FIR LINE OBSERVATIONS OF PHOTON DOMINATED REGIONS AROUND COOL STARS

we propose to observe the farinfrared cooling lines of cii (158 mu), oi (63 and 145 mu), h2 s(1) (17 mu), s(2) (12 mu), and si ii (35 mu) in photondominated regions (pdrs) associated with reflection nebulae exposed to radiation from cool stars with t_eff5,00022,000 k. compared with classical pdrs illuminated by early o and b stars, these clouds close to a and f stars receive much less farultraviolet radiation. since the incident radiation field controls both the heating and the formation and destruction of atomic and molecular species, the observations will allow stringent tests of pdr models in a regime that has so far been unexplored. in particular, the relative contribution of the various types of grains (including polycyclic aromatic hydrocarbons (pahs)) to the heating can be investigated since the absorption cross section of pahs has a different wavelength dependence from that of large grains. the dominant cooling lines of pdrs occur in the farinfrared and hence cannot be observed from the ground. in general, the expected line strengths for the reflection nebulae in our sample are 12 orders of magnitude less than those observed in these classical pdrs. hence, the high sensitivity of iso is essential for this study. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]