza del Parlamento 1, 90134 Palermo, Italy
3 COMETA, via S. Sofia, 64 95123 Catania, Italy
4 Astrophysics Div. – Research and Science Support Dept. of ESA, ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands
5 European Space Agency Community Coordination and Planning Office, 8–10 rue Mario Nikis, 75738 Paris Cedex 15, France
6 Nordic Optical Telescope Apartado 474, 38700 Santa Cruz de La Palma, Santa Cruz de Tenerife, Spain
7 Onsala Space Observatory, 439 92 Onsala, Sweden
Accepted: 19 March 2008
Context. The jet coming from the YSO binary L1551 IRS5 is the closest astrophysical jet known. It is therefore a unique laboratory for studies of outflow mechanisms and of the shocks occurring when expanding material hits the ambient medium as well as of how the related processes influence the star- (and planet-) forming process.
Aims. The optical data are related to other data covering the spectrum from the optical band to X-rays with goal of understanding the energetics of low-mass star jets, in general, and of this jet in particular. We study the time evolution of the jet, by measuring the proper motions of knots as they progress outwards from the originating source.
Methods. The nebulosities associated with the jet(s) from the protostellar binary L1551 IRS5 were imaged in a number of spectral bands using the Hubble Space Telescope. This allows the proper motion to be measured and permits a simple characterization of the physical conditions in different structures. To this end we developed a reproducible method of data analysis, which allows us to define the position and shape of each substructure observed within the protostellar jet. Using this approach, we derive the proper motion of the knots in the jet, as well as their flux variability and shock emission.
Results. The time base over which HST observations were carried out is now about ten years. The sub-structures within the jet undergo significant morphological variations: some knots seem to disappear in a few years and collision between different knots, ejected at different epochs and maybe with different speed, may occur. The velocities along the jet vary between ~100 km s-1 and over 400 km s-1, with the highest speed corresponding to the knots at the base of the jet.
Conclusions. There are indications that the HH 154 jet has been active relatively recently. Our results suggest the presence of a new shock front at the base of the jet identified with an internal working surface. From the analysis of the terminal and internal working surfaces within the jet, we find that the more likely scenario for the HH 154 jet is that of a jet traveling through a denser ambient medium (a “light jet”). These results are consistent with the Bonito et al. (2007) model predictions. Furthermore, there is strong evidence that the knots at the base of the northern jet correspond to the location where the highest velocity and the highest excitation component are measured along the jet. More important, this is the location where the X-ray source has been discovered.
Key words: shock waves / ISM: Herbig-Haro objects / ISM: jets and outflows / X-rays: ISM
Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with programs #6127, #6411 & #10351.
© ESO, 2008