Abstract

The formation of stars is inextricably linked to the properties of their parental clouds. It is still not entirely understood what causes different modes of star formation (clustered or isolated) in giant molecular clouds. For example, are the turbulent properties the determinant factor or are feedback mechanisms decisive? Here we will give a brief overview of possible observational techniques (molecular line observations, dust continuum emission, scattered infrared light, extinction mapping techniques) to study the structure and properties of giant molecular clouds and will brie°y discuss their advantages and disadvantages. We will then concentrate on our recent efforts to determine the first ever all-sky extinction maps based on near infrared excess of star light. First results of our investigation of the structure of all giant molecular clouds in the solar neighbourhood show that there are significant differences in the column density distribution between clouds in the low AV regime at a spatial scale of 0.1 pc. At higher extinction values, dominated by material most likely involved in ongoing star formation, the column density distributions are very similar. We also find that star formation has a typical threshold of 4-8 mag of optical extinction and that the overall star formation efficiency of giant molecular clouds is in the order of a few tenths to a few percent.