  The determination of the physical conditions in molecular clouds is a key
step towards our understanding of their formation and evolution of associated
star formation. We investigate the density, temperature, and column density of
both dust and gas in the photodissociation regions (PDRs) located at the
interface between the atomic and cold molecular gas of the NGC 7023 reflection
nebula. We study how young stars affect the gas and dust in their environment.
Our approach combining both dust and gas delivers strong constraints on the
physical conditions of the PDRs. We find dense and warm molecular gas of high
column density in the PDRs.
