Proper motions of radiative knots in simulations of stellar jets - An alternative to pulsating inflow conditions

TL;DR

This study uses numerical simulations to explore how internal oblique shocks within stellar jets can produce observable knots with proper motions, offering an alternative explanation to pulsating inflow conditions.

Contribution

The paper demonstrates that internal oblique shocks caused by pressure gradients can generate moving knots in stellar jets without requiring pulsating inflow conditions.

Findings

Knots are formed by internal oblique shocks due to pressure gradients.

Knots move downstream with the jet flow, increasing velocity and decreasing brightness.

Pulsating inflow is not necessary to reproduce observed knot properties.

Abstract

Elongated jets from young stellar objects typically present a nodular structure, formed by a chain of bright knots of enhanced emission with individual proper motions. Though it is generally accepted that internal shocks play an important role in the formation and dynamics of such structures, their precise origin and the mechanisms behind the observed proper motions is still a matter of debate. Our goal is to study numerically the origin, dynamics, and emission properties of such knots. Axisymmetric simulations are performed with a shock-capturing code for gas dynamics, allowing for molecular, atomic, and ionized hydrogen in non-equilibrium concentrations subject to ionization/recombination processes. Radiative losses in SII lines are computed, and the resulting synthetic emission maps are compared with observations. We show that a pattern of regularly spaced internal oblique shocks, characterized by individual proper motions, is generated by the pressure gradient between the propagating jet and the time variable external cocoon. In the case of under-expanded, light jets the resulting emission knots are found to move downstream with the jet flow, with increasing velocity and decaying brightness toward the leading bow shock. This suggests that the basic properties of the knots observed in stellar jets can be reproduced even without invoking ad hoc pulsating conditions at the jet inlet, though an interplay between the two scenarios is certainly possible.