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Astron. Astrophys. 358, 299-309 (2000)

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6. Discussion of the results. Conclusions

We computed positions, distances, and radial velocities for test particles, which were ejected almost isotropically from a small volume centered at E through the effects of an explosive event. Ballistic orbits were assumed. We adopted a standard solution having [FORMULA], and E at [FORMULA] pc in the direction [FORMULA] (Olano 1982), and a mean altitude [FORMULA] pc above the galactic plane. The assumed initial parameters include a cutoff at [FORMULA] (cf. Table 1). At low latitudes a large fraction of the observed OCCs can be fitted approximately by our simple standard solution. However, in the GQs I and IV at [FORMULA] the ballistic approximation is not satisfactory and Olano's model 1 makes a better fit. The motions of the fitted OCCs are consistent with an expansion . Among the remaining non-fitted OCCs, a significant fraction can be understood as relatively distant objects, which are moving mainly on circular orbits about the galactic center C. At intermediate and high latitudes the ballistic approximation becomes more satisfactory and the fraction of the OCCs fitted by the standard solution increased considerably. Among the fitted OCCs the abundance of negative velocities becomes overwhelming at the high latitudes. The corresponding motions are consistent with a falling to the galactic plane.

A significant fraction of the non-fitted OCCs belongs to regions, which appear to have been disturbed by the ulterior processes of star formation (e.g. the Sco-Cen shells and the Ori-Eri superbubble). As a check, we considered the effects of three different disturbance centers, each one producing a radial acceleration during a time interval t, which is short, as compared to [FORMULA]. Their parameters were chosen as to be very roughly consistent with those of the Loop I, the Ori OB1 association, and the Her-shell, respectively (cf. Table 2). The corrections obtained for the test particles were in the sense of improving the fit.

Our simple standard solution also fits very roughly the shape of the well-known northern hole in the distribution of the HI of low velocities, by assuming, that it was produced by particles ejected with altitudes [FORMULA]. We suggest that the hole is a signature of the explosion, indicating the ionization and transfer of gas into the lower halo (cf. Paper I). The resulting interaction could be the origin of the observed northern polar soft X-ray enhancement, which is not related to the North Polar Spur. We quote that the hole is also apparent in the data of Westphalen (1997) regarding the diffuse high-dispersion HI-component detected in the ISM (Kalberla et al. 1998) (cf. Fig. 4). Presumably, this component is very turbulent and extends into the halo. Moreover, samples of the HVC complex M were fitted by assuming test particles ejected beyond the cutoff [FORMULA] with very high initial velocities. Analogously, the bulk of the OCCs with [FORMULA](km [FORMULA]) [FORMULA] related to the IVCs at [FORMULA] and [FORMULA] in the GQs II and III could be fitted by particles with [FORMULA] not larger than about 65 km [FORMULA] and [FORMULA].

[FIGURE] Fig. 4. Distribution of the column densities [FORMULA] of the large velocity dispersion component of the ISM at the northern Galactic Pole cap, as derived from Westphalen's (1997) data. The five intervals considered are (in units of [FORMULA] H atoms cm-2): 1: 10.0-12.9; 2: 13.0-15.9; 3: 16.0-18.9; 4: 19.0-21.9; 5: 22.0-24.9.

Our main conclusion is that the global characteristics of the kinematics of the local CNM at [FORMULA] [FORMULA] appear to be well consistent with the assumption of an energetic explosive event, of characteristics like those assumed in Olano's models (cf. Table 1).

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© European Southern Observatory (ESO) 2000

Online publication: June 26, 2000