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

Departamento de Física – ICEx – UFMG, Caixa Postal 702, 30.123-970 Belo Horizote – MG, Brazil e-mail: [wilsonr;wag]@fisica.ufmg.br

Received: 24 May 2007
Accepted: 23 April 2008

Abstract

Context. The Sun is located inside an extremely low density region of quite irregular shape called the Local Bubble or Local Cavity. The fraction of this cavity filled with extremely hot gas is known as the Local Hot 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.

Aims. Our goal is to identify the interface between the Local and Loop I Bubbles and discuss the ring's existence using Strömgren uvbyHβ data.

Methods. We have used the uvbyHβ data of the General Catalogue of Photometric Data, covering the region defined by the Galactic coordinates: 250° ≤ l ≤ 50° and -60° ≤ b ≤ 60° to obtain  colour excess and distances. A set of exclusion criteria have been applied to eliminate the values inappropriate to the study of the interstellar reddening distribution. The final sample is composed of 4346 stars located up to 500 pc from the Sun.

Results. The expected transition to ≈ - , 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 ≥ on the western side at about 100–150 pc and about 120–180 pc on the eastern side. Beyond these distance ranges the number of unreddened stars decreases considerably indicating the location of these large dark cloud complexes. In the southernmost part of the ring the reddening remains very low, typically = 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 ≤ ≤ becomes predominant only after d = 120 ± 15 pc.

Conclusions. 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.