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Astron. Astrophys. 333, 1001-1006 (1998)

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2. Observational constraints

For the general properties of knots we mainly refer to the HH34 and HH111 jets (see e.g. Bührke et al. (1988), Cohen & Jones (1987), Eislöffel & Mundt (1992), Heathcote & Reipurth (1992), Morse et al. (1993a)), that have been observed with the utmost detail (see Table 1 for a summary). Spectroscopical data taken at different epochs lead to somewhat different results that may imply a degree of variability in the sources; however the relevant morphological, spectral and kinematical features can be summarized as follows:


Table 1. Observational constraints for jets in HH34 and HH111

  1. These objects show one-sided jets displaying a chain (10-13) of quasi-equally spaced emission knots, starting from few jet radii from the central YSO ([FORMULA]) up to [FORMULA] ; in both cases the farthest knot ('O' in HH34 and 'P' in HH111, in Morse et al. 1993a) appears more detached and shows a distinct, wide 'bow-shock like' form. Where the instrument resolution suffices, all knots show the apex in the forward direction (Ray et al. 1996), indicating a common 'bow-shock like' shape.
  2. A region of very low emission (a 'gap') is present in HH34 and HH111 between the central source and the first knot. This is also true for the jet of HH1/2, but perhaps due to a reflection nebula (Reipurth et al. 1993). HH30 instead shows intense emission right from the beginning then a decline (see Lopez et al. 1995, Mundt et al. 1990).
  3. The intensity in the two prominent emission lines, [FORMULA] and [FORMULA], typically increases along the jet from the first knot, reaches a maximum in the middle of the chain and then decreases.
  4. The line intensity ratio [FORMULA] shows a similar behavior, but Cohen & Jones (1987) find a higher value in knot 'O' of HH34 with respect to Morse et al. (1993a) (variability?).
  5. The electronic density grows with the distance from the origin, attains a maximum value of [FORMULA], then decreases.
  6. Measures of knots proper motions in HH34 show tangential (projected) velocities that increase with distance along the jet from [FORMULA] to [FORMULA]. Estimates of the real knot pattern and jet velocities, i.e. disentangled from projection effects, have been attempted by Eislöffel & Mundt (1992) who found a knot pattern speed to jet speed ratio again increasing steadily with distance from [FORMULA] to [FORMULA].
  7. The intra-knot spacing is larger, by a factor of [FORMULA], in HH111 with respect to HH34, and the the ratio [FORMULA] is smaller in the former by a similar amount. Therefore, whatever physical mechanism is proposed for the origin of knots, this must agree, at least qualitatively, with the above constraints.
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© European Southern Observatory (ESO) 1998

Online publication: April 28, 1998