The Andromeda nebula M 31 is the closest spiral galaxy and offers the best
opportunity to explore in detail the distribution of the dust emission and the
major cooling lines in extragalactic photo-dissociation regions. This proposal
will complement on-going ISOCAM observations on a prominent region of star
formation activity located SW in the 10-kpc ring of M 31. Whereas ISOCAM
will probe the mid-IR emission of the smallest dust grains, we propose to
measure the major cooling line of the neutral interstellar medium, the CII line
at 157 microns, by using LWS in the grating mode. The NII line at 122 microns
will also be observed as a check of ionization, and the OI line at one point.
This will provide a detailed picture of the cooling mechanisms. In addition,
we propose to explore a few of the molecular clouds which were found in the
inner regions of M 31 in order to better understand the excitation of CO, the
kinetic temperature and the total mass in those clouds.