The far infrared line spectrum of the protostar IRAS 16293-2422

Abstract

We report mid-IR wavelength observations toward the low mass star forming region IRAS 16293-2422 between 45microns - 197microns with the Long Wavelength Spectrometer (LWS) on board ISO, and of the CI(609microns ) line observed with the JCMT. A map of the CII(157microns ) line shows that the region is relatively uncontaminated by Photo-Dissociation Region-like emission; there is only weak diffuse CII emission, which results from the illumination of the cloud by a faint UV field (G_o ~ 6). The observed CI(609microns ) line intensity and narrow profile is consistent with this interpretation. On-source, the LWS detected the OI(63microns ) and several molecular lines. In this work we report and discuss in detail the lines which dominate the 43microns - 197microns spectrum, namely CO, H_2O and OH rotational lines and the OI(63microns ) fine-structure line. Combining the CO J_up=3D14 to 25 observations with previous J_up=3D6 measurements, we derive stringent limits on the density ( ~ 3 * 10(4) cm(-3) ), temperature ( ~ 1500 K), and column density ( ~ 1.5 * 10(20) cm(-2) ) of the emitting gas. We show that this warm gas is associated with the outflow and that a low velocity, C-type shock can account for the characteristics of the CO spectrum. If the observed H_2O and OH lines originate in the same region where the CO lines originate, the H_2O and OH abundance derived from the observed lines is [H_2O] / [H_2] ~ 2 * 10(-5) and [OH] / [H_2] ~ 5 * 10(-6) respectively. Given the relatively high temperature of the emitting gas, standard chemistry would predict all the gas-phase oxygen to be in water. The relatively low water abundance we observed may mean either that most of the oxygen is locked into grains or that the time scale required to convert the gas-phase oxygen into water is higher that the outflow time scale, or both. The relatively high abundance of OH with respect to H_2O gives support to the latter hypothesis. Finally, we speculate that the OI(63microns ) line emission originates in the collapsing envelope that surrounds the central object. The successful comparison of the observed flux with model predictions of collapsing envelopes gives a mass accretion rate toward the central object >= 3 * 10(-5) M_sun yr(-1) and an accretion shock radius larger than three times the protostar radius. 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.