We have presented data from longslit ro-vibrational spectroscopy of molecular hydrogen in the outflow from the protostellar source in L483. The jet shows a bow shock, and a series of emission knots leading back to the source. The excitation of the leading bow shock is consistent with a C-shock, with a speed of between 40 km s-1 and 45 km s-1. Slower J-shocks can be ruled out, because of data from CO observations that gives a strict lower limit of 30 km s-1 to the shock velocity. The shock velocity indicates that the jet from L483 has a density of about 10 times less than the medium into which it is propagating.
The excitation in the emission clumps along the jet is consistent with shock excitation, and so the emission may be due to internal jet shocks. These shocks could be fast C-shocks, as at the jet head, or they could be much slower J-shocks, with speeds 11 km s-1. However, other mechanisms such as jet instabilities or episodic activity cannot be ruled out.
In addition to the brighter emission clumps, molecular hydrogen emission is seen along the full length of the jet. This may be due to emission from in the jet, in which case the jet must contain a small fraction, less than a few percent, of molecular hydrogen. Alternatively, material may be being entrained in a mixing layer along the jet. If this is the case, then the observations clearly show that the total mass entrained in mixing layers is much less than the mass swept up by the leading bow shock. A third alternative is that the emission comes from unresolved sub-knots.
The in the clumps and along the jet has similar excitation temperatures; the emission along the jet is mostly distinguished by its low column density. The bow shock at the jet head has by far the greatest column density of . The higher excitation lines, which are weak and therefore only well detected at the jet head, have a higher excitation temperature than the lower energy lines suggesting the presence of a second, warmer, gas component. Increasing excitation temperature with vibrational energy level is a feature of bow C-shock models, suggesting that the jet head is a bow C-shock.
Finally, using lines at different wavelengths which originate in the same upper state energy level, the extinction to the emitting material was estimated. An extinction of mag is derived towards the K band reflection nebula and extinctions about a factor of three less than this towards the jet and jet head. These results indicate that the protostellar source is still very deeply embedded, consistent its inferred young age, and that the source is embedded in a centrally condensed region.
Although this source is the youngest yet studied using this technique, it shows many of the characteristics displayed in older sources. The excitation - and therefore the shock conditions - are similar to more evolved sources and the flow already shows a complex, knotty structure. The outflow-driving mechanism clearly begins very early in the star formation process.
© European Southern Observatory (ESO) 1999
Online publication: July 26, 1999