Astron. Astrophys. 333, 1001-1006 (1998)
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
Table 1. Observational constraints for jets in HH34 and HH111
- 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 () up to
; 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.
- 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).
- The intensity in the two prominent emission lines,
and , typically increases
along the jet from the first knot, reaches a maximum in the middle of
the chain and then decreases.
- The line intensity ratio 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?).
- The electronic density grows with the distance from the origin,
attains a maximum value of , then decreases.
- Measures of knots proper motions in HH34 show tangential
(projected) velocities that increase with distance along the jet from
to . 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 to
- The intra-knot spacing is larger, by a factor of
, in HH111 with respect to HH34, and the the
ratio 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
© European Southern Observatory (ESO) 1998
Online publication: April 28, 1998