2012A&A...545A..21P

A detailed three-dimensional (3D) distribution of interstellar matter in the solar neighborhood is increasingly necessary. It would allow a more realistic computation of photons and cosmic rays propagation, which is of prime importance for microwave and gamma-ray background emission interpretation, as well as a better understanding of the chain of phenomena that gave rise to the main local structures. As part of a 3D mapping program, we aim at assigning a precise distance to the high-latitude HI gas, in particular the northern part (b≥55°) of the shell associated with the conspicuous radio continuum Loop I. This shell is thought to be the expanding boundary of an interstellar bubble inflated and recently reheated by the strong stellar winds of the nearby Scorpius-Centaurus OB associations. We recorded high-resolution spectra of 30 A-type target stars located at various distances in the direction of the northern part of Loop I. Interstellar neutral sodium and singly ionized calcium absorptions (NaI 5889-5895 and CaII K-H 3934-3968 Å) are modeled and compared with the HI emission spectra from the LAB Survey. About two-thirds of our stellar spectra possess narrow interstellar lines, while the remaining spectra show no absorption at all. Narrow lines are located at the velocity of the main, low-velocity Loop 1 HI shell ([-6, +1]km/s in the LSR). Using Hipparcos distances to the target stars, we show that the closest boundary of the b≥+70° part of this low-velocity Loop I arch is located at of 98 ±6 pc. The corresponding interval for the lower-latitude part (55≤b≤70°) is 95-157pc. However, since the two structures are apparently connected, the lower limit is more likely. At variance with this shell, the second HI structure (b≥+85°), which is characterized by LSR Doppler velocities centered at -30km/s, is NOT detected in any of the optical spectra. It is located beyond 200 parsecs or totally depleted in NaI and CaII. We discuss these results in the light of spherical expanding shells and show that they are difficult to reconcile with simple geometries and a nearby shell center close to the plane. Instead, this high-latitude gas seems to extend the inclined local chimney wall to high distances from the plane.