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

5. Further peculiarities of the observed cold clouds

5.1. Disturbance centers

In the last Section we pointed out several dense groups of clumps of OCCs having peculiar velocities, which are not fitted by the standard solution. Many of these OCCs should belong either to the Sco-Cen shells or to the Ori-Eri bubble. These objects are driven by the associations Sco-Cen and Ori OB1 respectively, which are not older than 15 Myr (Blaauw 1991). This suggests the production of local changes in the original densities and velocities of the expanding shell during the formation and evolution of the associations, assuming that they interacted with the shell.

In order to test this possibility, we assumed the appearance of single isotropic disturbance centers . A given disturbance center D can arise in the direction at a distance from the Sun, producing a spherical disturbance of radius s during a time interval t, which is short compared with . The force components , and will not vanish now. We assumed a solution of the form

where the subindex 1 refers to the ballistic solutions while 2 refers to small corrections produced by the disturbance center. We considered a test particle located at , at a distance from D. A linear isotropic mean radial acceleration of magnitude was assumed to act on the test particle, during a time interval . The intensity parameter k was assumed to be in the range 0-1. Simple solutions and were obtained. In this manner, small corrections could be fitted changing the position and the velocity of the test particle.

Three independent disturbance centers were considered, namely in Orion, in Hercules, and at the Loop I center. The adopted parameters are listed in Table 2. The results were plotted in Figs. 1-2 replacing the original test points from the standard solution. In the region of the Ori-Eri bubble the disturbance produced small bumps at and , with a significant improvement of the fit. This was also the case for the spike in Hercules at . The Loop I disturbance acted on the Sco-Cen region improving the fit at and . We conclude that our results are well consistent with the assumption of significant modifications of the original distribution of the OCCs in Lindblad's ring by the action of the new star formation activity.

Table 2. Adopted parameters for the disturbance centers

5.2. The HI-hole

We have seen that a cutoff at produces a large gap in the distribution of the northern test particles in GQ II. Globally, the gap seems consistent with the large HI-hole mentioned in Sect. 1. To test this more closely, we considered the locus of the test particles ejected at three selected values of . The adopted parameters were those of our standard solution with km and pc (cf. Table1). The values of are shown in Table 3. For each locus we indicate the coordinates , velocity , distance , and altitude of the test point having the lowest b. Similar quantities are also given for the one having the highest b (subindex 2). The three loci are plotted in Fig. 3 superposed on an HI-contour map derived by Kuntz & Danly (1996) for radial velocities in the range -5 to 0 km , where the HI-hole is well apparent. As can be seen, each locus forms a loop, the outer one corresponding to . It seems to enclose roughly the HI-hole. For each computed locus its top border is nearer to us and has approaching velocities, while the bottom border has receding velocities (cf. Table 3). In contrast, the values of z are similar for both borders. These characteristics are expected for a hole produced by a nearby explosive event, It should be interesting to check them observationally . Moreover, Fig. 3 suggests that our standard solution is a simplification in the sense, that the cutoff-values of could be a function of . We quote that for the locus encloses only partially the large area of the most intense X-ray enhancement detected at high positive latitudes in the ROSAT 1/4 keV band (Snowden et al. 1995, cf. their Fig. 5c). Thus, the large northern X-ray enhancement appears related to two different sources at least. They could be the HI-hole and the North Polar Spur.

Fig. 3. Locus of the tests particles ejected with pc; km ; -, and the values of given in Table 3. The loci are superposed on a contour map derived by Kuntz & Danly (1996) for radial velocities in the range -5 to 0 km . The grid correspond to galactic coordinates with a space of in l and b. See the text for more details.

Table 3. Characteristic parameters of the loci of particles ejected with large values of , pc and km

Furthermore, it was not possible to fit the gas with related to the NCPL at and , neither by means of the standard solution nor by varying . Computations for particles ejected beyond the cutoff (i.e. with ) produced only negative velocities at . Therefore, we computed the effects of an isotropic disturbance center acting on test particles, whose initial parameters were

and else, those of Table 1. The adopted disturbance parameters are shown in Table 2. In these conditions, at we obtained test particles having positions and velocities qualitatively similar to those of the OCCs related to the NCPL (cf. Fig. 1c).

5.3. High and intermediate velocity complexes

In Paper I it was shown, that the scenario of an explosive event is consistent with large z-extensions of the IS gas, as well as with the observed parameters of some nearby intermediate velocity (IV) and high velocity cloud (HVC) complexes. Therefore, we tried to fit some sample positions of the HVC complex M (Herbstmeier et al. 1995) and the IV cloud bridge (Kuntz & Danly 1996) by means of test particles ejected from E with the general parameters of the standard solution (cf. rows 1-4 of Table 1, with pc), and particular values of and . The results are given in Table 4. We list the name of the complex, the coordinates and velocity V at the sampled position; the initial parameters and of the fitting test particle; its distance r and altitude z. As can be seen, the fitting requires altitudes , (i.e. ejections through the HI-hole), and very large values of .

Table 4. Adopted parameters for the fit of IV and HVC complexes

On the other hand, velocities and coordinates similar to those of the OCCs with (km ) at and in the GQs II and III could be produced with test particles ejected beyond the cutoff value with velocities not larger than about 65 km .

© European Southern Observatory (ESO) 2000

Online publication: June 26, 2000
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