We propose to study the transition between the region of enhanced
 star formation and the more quiescent disk in the prototypical
 starburst galaxy M82, by mapping a number of diagnostic emission
 lines along the major axis. This will provide a characterization
 of the "super photon-dominated region" that likely marks the
 transition from the central starburst to the molecular ring and
 undisturbed parts of M82.

 M82 represents the unique case of a starburst galaxy suitable
 for such a spatially resolved study with ISO.

 We have chosen diagnostic lines, mainly from the 4-12 micron
 wavelength range offering best spatial resolution, that will
 allow to
 1) Determine the excitation of the ionized medium by observation
    of lines of [ArII] and [ArIII].
 2) Study emission of several transitions of warm molecular
    hydrogen, constraining its temperature, and of [SiII] from
    photon-dominated regions.
 3) Determine the temperature and density character of the
    [FeII] emitting medium, to which HII regions, photon-
    dominated regions and supernova remnant shocks possibly
    contribute significantly.

 During phase 2 proposal entry, the spatial sampling had to be decreased
 to meet the allocated time. If overheads decrease in the future, this should
 be restored at least partly to fully achieve the scientific goal.