1. INTERSTELLAR EXTINCTION TOWARDS THE INTERACTION ZONE BETWEEN THE LOCAL AND THE LOOP I BUBBLES
Wagner Corradi, Marcelo Guimarães and Sérgio Vieira
Departamento de Física/UFMG - Brazil
The various observacionational data on the Local Interstellar Medium suggest that the Sun is located inside a low-density (nHI < 0.1 cm-3), warm (T8000 K) and partially ionized cloudlet, whose diameter ranges from 2 to 30 pc. Surrounding this cloudlet, there is an irregularly shaped region of very low density (nHI < cm-3), filled with hot interstellar gas (T106 K) that is know to coexist with neutral and molecular clouds inside its interior. Usually termed the Local (Hot) Bubble, this region may have been created by the explosion of one or more supernova explosion near the Sun some million years ago. Its radius varies from 30 to 300 pc, depending of the direction under consideration (Cox & Reynolds 1987).

2. Towards the line-of-sight to the Scorpio-Centaurus OB Association (Sco-Cen) there is an even larger cavity, called Loop I. Its density is also very low (nHI < cm-3) and is filled with gas whose temperature is about 106 K. Loop I is believed to have been formed from the subsequent supernova explosions of the OB stars of the Sco-Cen association, that acting on the insterstellar matter left over of the star formation, left an interstellar bubble concentric to Sco-Cen (Iwan 1980).
Fig.1 (a) Schematic view of the two bubbles, according to the Iwan (1980) model
Fig.1 (b) side view of the interaction zone of the two bubbles

3. Given its proximity these two bubbles are believed to interact with each other. According to the simulation of expanding plasma (Yoshioka & Ikeuchi 1990), if one of the two bubbles has reached the radiative stage prior to the collision, the two interiors should not merge. Intead, at the region of significant compression between them, it should appear a thin dense wall, with an thick neutral annular ring-like feature around it.
Observational support for the existence of such ring-like feature has been found on ROSAT data by Egger and Aschenbach (1995, EA95). They have discovered a soft X-ray shadow on the edges of Loop I bubble, cast by a warped annular volume of dense neutral matter, that supposedly formed during the collision of the bubbles.
There is, however, a controversy on the distance to the ring. According to EA95 , the column density of the HI shadow counterpart, in some directions, jumps from less than to 7 x cm-2, at a distance of 70 pc from the Sun. But, other studies (Corradi et al. 1997, Franco 1990) suggest that the interaction zone is located, instead, at pc from the Sun.
Fig.2 Face-on view (as seen from the Sun) of the annular ring-like feature with the nearby dark clouds that are seen towards the interaction zone between the two bubbles.

4. In this case, one might think that the interstellar material has been compressed from both sides by the expanding bubbles. From the far side, by the action of the stellar wind of the OB stars of the Sco-Cen, and from the near side, by the energetic events that created the local low density region.
Corradi et al. (1997,1998) have shown that the Southern Coalsack and the Chamaeleon -Musca dark cloud complex (SCCM) are physically associated, being part of a large complex of clouds about 150  30 pc from the Sun. They also suggest that this complex might have its origin related to the interface between the Local and Loop I Bubbles.
Although being apart by more than 50° in the sky, there are other nearby dark clouds, comprising, e.g.  Oph, Lupus, R CrA, and Serpens, that are located almost at the same mean distance of 150 pc from the Sun that seemingly define a wall of molecular interstellar material (Dame et al. 1987).
Fig.3 Molecular clouds distribution seen from above the Galactic Plane

5. The main goal of this work is to investigate the distribution of the interstellar reddening caused by the presence of the interface, aiming to establish the distance to this large scale feature. As well we intend to look for a connection between the nearby molecular clouds and the interface of the Local and Loop I bubbles in the mentioned direction.
Using the Strömgren uvby and H data of the General Catalog of Photometric Data (GCPD) compiled by Mermilliod et al. (1997) and Hauck et al. (1998) we have selected more than stars, within 500 pc from the Sun, covering the region defined by the Galactic coordinates 60° > l > 250° and -60° < b < 60°.
Intrinsic colours, E(b-y) colour excess and distances have been computed to all the stars. When available Hipparcos distances were used.
The most important step is the application of a set of selection criteria to exclude the stars whose colour excess and distances are not properly adequate to this study of the interstellar reddening distribution. Further details of the used method can be found in Corradi et al. (1997).
Fig.4 The sampled region. The color code can be seen on the next figure.

6. After the analysis of the various colour excess vs
After the analysis of the various colour excess vs. distance diagrams we have plotted the stars according to their position in the sky, and divideb by intervals of colour excess and distances. On can see that:
 Up to 60 pc from the Sun the colour excess is very low E(b-y) < 0m.017, which corresponds to NHI = 1020 cm-2, indicating the local cavity is deficient in interstellar matter.
Fig.5 -0m.050 < E(b-y) < 0m.017
0m.017 < E(b-y) < 0m.034
0m.034 < E(b-y) < 0m.050
Spectral Type Distance interval Spectral Type Distance interval
AF B AF B
+ * < d (pc) < * < d (pc) < 150
+ * < d (pc) < * < d (pc) < 200
+ * < d (pc) < 250
Colour code

7. while the distances follow the caption below the figures.
In Fig. 6, the maps are ordered by colour excess values in the following way:
(a) -0m.050 < E(b-y) < 0m (b) 0m.017 < E(b-y) < 0m (c) 0m.034 < E(b-y) < 0m.050
while the distances follow the caption below the figures.
 Between 60 < d (pc) < 90, as can be seen in the first column of Figure 6, the colour excess is still very low, but observing Figures 7 and 8 we will be able to find the first signs of the northwest side of the interface.
 The second column in Figure 6 shows that between 90 < d (pc) < 120 there is a transition of the colour excesses from 0m.034 < E(b-y) < 0m.050 (from NHI = 4 x 1020 to 9 x 1020 cm-2) indicating that the northwest side of the interface is being crossed.
 Looking at the figure with the distance interval 120 < d (pc) < 150 one can see that the nortwest side of the interface has been completely crossed, since the mean colour excess grows from less than E(b-y) = 0m.050 to E(b-y) = 0m.120. Some objects even show very large colour excess values  E(b-y) > 0m.120  as indicated by the third column of Fig. 6 and Figures 7 and 8.
 The southern parts of the interface only show up about 120 < d (pc) < 150.

8. Fig.6 a b c 60 < d (pc) < 90 90 < d (pc) < 120

9.  Beyond 150 pc we can see the extinction caused by the molecular clouds  fourth colum of Figure 6  and by the higher values of colour excess  Figures 7 e 8.
 Around 180 pc the southeast side of the interface shows up, since the mean colour excess jumps to intermediate values of 0m.034 < E(b-y) < 0m.050, although being still rather low in the northeast line-of-sight.
 Only beyond 180 pc, the mean colour excess of the northeast side of the interface jumps to 0m.034 < E(b-y) < 0m.050, showing a definite transition.
180 < d (pc) < 210

10. Fig.7 0m.050 < E(b-y) < 0m.120 Fig.8 E(b-y) > 0m.120
60 < d (pc) < 90
90 < d (pc) < 120
60 < d (pc) < 90
90 < d (pc) < 120
120 < d (pc) < 150
150 < d (pc) < 180
120 < d (pc) < 150
150 < d (pc) < 180
180 < d (pc) < 210
180 < d (pc) < 210

11. Stars with low polarization, P < 0.1% Polarized Stars, P > 0.1%
0 < d (pc) < 60
60 < d (pc) < 90
90 < d (pc) < 120
Fig.9

12. Our findings on the interstellar reddening distribution are consistent with the results by Leroy (1999) that used polarimetric data to study the Local Bubble. Figure 9 show the polarization P(%) of the data covering the region of the interface. As one can see the transition to higher polarization values occurs at distances similar to ours.
Summing up, our results suggest that the annular volume, in fact, is twisted and folded, with different directions having different distances. The remarkable feature is that the western side is nearer than the eastern side, consistent with the fact that we are not located at right angles with the direction to the center of the Loop I bubble.
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