Astron. Astrophys. 318, 608-620 (1997)

Astron. Astrophys. 318, 608-620 (1997)

6. Discussion and conclusions

We have obtained Fabry-Pérot images of the OMC-1 star forming region in various K band transitions of the H₂ molecule. These images show a network of narrow linear structures, reminiscent of jets with very small opening angles. From Figs. 9 & 10 it is clear that these jet structures cannot be explained as a result of temperature variations but are due to density fluctuations. This conclusion is reinforced by the wavelet analysis of the temperature map which shows very little structure at small or intermediate scales. Instead the temperature makes only a large scale contribution to the structure of the region. The column density map, on the other hand, shows structure on all of the scales we have used.

From the observed column density and the fact that H₂ needs to be thermalised, we deduce that the H₂ emission comes from a thin sheet only. This argument has already been presented earlier (e.g. Ridgway 1983) but with the higher resolution images now available, the thin sheet is not anymore uniformly covering the lobes of Beckwith et al. (1978) but rather consists of a network of long tubes which are wrapped in a layer of shock excited molecular hydrogen. This molecular material was probably entrained into the path of the passing shocked material which in itself is not visible on H₂ images. The molecular gas is heated to a temperature of up to [FORMULA] 3000 K. The fact that we observe this upper limit of the H₂ temperature is due to the strong increase in the collision-induced dissociation coefficient in this temperature range (Roberge & Dalgarno 1982).

6.1. Structure formation mechanisms

The question arises of how the numerous linear structures have been formed. Two types of formation mechanisms can be envisaged: one is jet-driven and the other wind driven. The former process invokes bullets of material which are ejected in various directions (Allen & Burton 1993) and the latter produces the observed linear structures with instabilities in the wind-molecular cloud interaction zone (e.g. Stone et al. 1995).

6.1.1. The jet scenario

In the jet scenario, OMC-1 is a superposition of jets as they are observed in Herbig-Haro objects. A cluster of young stars or pulsed ejection from a single strongly precessing source could in principle produce the observed wealth of linear structures. Regular chains of knots have been detected in many outflows (e.g. in HH 7-11 see Mundt 1985, Bachiller & Cernicharo 1990 or in IRAS 3282 see Bachiller et al. 1991) which indeed have been interpreted as due to a pulsed matter ejection (e.g. Bachiller et al. 1990, Guilloteau et al. 1992). On the other hand, such a knotty structure could also be the consequence of hydrodynamical instabilities in a jet.

If IRc2 is indeed the central object driving the dozens of radially expanding jets, then the formation mechanism must be different from "normal star-forming" in which a bipolar outflow is produced by the collimation of the proto-stellar wind by the circumstellar disk. If they are jets produced by bullets with a speed comparable to the H₂ line width of [FORMULA] km/s (Nadeau & Geballe 1979), tracks such as we observe could be produced in years. This compares with the estimate of 1000 years by Allen & Burton (1993) for the age of their jets, which are located further outside of OMC-1, to the north-west. At least in the case of the shorter and more numerous jets, it seems difficult to generate them solely with "bullets" from one or several central sources. The bright linear structures look rather similar, have similar lengths and therefore presumably also not very divergent ages. If the "bullets" were to be produced by numerous stellar sources they would have to have passed through the same ejection phase almost simultaneously in order to produce the morphology now observed. If, on the other hand, one single object emitted the "bullets", the precession rate would have to be extremely large, which in turn, would lead to warped jets which however are not observed.

6.1.2. The wind instability scenario

We find a rather smooth temperature distribution for OMC-1 which only varies radially and not as a function of position angle. Also kinematic observations of the S(1) 1-0 line profiles indicate that we see a spherically expanding shell with a velocity of a few ten kilometers per second (Scoville et al. 1982). This is more indicative of an uncollimated stellar wind which could be produced by a compact group of young massive stars in the IRc2 area. The collision of this wind with the surrounding dense material creates two shocked layers: The outer one consists of matter from the surrounding material and the inner one of the shocked wind (Pikel'ner 1968, Pikel'ner & Shcheglov 1969). Under various conditions instabilities can occur in the shell of swept up material and it will fragment (e.g. Silk 1983). Recently, Stone et al. (1995) proposed a model where an early stellar wind coupled with gravitational deceleration produces a thin shell of material. This material is then shocked by a second, faster wind. In this situation, Rayleigh-Taylor instabilities will develop which may produce a morphology similar to the one observed in OMC-1.

Alternatively, instabilities also occur in a non-variable wind if the shocked envelope material is rapidly cooled. In this case, a thin layer develops which is highly unstable and which might quickly generate radial extensions which morphologically look very similar to jets. This "thin layer instability" has been studied in other astrophysical contexts (Dgani et al. 1993) but we suggest that it is also likely to play a role in the environment of OMC-1.

6.2. Coincidence of H₂ O maser sources

A number of H₂ O masers have been found in OMC-1 which, unlike masers from other molecules, are widely scattered over the outflow region. Those masers further away from the central area are also distinct because of their large proper motions (Genzel et al. 1981). In Fig. 11 we have drawn maser positions and directions of movement onto a 2.12 µm image. Interestingly, some of the fast moving masers are found close to the inner edge of H₂ fingers, with their velocity vectors pointing outwards into the same direction as the fingers.

Fig. 11. H₂ O maser positions and movements superposed on an image of molecular hydrogen taken at 2.12 µ.

Water masers in star forming regions are thought to be produced by a clumpy outflow from one or several young stellar sources. The maser lifetime is only a few years whereas the H₂ fingers must have ages of a few hundred years. It is therefore unlikely that there exists a generic link between H₂ O masers and the H₂ linear structures unless the masers can be rejuvenated. The coincidence shown in Fig. 11 however indicates that both phenomena are linked and perhaps driven by the same mechanism.

6.3. Chemical evolution

The picture of OMC-1 where several supersonic flows interact with the surrounding molecular material could have important consequences for chemical models of this region. About a decade ago it was considered that warm (T [FORMULA] 100 K) cloud chemistry could account for what was observed (Herbst & Leung 1986). But more recent work by Herbst & Millar (1991) has stressed the need for the detailed physical conditions in dense clouds to be taken into account.

Reviewing chemical modeling of the region close to IRc2, they commented that existing models are based on a gross oversimplification of the physical conditions present in dense interstellar clouds. Our images indicate that explanations of the observed molecular abundances may have to be refined still further by taking into account the effect of narrow, highly collimated structures, giving rise to steep density gradients throughout the gas in which the resulting chemistry occurs.