High-latitude molecular clouds and near-by OB associations

The Galactic disribution of the high-latitude molecular clouds is considered. It is suggested that the majority of these clouds are clustered in two large shells around the two closest OB associations, Per OB3/Cas-Tau and Sco OB2. The most prominent shell of high-latitude clouds is centred around the Per OB3/ Cas-Tau association which is also at the centre of the Gould’s Belt. However, the Per OB3/Cas-Tau group of high-latitude clouds appears as an elliptical shell at nearly right angles to the plane of the Gould’s Belt. Its kinematic age (∼ 10 Myr) is much smaller than the expansion age (∼ 35 Myr) of the ring of intertellar matter associated with the Gould’s Belt. It is suggested that while, as is generally understood, the larger expanding ring of gas associated with the Gould’s Belt was created by stellar winds and supernova explosions of the more massive OB stars of the Per OB3/Cas-Tau association ∼ 35 Myr ago, the Per OB3 shell of high-latitude clouds was formed from the back-falling gas as it was swept up by a more recent supernova explosion of an early B type star in Per OB3 ∼ 10 Myr ago. More distant OB associations also produce similar shells of clouds at large heights from the Galactic plane having relatively smaller angular sizes due to their greater distances. These can be seen as higher latitude extensions of cloud complexes at lower Galactic latitudes as, for example, in the case of Vela OB2.