J. Lequeux – författare

In these lectures, I have discussed a number of basic concepts that provide the necessary background to the current studies of star formation. A ?rst partwas dedicatedto illustrate the conceptofa protostar,discussing con- tions and propertiesof the collapseof a molecular core. A secondpart deals with circumstellardisks. Disks areimportantnot only to the processofstar formation itself, but also because they are in all probability the site where planets form. The age range of pre-main-sequence stars coincides with the timescales for the formation of very large planetesimals, the building blocks of planets. Studies ofdisk properties in pre-main-sequencestars ofdi?erent age, located in star-forming regions of di?erent properties, may shed light on the characteristics of planet formation processes. ISO observations can provide important (in some cases, unique) inf- mation on the various stages of the star and planet formation. I have illustrated in detail some examples, when, to my knowledge, ISO data had been reduced and analyzed. Many other programs exist, and will certainly contribute to our understanding of star formation in the near future.

In these lectures, I have discussed a number of basic concepts that provide the necessary background to the current studies of star formation. A ?rst partwas dedicatedto illustrate the conceptofa protostar,discussing con- tions and propertiesof the collapseof a molecular core. A secondpart deals with circumstellardisks. Disks areimportantnot only to the processofstar formation itself, but also because they are in all probability the site where planets form. The age range of pre-main-sequence stars coincides with the timescales for the formation of very large planetesimals, the building blocks of planets. Studies ofdisk properties in pre-main-sequencestars ofdi?erent age, located in star-forming regions of di?erent properties, may shed light on the characteristics of planet formation processes. ISO observations can provide important (in some cases, unique) inf- mation on the various stages of the star and planet formation. I have illustrated in detail some examples, when, to my knowledge, ISO data had been reduced and analyzed. Many other programs exist, and will certainly contribute to our understanding of star formation in the near future.

The study of the interstellar medium is certainly one of the most recently developed astrophysical topics. Indeed, to understand the physical status of this medium, one should observe it at every wavelength, especially in radio and in the ranges which are only accessible from space such as Infra Red, X and gammas. Thanks to many new observations we now know that interstellar gas is quite inhomogeneous, since it is made from relatively cold and dense phases (interstellar clouds), surrounded by a relatively hot and tenuous medium. The summer school which took place from September 1 to 16 at the Institut d''Etudes Scientifiques of Cargese, and from which this book originates, was devoted to the analysis of this second diffuse, hot and tenuous component. The eleven sets of lectures which were presented at this school constitute the chapters of this book. They review different aspects (observations, determination of the physical properties and evolution) of this diffuse medium. In the first chapter, J. Lequeux (Marseille Observatory) introduces the study of the diffuse interstellar medium which plays a major role in galactic evolution. His contribution provides some historical references on this subject, such as a copious bibliography on the analysis and observations of the interstellar medium.