H2 mass–velocity relationship from 3D numerical simulations of jet-driven molecular outflows

Abstract

Context. Previous numerical studies have shown that in protostellar outflows, the outflowing gas mass per unit velocity, or mass–velocity distribution m(v), can be well described by a broken power law ∝ v−γ. On the other hand, recent observations of a sample of outflows at various stages of evolution show that the CO intensity–velocity distribution, closely related to m(v), follows an exponential law ∝ exp(−v∕v0). Aims. In the present work, we revisit the physical origin of the mass–velocity relationship m(v) in jet-driven protostellar outflows. We investigate the respective contributions of the different regions of the outflow, from the swept-up ambient gas to the jet. Methods. We performed 3D numerical simulations of a protostellar jet propagating into a molecular cloud using the hydrodynamical code Yguazú-a. The code takes into account the most abundant atomic and ionic species and was modified to include the H2 gas heating and cooling. Results. We find that by excluding the jet contribution, m(v) is satisfyingly fitted with a single exponential law, with v0 well in the range of observational values. The jet contribution results in additional components in the mass–velocity relationship. This empirical mass–velocity relationship is found to be valid locally in the outflow. The exponent v0 is almost constant in time and for a given level of mixing between the ambient medium and the jet material. In general, v0 displays only a weak spatial dependence. A simple modeling of the L1157 outflow successfully reproduces the various components of the observed CO intensity–velocity relationship. Our simulations indicate that these components trace the outflow cavity of swept-up gas and the material entrained along the jet, respectively. Conclusions. The CO intensity–velocity exponential law is naturally explained by the jet-driven outflow model. The entrained material plays an important role in shaping the mass–velocity profile.