Evolution of the atomic component in protostellar outflows

Abstract

Context. We present SOFIA/FIFI-LS observations of three Class 0 and one Class I outflows (Cep E, HH 1, HH 212, and L1551 IRS5) in the far-infrared [O I]63 μm and [O I]145 μm transitions. Spectroscopic [O I]63 μm maps enabled us to infer the spatial extent of warm (T ∼ 500−1200 K), low-excitation atomic gas within these protostellar outflows. Aims. Our main goal is to determine mass-loss rates from the obtained [OI]63 μm maps and compare these with accretion rates from other studies. Methods. The far-infrared [O I]63 μm emission line is predicted to be the main coolant of dense, dissociative J-shocks caused by decelerated wind or jet shocks. If proper shock conditions prevail, the instantaneous mass-ejection rate is directly connected to the [O I]63 μm luminosity. In order to unravel evolutionary trends, we analysed a set of 14 Class 0/I outflow sources that were spatially resolved in the [O I]63 emission. We compared these data with a sample of 72 Class 0/I/II outflow sources that have been observed with Herschel (WISH, DIGIT, WILL, GASPS surveys) without spatially resolving the [O I]63 μm line. Results. All our newly observed targets feature prominent [O I]63μm emission either close to the driving source (L1551 IRS5, HH 1, HH 212) or as extended jet-like or knotty emission region away from it (Cep E). The detected [O I]63 μm emission can mostly be attributed to dissociative shocks and photodissociation regions (PDRs). Flux values at 63 μm and 145 μm of all four associated continuum sources are presented. We calculated mass-loss rates connected to the low-excitation, atomic outflow component in the range of (5−50)×10−7 M⊙ yr−1. Estimated ratios between the mass loss in the outflow and the mass accretion onto the source (jet efficiency ratios) are largely in the range of Ṁout/Ṁacc ∼ 0.05 − 0.5 for the observed outflow sources, which are consistent with theoretical predictions and quoted Herschel data. Conclusions. Our new observations and a comparison with the 72 outflow sources observed with Herschel indicate that the bulk ejected material in outflows from Class 0 sources resides in the molecular component, that is mass-loss rates derived from the [O I]63 emission line significantly underestimate the total mass-loss rate during this and possibly also later phases of the star formation process.