Mapping the interface between the Local and Loop I bubbles using Stromgren photometry

Abstract

The Sun is located inside an extremely low density region of quite irregular shape called the Local Bubble. Close to the Local Bubble, there is an even larger cavity known as Loop I. A ring-like feature observed in X-ray and HI has been proposed as the contour of the bubbles interaction zone around 70 pc. Our goal is to identify the interface between the Local and Loop I Bubbles and discuss the ring's existence using Stromgren uvbyHβdata. We have used the uvbyHβdata of the General Catalogue of Photometric Data, covering the region defined by the Galactic coordinates: 250\degr <= l <= 50\degr and -60\degr <= b <= 60\degr to obtain E(b-y) colour excess and distances. The final sample is composed of 4346 stars located up to 500 pc from the Sun. The expected transition to E(b-y) ~ 0.070 - 0.100 mag, corresponding to the ring's column density, occurs on the western part of the ring at d = 110 +/- 20 pc, whereas on the eastern side it is not clearly seen before d = 280 +/- 50 pc. Near the Galactic plane the presence of the dark clouds is clearly established by E(b-y) >= 0.100 mag on the western side at about 100 - 150 pc and about 120 - 180 pc on the eastern side. In the southernmost part of the ring the reddening remains very low, typically E(b-y) = 0.020 in all its extension, except towards the Mensa constellation where a possible transition is observed at d = 200 +/- 20 pc. In the northernmost part the colour excess increases with distance in a gradual way, such that 0.020 <= E(b-y) <= 0.040 mag becomes predominant only after d=120 +/-15 pc. If the ring really exists the colour excess distribution indicates that it is very fragmented and distorted. However, the different characteristics of the reddening inside and along the ring do not support the existence of a ring.