Loop I Bubble

Abstract: Recent pulsar scintillation measurements from Ooty, in conjunction with those from Parkes and other large radio telescopes, are used for a systematic investigation of the Local Interstellar Medium (LISM) towards the general direction of the Loop I Bubble. For several pulsars, clear evidence is found for an enhanced level of scattering which is over and above what can be accounted for by the enhanced scattering model for the Local Bubble. These results are interpreted in terms of enhanced scattering due to turbulent plasma associated with the Loop I shell. Useful constraints are obtained for the scattering properties of the shell. The inferred value for the scattering measure for the Loop I shell is found to be ~0.3 pc m^{-20/3}. Assuming a shell thickness ~5-10 pc, this implies an average strength of scattering in the shell that is ~100-200 times larger than that in the ambient ISM. An alternative explanation, where the enhanced level of scattering is due to a possible "interaction zone" between the Local Bubble and the Loop I Bubble, is also considered; it is found to be somewhat less satisfactory in explaining the observations. The best fit value for the scattering measure for such an interaction zone region is estimated to be ~1.1 pc m^{-20/3}. Further, several pulsars beyond ~1 kpc are found to show enhanced levels of scattering over and above that expected from this "two-bubble model." For some of the low-latitude pulsars, this is found to be due to enhanced scattering from plasma inside the intervening Sagittarius spiral arm. We discuss the implications of our results for the interpretation of scintillation data and for the general understanding of the LISM.

Abstract: High-resolution observations of interstellar Na I towards the relatively nearby (180 pc) and lightly reddened, [E(B−V)=0.12] B8V star HD 174632 are presented. There are two distinct interstellar cloud components in this line-of-sight, with heliocentric velocities of about −4.4 and −17.6 km s−1. A third, much weaker, component at about −21.7 km s−1 also appears to be present. Na I column densities for these components have been obtained by line-profile fitting, and estimates have been made of the N(Na I)/N(H) ratios. These are found to be consistent with standard diffuse interstellar clouds. Arguments are presented that these clouds are associated with the boundary between the local low-density interstellar bubble and the neighboring Loop I bubble, at a distance between about 50 and 100 pc from the Sun.

Abstract: The present study of the local insterstellar medium toward the center of Loop I analyzes optical and UV spectra of nearby stars within the l = 310-330 deg, b = 15-25 deg area. The analysis of the UV spectra reveals a sudden gas-column density increase as a function of distance from 10 to the 18th to 10 to the 20th/sq cm, suggesting the presence of a wall of gas 40 + or {minus} 25 pc from the sun; this constrains the distance to the boundary of the soft X-ray emitting cavity around the sun, or 'Local Bubble', in the direction toward the Loop I center. At greater distances, the approximately constant N(H I), in conjunction with the lack of interstellar components in the optical spectra, are suggestive of the presence of another low-density gas column which can be identified with the Loop I Bubble. 51 refs.

Abstract: Recent 3D high-resolution simulations of the interstellar medium in a star forming galaxy like the Milky Way show that supernova explosions are the main driver of the structure and evolution of the gas. Its physical state is largely controlled by turbulence due to the high Reynolds numbers of the average flows. For a constant supernova rate a dynamical equilibrium is established within 200 Myr of simulation as a consequence of the setup of a galactic fountain. The resulting interstellar medium reveals a typical density/pressure pattern, i.e. distribution of so-called gas phases, on scales of 500–700 pc, with interstellar bubbles being a common phenomenon just like the Local Bubble and the Loop I superbubble, which are assumed to be interacting. However, modeling the Local Bubble is special, because it is driven by a moving group, passing through its volume, as it is inferred from the analysis of Hipparcos data. A detailed analysis reveals that between 14 and 19 supernovae have exploded during the last 15 Myr. The age of the Local Bubble is derived from comparison with Hi and UV absorption line data to be 14.5± 0.4 0.7 Myr. We further predict the merging of the two bubbles in about 3 Myr from now, when the interaction shell starts to fragment. The Local Cloud and its companion Hi clouds are the consequence of a dynamical instability in the interaction shell between the Local and the Loop I bubble.