The L1582B outflow -- A 5 parsec long molecular superjet

Optical emission

This outflow source is one of a select few that show both collimated molecular outflow and also optical emission associated with a so-called 'optical jet' (others that spring to mind are HH46-47, HH111 and perhaps NGC 1333, but the number is increasing steadily.) In the picture on the front cover the Ha optical emission is overlaid on the CO map. Since the Ha emission is generally thought to be indicative of shocks at a velocity of at least several tens of km/s, this seems like excellent evidence for a molecular outflow driven directly by the optical jet. The picture we have in mind is of a highly collimated, mostly neutral jet, which is ejected from the unseen star at a speed of about 300 km/s. As the jet propagates through the surrounding cloud it generates the familiar 2-shock structure: since the shocks are highly radiative the shocked material cools rapidly into a dense swept-up shell, which then generates the observed optical emission. As the shell cools further and continues to expand away from the jet it sweeps up molecular material, which forms the observed CO outflow.

Highly collimated CO bowshocks

In the past it has been common to interpret similar pictures, in H2 S(1) for example, as lumps or instabilities in a rather poorly collimated outflow. Here the overall north-south symmetry, and the association of collimated optical emission with individual regions of excited CO, persuade us that in fact each of these excited regions is itself a separate bowshock.

In fact there is lots of less circumstantial evidence that this is the case. In particular, position-velocity diagrams along the axes joining the individual regions to the supposed exciting source all show patterns characteristic of those expected from prompt entrainment in the working surface of a highly supersonic jet. But if this is the case, then the collimation of the CO outflow is really very high, and we need to invoke a fairly rapid variation in the original jet direction to explain the spread in the position angles. The overall 'envelope' of the emission appears to be biconical, with an opening angle of 20 degrees, and we are investigating mechanisms which might produce this 'precession' -- currently torques exerted by a binary companion look very promising. It is interesting to note that the velocity widths at the bowshocks are highest towards the centre of the apparent cone, where the jet lies closest to the line-of-sight, and symmetrically less at the outer edges.

Bullets?

We have avoided this term so far. But one remarkable feature of the CO map is just how lumpy the outflow is. Channel maps show that there are very high velocity features associated with each of the bowshocks, and that the CO excitation is high there. So it looks as though, with further observations, these bowshocks will turn out to be rather similar to the 'bullets' claimed in sources such as L1448 and Orion-S. On the other hand, there doesn't appear to be any need to hypothesise that these are semi-solid lumps expelled from the central source. All that is necessary -- as shown by Alex Raga and collaborators in the UK, and numerically by Stone and Norman in the US -- is for a slight modulation of the basic jet, either in density or velocity. Internal working surfaces (shocks) then rapidly form downstream, in the jet as stuff piles up non-linearly at particular places. Add in precession, and you have a recipe for something very similar to that observed.

It is worth noting that the tails of individual CO bow shocks don't extend all the way back to the source. In our model this is a consequence of the relatively short cooling time for the hot molecular material: assuming a velocity of 300 km/s and a distance of 450 parsec the 150 arcsec tail corresponds to a cooling time of only 1000 years, which is much less than the dynamical timescale. The observed hotspots are therefore the sites of current interaction between jet-fragments and the ambient cloud. It looks as though the jet fragment is rather denser than the environment, and therefore should appear to move with something like the true jet speed. Which all fits in rather well with the Ha proper motions for the three regions where they have been measured.

The Inner Region

The inner region has been mapped with almost full sampling (a luxury otherwise denied to us -- until very recently -- because of the overheads on single-point observations), as shown in the blow-up on the cover. To those of us conditioned by years in the company of extragalactic radio astronomers, this is the spitting image of Cygnus-A, right down to the hot-spots and the essentially unresolved jet symmetrically joining the two lobes.

Numerical modelling shows that the nice fluffy cocoons that you see around the bowshocks in Cygnus, as in most extragalactic sources, result from the interaction of an underdense jet with the surrounding medium when the cooling time scale is long with respect to the dynamical time. We argued above for a (molecular) cooling time of around a 1000 years, and since this is about the same as the dynamical timescale of the 'inner' outflow, this also seems fairly reasonable. Richer et al. hypothesised that for NGC 2024, the jet in the inner regions of the source is less dense than the surrounding cloud, and perhaps the same is true here; presumably the cloud density falls off fairly rapidly away from the FIR source, but this is something that can be fairly readily checked.

And now?

Whatever the final extent of the L1582B outflow may prove to be, it is clear already that it has managed to shock a very large volume of its parent cloud, and that the cloud retains a memory of this shock, in the form of broadened lines, long after any particular shock has passed by. So indeed it looks as though outflows can do a lot to keep the ISM stirred up.

The case for the molecular outflow being driven directly by the optical jet is so strong in this source that we really have to ask if other mechanisms are necessary. We could, with some confidence, turn the usual argument on its head, and claim that we can determine the momentum flux in the optical jet from observations of the swept-up molecular gas. The number is clearly bigger than has hitherto been assumed. It seems to be time to forget about looking for the driving sources of molecular outflows, and to concentrate on seeking mechanisms for driving the optical jets.