Polarização interestelar na região de interação entre as Bolhas Local e Loop I

Abstract

The interstellar medium in the proximity of the Sun, between 65 and 250 pc, is composed by a quite irregular, low density cavity (nHI < 0:005 cm..3), known as the Local Bubble. The fraction of this cavity filled with high temperature gas (up to 106 K) is known as the Hot Local Bubble. In the direction of the Galactic Center there is a nearby superbubble, known as Loop I, which was sculpted by the action of stellar winds and supernovae explosions from the OB stellar association Scorpio- Centaurus (at a distance of ~ 170 pc). Considering that the distance to Loop I is close to the size of the Local Bubble, it is possible that both bubbles may be interacting. According to this view, a wall of neutral and dense material would be formed, surrounded by a ring shaped feature at the interaction zone. From the HI column density and X-ray absorption analysis, evidence of such annular structure were obtained, at a distance of about 70 pc. The more recent analysis of the color excess E(b .. y) distribution as a function of distance along the interaction zone has shown, however, that the ring is highly fragmented and distorted, with very diferent distances between the western (90 pc) and eastern ( 280 pc) portions. The diferent properties of the interstellar color excess between the annular region and the interaction wall do not support the existence of a unique large-scale feature. To confirm these evidences, we used the IAG 60cm and PE 1:60m telescopes from Laboratorio Nacional de Astrofíica" (LNA, Brazil), equipped with the IAGPol imaging polarimeter to collect V filter data to =~ 900 stars from the Hipparcos catalogue. From these data, we have obtained the degree and position angle of polarization of these objects, which, taken together with the literature data, allowed us to investigate the distance and the extension of the proposed annular region. Through the study of the polarization asa function of distance we were able to map in a consistent way the dust distribution and the geometry of the Galactic magnetic eld in these directions. Our results conform the indications from the photometric analysis that the distances to the different parts of the annular structure are very discrepant, suggesting a highly irregular nature. The polarization vectors' orientation along the ring doesn't display the expected pattern of a single large-scale annular structure. These evidences are a strongargument against the existence of the interaction ring between the bubbles, suggesting distinct formation histories among its different parts. Thus, it seems more plausible a scenario in which the Local Bubble would be a portion of the Loop I superbubble, which would have been created by different successive shock fronts associated to different epochs of star formation in the Scorpio-Centaurus association. During this work, we have also collected polarimetric data at the UBVRI filters to study the spectral dependence of the polarization towards three dark clouds situated in the direction of the supposed interaction zone. The preliminary analysis for R CrA shows that the polarization vectors' direction is approximately parallel to the filament associated to the cloud. We detected the rotation of polarization angle with y-1 for a star located in the line-of-sight of the denser part of the cloud, suggesting the presence of sub-condensations in this direction. xi For the Southern Coalsack, the study of nine objects located behind the cloud indicated that most of them do not show the rotation of polarization angle with ..1, suggesting the presence of a single sheet structure. The max values concentrate mostly in the 0:58 .. 0:62m band. The study of a diuse interstellar structure in the direction of the Mensa constellation showed that the polarization vectors are located approximately perpendicular to thedirection of the filament. We found a distance of ~ 200 pc to this object, consistent with previous photometric results.