Interferometry occupies a privileged place amongst the high angular resolution techniques. Its angular precision allows one to zoom in on the spectrum of the different parts of a protoplanetary disk, whilst a regular spectrum results from an integration of the whole of the disk. Amongst the ‘astronomical’ fruits of interferometry let us highlight one result in particular: the study of the central section of these disks has enabled adjustments to be made to the modelling of this hitherto little known region: ‘On the basis of these interferometric measurements,’ describes Olivier Absil, ‘it turns out that the radius of this region is greater that the models predicted. We can only explain it by supposing that the gas which fills this region is less opaque than anticipated, and thus it does not form an obstacle to the star’s light (cf: see the diagram opposite). It thus very strongly heats the disk at this frontier. There the disk takes on thickness, forming like a wall which will protect the part of the disk behind it from the star’s direct light. Interferometry thus allows us not only to check out the type of dust in the central region but also to more precisely characterise the protoplanetary disk., which is divided into three regions: a very hot central area, an intermediary one which is cooler, and an exterior one which is once against hot.’

If the large scale study of protoplanetary disks has made giant steps forwards with the advent of high angular resolution techniques, there remain questions which are yet to be answered.  For example: ‘after ten million years, most of these disks have disappeared,’ declares our astronomer. ‘It’s as if they have evaporated. We still do not know through what processes, but it is typically this type of question which could in the future be tackled by high angular resolution techniques as they can see which part of the disk is the first to disappear and thus provide clues as to the physics which hides behind it.’