Solving the excitation and chemical abundances in shocks: the case of HH1

Abstract

We present deep spectroscopic (3600 - 24700 A) X-shooter observations of the bright Herbig-Haro object HH1, one of the best laboratories to study the chemical and physical modifications caused by protostellar shocks on the natal cloud. We observe atomic fine structure lines, HI, and He, recombination lines and H_2, ro-vibrational lines (more than 500 detections in total). Line emission was analyzed by means of Non Local Thermal Equilibiurm codes to derive the electron temperature and density, and, for the first time, we are able to accurately probe different physical regimes behind a dissociative shock. We find a temperature stratification in the range 4000 - 80000 K, and a significant correlation between temperature and ionization energy. Two density regimes are identified for the ionized gas, a more tenuous, spatially broad component (density about 10^3 cm^-3), and a more compact component (density > 10^5 cm^-3) likely associated with the hottest gas. A further neutral component is also evidenced, having temperature lass than 10000 K and density > 10^4 cm^-3. The gas fractional ionization was estimated solving the ionization equilibrium equations of atoms detected in different ionization stages. We find that neutral and fully ionized regions co-exist inside the shock. Also, indications in favor of at least partially dissociative shock as the main mechanism for molecular excitation are derived. Chemical abundances are estimated for the majority of the detected species. On average, abundances of non-refractory/refractory elements are lower than solar of about 0.15/0.5 dex. This testifies the presence of dust inside the medium, with a depletion factor of Iron of about 40%.