Far infrared mapping of the gas cooling along the L1448 outflow

Abstract

The molecular outflows associated with the two Class 0 sources L1448-mm and L1448-IRS3 have been mapped with the spectrometers on board the ISO satellite allowing to study in detail the physical and chemical structure of the shocked gas. The far infrared cooling is mainly due to the emission from pure rotational lines of CO, H₂O and H₂ excited at temperatures between 500 and 1200 K. [O I]63μm emission is also widespreadly observed along the flows. Additional ground based observations of the ¹²CO 4-3 and 3-2 transitions in the surroundings of the L1448-mm source allow us to localize this warm emission in the extreme high velocity clumps forming the collimated molecular jet responsible for the entrainment of the outflow. Our analysis shows therefore that this jet is hotter than previously thought on the basis of millimeter observations alone. A comparison with existing models suggests that the excitation along the outflow from L1448-mm is mainly due to low velocity (V_s <= 20km s^-1) non-dissociative shocks (C-shocks) probably developed as the jet proceeds through a medium already put into motion by previous episodes of mass loss. Excitation from turbulent mixing layers along the molecular jet axis seems not able to explain the observed cooling ratios among the different molecular components. An higher excitation shock component is likely present in the direction of the source L1448-IRS3, as testified by the detection of the [Si II] 35μm line and by a larger contribution of the [O I] emission. Finally the abundance of gas-phase H₂O is largely enhanced with respect to its interstellar value all along the flow. Both the total luminosity of water and its abundance correlate with SiO at high excitation, implying that both H₂O and SiO are released in the low-velocity shocks developed along the outflow. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of ISAS and NASA