6. On the nature of the GGD 34 envelope
The most puzzling characteristic of GGD 34 is the presence of the extended faint envelope forming a bubble like shell between GGD 34 B and C. The envelope emits more strongly in [S II] than in H and it is better observed in images with low spatial resolution (see e.g. Ray 1987). The new CFHT images show that the envelope connects smoothly with the back tails at the head of the jet suggesting that it can be tracing, at least partially, the backflow.
Strong backflows are expected particularly in low density, high Mach number jets. The jet material, which is still travelling forwards after passing through the jet-shock, is bent back towards the source by the pressure gradient between the shocked molecular cloud at the head of the jet (i.e. material at the stand-off shock in the jet) and the thermal pressure of the material jet. The strength of the backflow depends both on the density ratio and the Mach number of the jet, however for sufficiently light jets the shock front can be considered as stationary and therefore the dependence of the backflow strength on density ratio is very weak. Williams (1991) has computed the expected relationship between the Mach number of the backflow () and the jet Mach number () in the case of no turbulence and a tangled magnetic field , which for the adiabatic case (i.e. we are neglecting radiative cooling at the head of the jet) with adiabatic index = 5/3 is:
The Mach number of GGD 34 is very high. The velocity of the jet, is:
where is the mean radial velocity which is -180 km s-1 according to Gómez de Castro et al. (1993) and is the velocity in the plane of the sky determined by Ray et al. (1990) from the proper motion of the head of the jet. Note that determined in this way represents a lower limit to the true jet speed since given the variability and the complex structure of the head an accurate determination of the proper motion requires the comparison between high resolution images. The jet sound speed is 11 km s-1 assuming a fiducial temperature of 104 K. Therefore, the Mach number of GGD 34 is:
As a consequence the expected Mach number of the backflow is,
This implies a backflow speed of km s-1. This flow is clearly supersonic in the molecular environment; the sound speed inferred from the CO lines excitation temperatures in this area of NGC 7129 is 0.4 km s-1 (Bechis et al. 1978). Shocks at this velocity are expected to emit primarily in [S II] (see Heathcote et al. 1996for a detailed discussion on the structure of the shocks at these low velocities) and this could explain why, in general, the envelope is brighter in [S II] than in H. No indications of such a low velocity gas have been found in the optical spectra but this could be due to the low surface brightness of the envelope which would require much longer integration times in order to detect its spectroscopic signature.
Additional constraints to this interpretation can be derived from a comparison between the dynamical time-scale for the formation of the envelope and the dynamical time-scale for the formation of the jet; the envelope extends roughly over a half of the jet length and the velocity of the backflow is a factor of 0.15 the jet speed. The dynamical time for the formation of GGD 34 is:
where represents the length of the jet. The proper motion measured by Ray et al. (1990) indicates that GGD 34 is highly inclined with respect to the plane of the sky. We derive an inclination of:
and a length of the jet of:
This implies a dynamical time of yrs. However the time required by the backflow to form the envelope is:
where represents the length of the envelope (0.16 pc) and the backflow speed. This time is a factor of 2 larger than the jet dynamical time scale which, in principle, is inconsistent with the envelope being produced by the jet backflow. Note however, that the dynamical time scale of the flow is just a lower limit to the jet true age. If the jet is light and the head has remained stationary during most of the jet life, the age of the jet could be significantly larger than the calculated above. A good estimate of the proper motion of the head based on high resolution images is clearly required.
© European Southern Observatory (ESO) 1999
Online publication: March 18, 1999