2009ApJ...692....1D

We present Spitzer-IRS spectra obtained along the molecular jet from the Class 0 source L1448-C (or L1448-mm). Atomic lines from the fundamental transitions of [Fe II], [Si II], and [S I] have been detected showing, for the first time, the presence of an embedded atomic jet at low excitation. Pure rotational H₂ lines are also detected, and a decrease of the atomic/molecular emission ratio is observed within 1' from the driving source. Additional ground-based spectra (UK Infrared Telescope/UIST) were obtained to further constrain the H₂ excitation along the jet axis and, combined with the 0-0 lines, have been compared with bow shock models. From the different line ratios, we find that the atomic gas is characterized by an electron density n_e∼ 200-1000/cm3, a temperature T_e< 2500 K, and an ionization fraction ≲ 10^–2; the excitation conditions of the atomic jet are thus very different from those found in more evolved Class I and Class II jets. We also infer that only a fraction (0.05-0.2) of Fe and Si is in gaseous form, indicating that dust still plays a major role in the depletion of refractory elements. A comparison with the SiO abundance recently derived in the jet from an analysis of several SiO submillimeter transitions shows that the Si/SiO abundance ratio is ∼ 100, and thus, that most of the silicon released from grains by sputtering and grain-grain collisions remains in atomic form. Finally, estimates of the atomic and molecular mass flux rates have been derived: values of the order of ∼ 10^–6 and ∼ 10^–7 M_☉/yr are inferred from the [S I] 25 µm and H₂ line luminosities, respectively. A comparison with the momentum flux of the CO molecular outflow suggests that the detected atomic jet has the power to drive the large-scale outflow.