Massimo De Luca


Diffuse Interstellar Medium in the Galactic Plane:

New results from the PRISMAS Herschel key program.

Maryvonne Gerin, Harshal Gupta, Carina Persson, David Neufeld, Bhaswati Mookerjea, David Teyssier, A.O.H. Olofsson, J.C. Pearson, J.H. Black, B. Godard


HIFI spectroscopic surveys occupy a fundamental place in our representation of the Milky Way in the Herschel era, thanks to the precious information they give us about the physical and chemical conditions of both dense clouds and diffuse gas. Interesting results are emerging and will likely constitute an important legacy for the incoming ALMA era. Among these are: the discovery of new molecular species, the role played by some molecules as chemical pivots or tracers of physical conditions, the formidable constraints on source modeling given by abundances of several species and transitions.

PRISMAS is a very successful Herschel program aimed at the characterization of the diffuse medium through the observation of absorption features against strong continuum sources represented by high-mass star forming-regions. Interstellar hydrides are at the root of the reaction networks theorized to form gas phase molecules and observing their ground state rotational transitions toward eight Galactic Plane targets gives us the opportunity to sample different lines of sight and to collect, at the same time, information on the chemically-rich background sources.

We will report on some of the newest results of the PRISMAS program, focusing the analysis on tracers of different environments within the diffuse medium, namely:

- the new detection of HCl+, one of the main molecular ions playing a key role in the chlorine chemistry, likely arising from the clouds of small extinction, where the molecular hydrogen fraction is ~1%;

- the methylidyne radical (CH), established as a powerful tracer of molecular hydrogen, showing a correlation with H2 over nearly two decades of column density; and

- the nitrogen hydrides NH, NH2, and NH3, which reside primarily within clouds of high molecular fraction and are fundamental diagnostics of the poorly understood nitrogen chemistry.