PRISMAS: PRobing InterStellar Molecules with Absorption line Studies

Maryvonne Gerin (Observatoire de Paris and Ecole Normale Superiure)

We will carry out a comprehensive spectroscopic study of key molecular line carriers, probing interstellar hydrides and carbon chains. Our investigation will entail high-resolution HIFI spectroscopy of some 25 molecules towards 8 sources, and full spectral scans with PACS. The target hydrides contain the elements H, D, C, N, O, F and Cl. We will take advantage of the strong dust emission from massive star forming regions to detect multiple absorption components from foreground clouds of diverse properties that are known to intersect the selected sight-lines, along with emission and absorption intrinsic to the background sources. Our investigation will provide a wealth of new information about interstellar hydrides -addressing key puzzles posed by previous observations from the ground since the 1940's, and recently with ISO, SWAS, and ODIN- and leaving an important Herschel legacy to astrochemistry and ISM science. We will address the role of high temperature chemical reactions in the formation of interstellar molecules, and the question of how such reactions might be driven. We will also investigate the role of grain surface reactions in interstellar chemistry, and the growth of carbon molecules, bridging the gap between molecules and aggregates, as unique spectroscopic signatures of carbon chains and rings, are accessible in the FIR. Many of the lines that are detectable with Herschel in the local Universe become accessible to ground-based observatories for redshifted sources. Our programme will provide an unique benchmark for the studies of molecular gas at high redshift with ALMA.