2013A&A...549A..16N

Water is a key probe of shocks and outflows from young stars because it is extremely sensitive to both the physical conditions associated with the interaction of supersonic outflows with the ambient medium and the chemical processes at play. Our goal is to investigate the spatial and velocity distribution of H₂O along outflows, its relationship with other tracers, and its abundance variations. In particular, this study focuses on the outflow driven by the low-mass protostar L1448-C, which previous observations have shown to be one of the brightest H₂O emitters among the class 0 outflows. To this end, maps of the o-H₂O 1₁₀-1₀₁ and 2₁₂-1₀₁ transitions taken with the Herschel-HIFI and PACS instruments, respectively, are presented. For comparison, complementary maps of the CO(3-2) and SiO(8-7) transitions, obtained at the JCMT, and the H₂ S(0) and S(1) transitions, taken from the literature, were used as well. Physical conditions and H₂O column densities were inferred using large velocity gradient radiative transfer calculations. The water distribution appears to be clumpy, with individual peaks corresponding to shock spots along the outflow. The bulk of the 557GHz line is confined to radial velocities in the range ±10-50km/s, but extended emission at extreme velocities (up to v_r∼80km/s) is detected and is associated with the L1448-C extreme high-velocity (EHV) jet. The H₂O 1₁₀-1₀₁/CO(3-2) ratio shows strong variations as a function of velocity that likely reflect different and changing physical conditions in the gas that is responsible for the emissions from the two species. In the EHV jet, a low H₂O/SiO abundance ratio is inferred, which could indicate molecular formation from dust-free gas directly ejected from the proto-stellar wind. The ratio between the two observed H₂O lines and the comparison with H₂ indicate averaged T_kin and n(H₂) values of ∼300-500K and 5x10⁶cm³, respectively, while a water abundance with respect to H₂ of about 0.5-1x10^–6 along the outflow is estimated, in agreement with results found by previous studies. The fairly constant conditions found all along the outflow imply that evolutionary effects on the timescales of outflow propagation do not play a major role in the H₂O chemistry. The results of our analysis show that the bulk of the observed H₂O lines comes from post-shocked regions where the gas, after being heated to high temperatures, has already been cooled down to a few hundred K. The relatively low derived abundances, however, call for some mechanism that diminishes the H₂O gas in the post-shock region. Among the possible scenarios, we favor H₂O photodissociation, which requires the superposition of a low-velocity nondissociative shock with a fast dissociative shock able to produce a far-ultraviolet field of sufficient strength.