Emeric Bron (Laboratoire d’Etudes du Rayonnement et de la Matière en Astrophysique et Atmosphères, Observatoire de Paris)

Vendredi 28 février à 14h (salle Magat) :

"Learning from model grids : what are the best tracers of the ionization fraction in the ISM"

Résumé

The ionization fraction controls several key processes of the ISM : fast ion neutral reactions allowing a rich interstellar chemistry, gas coupling to the magnetic field controlling the collapse of prestellar cores, collisional excitation of key molecular tracers (such as HCN). Determining the ionization fraction across the different regions of a giant molecular cloud (GMC), from its diffuse enveloppe to its densest cores, is thus a mandatory step towards understanding the structure of GMCs, and its link to star formation. Our ORION-B IRAM-30m large program (PIs : M. Gerin, J. Pety) provides an exhaustive map of a GMC in the full 100GHz band, containing tens of molecular lines. However, classical tracers of the ionization fraction, such as the DCO+/HCO+ ratio, are only detectable in the densest cores.

We propose a statistical approach, based on Random Forest regression models, exploiting large grids of astrochemical models to automatically find the best observable tracers of the ionization fraction among hundreds of species included in the model. We find several new tracers of the ionization fraction, with strong predictive power despite large uncertainties in the other unobservable physical parameters (H2 OPR, depletion...) and detectable in the extended enveloppe of the cloud.