Molecular emission in regions of star formation

Abstract

Recent observations show that young stars being formed eject matter at several tens of kilometers per second, in the form of jets and outflows that impact the matter whose collapse is at the origin of the formation of the star. The supersonic impact between this jet and the parent interstellar cloud of the star generates a shock front, in the form of a bow-shock, which propagates in the collapsing interstellar gas, and also an inverse shock that propagates along the jet itself. The structure of these shocks depends on their velocity as well as on the physical properties of the gas in which they propagate. Numerical MagnetoHydroDynamİGal (MHD) simulations of the propagation of such shocks are a way to model the molecular emission arising from these regions, and thus to constrain the physical and chemical properties of the gas in which these molecular lines are emitted. A large grid of shock models is ran, for different values of key parameters such as the shock velocity, the pre-shock density, the magnetic field, ad the shock age. The emission of molecular hydrogen (whose treatment is included inside the shock code) is studied first. Pure rotational and rovibrational excitation diagrams are built for each model, and compared to the available observations of the bipolar outflow L1157. These comparisons confirm the necessity to use non stationary models to be able to interpret the observed column densities of H(_2). The emission of other characteristic molecules in the shocked region is then studied. The radiation transfer is computed thanks to a program based on the LVG (Large Velocity Gradient) approximation. In the case of SiO, comparisons with observed integrated intensities in L1157 are done, independently from the molecular hydrogen results, with a good agreement for stationary shock models and under diverse assumptions regarding the initial repartition of silicon in the dust grains, and oxygen in the gas phase. An attempt to simultaneous fitting of SiO and H(_2) observational data is then done, that is their fit by a very same (non stationary) shock model, with encouraging results. To complete this study, CO emission is treated similarly as SiO, and studied over the whole models grid. CO is then added to the list of molecules whose production and emission can be modelled by the same shock model as H(_2) and SiO with a satisfying agreement, even if this addition does not yield further constrain on the shock and medium properties.