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

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2. Kinematics and distribution of the OCCs at [FORMULA]

We consider velocities in the range [FORMULA] (km [FORMULA] [FORMULA] and the positions of the OCCs at b-intervals of [FORMULA] about each of the following mean values:


For each b-interval we adopt plane polar diagrams ([FORMULA]) for plotting the clouds (in the following we supress the subindex c for simplicity). A scale of colors is used for [FORMULA]. The radial scales are proportional to cos [FORMULA] as to maintain a constant ratio of pixel to solid angle on the sky. To improve the statistical significance of our sample of OCCs we eliminated all those having [FORMULA] cm-2. Such weak objects are more susceptible to observational errors and spurious effects. Moreover, we eliminated a small set of about 2.2% of OCCs having [FORMULA] cm-2. Our final database consists of 13,961 OCCs in the north and 17,188 in the south.

The results are given in Figs. 1-2. For the sake of comparison we also made plots including all the eliminated OCCs (not shown here). A careful comparison with Figs. 1-2 confirmed that the differences were very small and of negligible statistical significance. According to our assumptions in Sect. 1, we consider Figs. 1-2 as a uniform statistical sampling of the CNM over the entire sky at [FORMULA]. In the same figures we included the distribution of the high-latitude molecular clouds at [FORMULA] according to the catalogs of Magnani et al. (1996), and Hartmann et al. (1998). The association of this sort of molecular clouds with the cold HI-gas is well apparent in the Figures (cf. Gir et al. 1994). As can be seen from Figs. 1-2, the distribution of the OCCs is rather clumpy. In the following we point out some general characteristics of the OCCs.

[FIGURE] Fig. 1a-d. Polar diagrams for [FORMULA], [FORMULA], [FORMULA] and [FORMULA], showing angular distribution and velocities. Galactic longitudes l increase counterclockwise, [FORMULA] points downwards. The three large concentric circles indicate V(km [FORMULA]) = +20, 0 and -20, respectively in order of decreasing radii, whereas -40 is at the center. Filled circles : OCCs. The scale of [FORMULA] is indicated by means of a wedge of 10 colors. From left to right the following intervals are considered (in units of [FORMULA] H at cm-2): 1: 5.2-15.0; 2: 15.1-25.0; 3: 25.1-35.0; 4: 35.1-44.9; 5: 45.0-54.8; 6: 54.9-64.7; 7: 64.8-74.6; 8: 74.7-84.6; 9: 84.7-94.5; 10: 94.6-104.4. Open black rectangles (only for [FORMULA]): Olano's (1982) model 1. Open red rectangles (only for [FORMULA]): high-latitude molecular clouds. Small open circles : a sample of the test particles fitted by the standard solution (cf. Table 1). The sample is given for [FORMULA] (in red) and [FORMULA] pc (in black), both with [FORMULA] and 12.5 km [FORMULA]. The interval of [FORMULA] is 10o. The interval of [FORMULA] is [FORMULA] in Figs. 1a-c, and [FORMULA] in Fig. 1d. Large open circles : positions of test particles corrected by the action of asumed disturbance centers. The uncorrected positions were not plotted, except in the case of the Her disturbance center at [FORMULA].

[FIGURE] Fig. 2a-d. Same as Figs. 1a-1d for [FORMULA], [FORMULA], [FORMULA] and [FORMULA]. The interval of [FORMULA] used for the sample of test particles is [FORMULA].

[FIGURE] Fig. 1e-h and 2e-h. Figs. 1e-1h (on the left). Same as Figs. 1a-1d for [FORMULA],..., [FORMULA]. The interval of [FORMULA] used for the sample of test particles is [FORMULA]. Figs. 2e-2h (on the right). Same as Figs. 1a-1d for [FORMULA],..., [FORMULA]. The interval of [FORMULA] used for the sample of test particles is [FORMULA].

First, we quote that at [FORMULA] there are numerous OCCs in the galactic quadrants (GQs) II and IV having V in the range -40 to 0 km [FORMULA], as well as in GQs I and III in the range 0 to +40 km [FORMULA]. OCCs with [FORMULA] km [FORMULA] are more loosely clumped. Given their low latitudes, the main velocity contributions of most of this sort of OCCs should stem from the galactic differential rotation. The corresponding rotational distances extend up to about 2.7 kpc from the Sun. At [FORMULA] this sort of OCCs is less abundant whilst the lower absolute values of their velocities suggest smaller kinematical distances.

Second, we mention two well-known large-scale features, which contain important amounts of cold HI and are expected to be very prominent in our maps, namely i) the loop-like structures or shells surrounding the Sco-Cen association, (de Geus 1992, cf. also Weaver 1979, and Paper II), and ii) the Ori-Eri bubble, (Brown et al. 1994, 1995).

The Sco-Cen HI-shells are seen in the region [FORMULA] to about [FORMULA], [FORMULA] to [FORMULA]. The main parts are concentrated in the GQ IV. The shells have low velocities and should consist of expanding material swept up by the association from the remnants of the original giant molecular cloud in which the association was formed (de Geus 1988, 1992, Blaauw 1991). We can tentatively identify the cold HI in the Sco-Cen shells in our maps for [FORMULA] to [FORMULA], at least. In these maps there is a very dense and clumpy stripe of OCCs with [FORMULA] km [FORMULA] covering the GQ IV. At [FORMULA] and [FORMULA] the stripe is shorter and presents gaps. At the other latitudes it extends continuously into the GQ III down to about [FORMULA] without any change of the sign of V. At the higher longitudes the stripe extends continuously into the GQ I, at least up to [FORMULA] in most of the maps, exceeding somewhat the limits given in de Geus' maps and merging with other OCCs. There is no change of the sign of V at [FORMULA]. Obviously, the persistance of the sign of V at [FORMULA] and [FORMULA] suggests the presence of systematic peculiar motions .

According to Brown et al. the Ori-Eri bubble could be understood as the product of the winds and supernovae from the Ori OB1 association. The resulting HI-shell is expanding. It extends within the boundaries [FORMULA]- [FORMULA] to [FORMULA], with V in the range -40 to [FORMULA] km [FORMULA]. In our maps for [FORMULA] to [FORMULA] we can tentatively identify the most prominent parts of this HI-shell with a very dense and clumpy stripe of OCCs extending along both sides of [FORMULA]. The stripe has positive velocities [FORMULA] km [FORMULA] as well as extensions with [FORMULA]. It extends down to [FORMULA], where it merges into a stripe of OCCs with mean velocities [FORMULA]. At the higher longitudes the stripe ends abruptly near [FORMULA]-[FORMULA]. At [FORMULA] it covers only the range [FORMULA]-[FORMULA] and has some dense extensions with negative velocities down to [FORMULA] km [FORMULA]. Such extensions exist also at [FORMULA] in the GQ II. Again, [FORMULA] at [FORMULA] and [FORMULA] in the GQ III are indicating systematic peculiar motions .

Third, in the GQ II at [FORMULA] and [FORMULA] we quote the presence of some very prominent clumps of OCCs with [FORMULA], indicating peculiar motions once more. Most of these clumps should be related to the well-known North Celestial Pole Loop (NCPL), which extends along [FORMULA]-[FORMULA], and [FORMULA] down to [FORMULA], at least. The region was observed in radio, IR, optical and soft X-rays. It is a candidate for a collision between high-velocity gas (from the "string" or "chain A", cf. Hulsbosch 1968) and low-velocity gas (cf. Meyerdierks et al. 1991, Meyerdierks 1992 and the references therein).

Fourth, we consider the latitudes [FORMULA] more closely. At [FORMULA] and [FORMULA] the velocities of the OCCs are overwhelmingly negative, the main exception being the NCPL at [FORMULA]. In contrast, at [FORMULA] and [FORMULA] the abundance of OCCs with [FORMULA] is still large. At latitudes [FORMULA] the general predominance of [FORMULA] is well apparent. Nevertheless, there are also some faint OCCs with [FORMULA]. We quote some remarkable asymmetries between both hemispheres:

  • i) A striking scarcity of OCCs along [FORMULA]-[FORMULA], at [FORMULA], and less prominently at [FORMULA]. This corresponds to the large northern HI-hole mentioned in Sect. 1.

  • ii) The presence of some clumps of OCCs with [FORMULA] km [FORMULA] in the range [FORMULA]-[FORMULA] at [FORMULA], and over all l at [FORMULA]. They correspond to the well-known intermediate velocity clouds (IVCs) described by Wesselius and Fejes (1973, cf. Kuntz & Danly 1996, Plate 22). The IVCs overlap partially with the HI-hole. This is consistent with Kulkarni & Fich's (1985) results for the mean HI-profiles observed toward the galactic polar caps. In both cases the profile is centered at negative velocities. The northern profile is highly asymmetric with extensions toward about -60 km [FORMULA], which are not seen in the southern profile.

  • iii) A gap in the distribution of the OCCs with low velocities at [FORMULA]-[FORMULA], for [FORMULA], with extensions up to [FORMULA]. At low latitudes this region overlaps with the elongated cavity of low density found by Frisch & York (1983) from IS line absorption measurements in the UV. Moreover, Ramesh (1994) concluded that at [FORMULA]-[FORMULA] there is a gap in the distribution of the nearby dark clouds extending up to more than 1 kpc.

  • iv) Several smaller gaps in the distribution of the OCCs, like those at [FORMULA], [FORMULA]-[FORMULA], and [FORMULA]-[FORMULA].

Summarizing, in our maps the OCCs offer a variety of well-defined characteristics regarding their distribution and kinematics. At low latitudes many loosely clumped OCCs appear to be mainly in galactic differential motion. On the other hand, very dense clumpy stripes of OCCs, which present peculiar motions, can be fairly related to prominent large-scale objects, such as the Sco-Cen shells, the Ori-Eri bubble and the NCPL. Moreover, at [FORMULA] several galactic north-south asymmetries are apparent in the distribution of the OCCs, as well as an overwhelming predominance of negative velocities.

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© European Southern Observatory (ESO) 2000

Online publication: June 26, 2000