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Astron. Astrophys. 325, 282-294 (1997)

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5. The case of G24.78+0.08

Among the objects of our sample, G24.78+0.08 turns out to be the most complex. In particular, we note that (see Fig. 6):

  • the ammonia clump extends over a region of [FORMULA]  pc, larger than in the other sources, although an optically thick core is seen towards the OH and H2 O masers;
  • a pointlike UC HII region (G24.78 A) is very likely embedded in such core, as suggested by the NH3 absorption seen towards it (Fig. 8);
  • the ammonia emitting region contains two centers of star formation, as indicated by the existence of two groups of H2 O maser spots, one (hereafter H2 O-A) coincident with the NH3 absorption (i.e. with the UC HII region G24.78 A) and close to the OH masers, the other (hereafter H2 O-M) located [FORMULA] (0.34 pc) to NE from the former, at the position called G24.78 M.

This scenario is reminiscent of the morphology of W3(OH), where OH maser emission is seen close to the UC HII region, whereas H2 O masers lie in a molecular core located [FORMULA] from it. This is commonly interpreted with the existence of two massive stars in different evolutionary phases, one still very young embedded in the molecular core, the other, older, at the center of the UC HII region. Such an interpretation can probably apply also to G24.78, with G24.78 A playing the role of W3(OH) and G24.78 M that of W3(H2 O). A few important differences are there though: first of all, although G24.78 M lies in the NH3 clump, no NH3 optically thick core is seen by us at its position; moreover, unlike W3(OH), H2 O masers are seen also towards the UC HII region G24.78 A; finally, such UC HII region is smaller than W3(OH) ([FORMULA] 0.01 pc as opposed to 0.02 pc). The difference in size can be explained in terms of an earlier evolutionary status of the former UC HII region with respect to the latter: this would also be consistent with the idea that H2 O masers (unlike OH masers) form prior to the appearance of an HII region and disappear when this evolves.

Another noticeable feature of G24.78 is that in each of the three groups (OH, H2 O-A, H2 O-M) the maser spots are aligned along a straight line. By using the positional and velocity information (Forster, priv. comm.), we have studied the velocity gradient of the masers. As a first step, a least square fit to the spots in each maser group has been done. Interestingly, the resulting three directions are almost parallel, being characterised by the following angles (from north to west): [FORMULA] (OH), [FORMULA] (H2 O-A), and [FORMULA] (H2 O-M).

This seems to indicate a preferred direction in the molecular clump where the masers are embedded. Such direction might be related to the magnetic field, as predicted by the models of Shu et al. (1993) and McKee et al. (1993) for the magnetically supercritical collapse. If this is the case, then two scenarios are possible:

  1. According to Elitzur et al. (1989) H2 O masers form behind shocks: in particular, in the assumption of a YSO emitting an isotropic wind through a medium threaded by a magnetic field, the model predicts that the brightest masers are those located in a ring around the magnetic equator (Heiles et al.  1993). In this case, the masers would trace a direction perpendicular to the magnetic field.
  2. If H2 O masers are strictly related to bipolar flows as suggested, e.g., by Felli et al. (1992), then the direction described by the maser spots should be parallel to the outflow axis and hence to the direction of the magnetic field. In fact, the NH3 clump in G24.78 looks slightly flattened perpendicularly to the masers: this might indicate that the maser distributions are parallel to the direction along which the collapse has occurred, i.e. the magnetic field direction.

In an attempt to further investigate this issue, in Fig. 9, we plot the LSR velocities of the maser spots against the projected distance along the direction given by the linear fit. The dashed line stands for the [FORMULA] of the NH3 (2,2) emission at the peak position, which is 110 km s-1 (Table 10). It is worth noting that this velocity does not differ from those of the ammonia absorption and emission seen respectively against G24.78 A and towards the H2 O-M group. Fig. 9 shows that the [FORMULA] of the maser spots changes steadily along each group, but around the centre it shows a spike. This is particularly evident in the H2 O-A group. Such effect suggests the existence of a systemic velocity field (like accretion, expansion, or rotation) close to the central object powering the masers. However, it is difficult to distinguish among different models, at least in the case of H2 O-M. As for the OH and H2 O-A masers, one sees that almost all spots are blue-shifted with respect to the NH3 gas. This can be explained in a model where the UC HII region G24.78 A has formed inside the NH3 core just at the SE border of it, on the opposite side of the core with respect to the observer: in this scenario, most of the molecular gas lies among the UC HII region and the observer (giving rise to NH3 absorption) and to the NW of G24.78 A (originating the NH3 emission). Therefore, if the maser spots are bullets shot away from the central stellar object against the surrounding dense molecular gas, then only those shot towards the observer can be seen, which hence have blue-shifted [FORMULA], as observed.

[FIGURE] Fig. 9. Plot of LSR velocity versus projected distance for the maser spots of the three maser groups of G24.78+0.08: OH (empty squares), H2 O-A (empty triangles) and H2 O-M (filled triangles). The projected distance is given in seconds of arc ([FORMULA] =0.037 pc) measured from a "center of mass" weighted according to the maser intensities. The dashed line indicates the LSR velocity of ammonia ([FORMULA]  km s-1)

Although the previous facts seems to favour the interpretation of masers as originating in outflows, we conclude that at present, both hypotheses above (1. and 2.) are highly speculative and in no way we can rule out any of them. Better signal-to-noise ammonia maps with higher spectral resolution are in order, to obtain detailed information about the physics of the molecular gas. Also, VLA maps of the H2 O masers done with higher angular resolution than that of Forster & Caswell (1989), would be worth to studying the proper motions of the maser spots and hence the intrinsic velocity field of the gas close to young massive stellar objects.

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© European Southern Observatory (ESO) 1997

Online publication: May 5, 1998

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