Velocity asymmetries in young stellar object jets

Abstract

Context. It is well established that some YSO jets (e.g. RW Aur) display different propagation speeds between their blue and red shifted parts, a feature possibly associated with the central engine or the environment in which the jet propagates. Aims. To understand the origin of asymmetric YSO jet velocities, we investigate the efficiency of two candidate mechanisms, one based on the intrinsic properties of the system and the other on the role of the external medium. In particular, a parallel or anti-parallel configuration between the protostellar magnetosphere and the disk magnetic field is considered, and the resulting dynamics examined both in an ideal and in a resistive magneto-hydrodynamical (MHD) regime. Moreover, we explore the effects of a potential difference in the pressure of the environment, as a consequence of the nonuniform density distribution of molecular clouds. Methods. Ideal and resistive axisymmetric numerical simulations were carried out for a variety of models, all of which are based on a combination of two analytical solutions, a disk wind and a stellar outflow. The initial two-component jet is modified by either inverting the orientation of its inner magnetic field or imposing a constant surrounding pressure. The velocity profiles are studied by assuming steady flows as well as after strong time variable ejection is incorporated. Results. Discrepancies between the speeds of the two outflows in opposite directions can indeed occur both due to unaligned magnetic fields and different outer pressures. In the former case, the asymmetry appears only on the dependence of the velocity on the cylindrical distance, but the implied observed value is significantly altered when the density distribution is also taken into account. On the other hand, a nonuniform medium collimates the two jets unevenly, directly affecting their propagation speed. A further interesting feature of the pressure-confined outflow simulations is the formation of static knots whose spacing seems to be associated with the ambient pressure. Conclusions. Jet velocity asymmetries are anticipated both when multipolar magnetic moments are present in the star-disk system and when nonuniform environments are considered. The latter is an external mechanism that can easily explain the large timescale of the phenomenon, whereas the former naturally relates it to the YSO intrinsic properties.

The outflow history of two Herbig-Haro jets in RCW 36: HH 1042 and HH 1043

“…However, the values for the period and amplitude of this orbit render an unrealistically high companion mass. Additional explanations for asymmetries in the launching mechanism are different ISM conditions or magnetic field configurations (Matsakos et al 2012). However, these generally lead to a systematic asymmetry in the velocity of the blue and red lobes, while we observe a temporal asymmetry in the velocity modulations.…”

LBT/LUCIFER near-infrared spectroscopy of PV Cephei

“…It might be intrinsic to the source and strictly linked to the launching mechanism (e.g., asymmetry in the magnetic field configuration), or extrinsic, i.e. caused by different physical conditions in the circumstellar medium, such as an inhomogeneous medium (for a detailed discussion see, Matsakos et al 2012). In principle, the former scenario could explain both the observed YSO variability (which cannot be triggered by the presence of a close companion) and the asymmetries observed in the jet.…”

Young stellar object jet models: From theory to synthetic observations

“…Three main approaches have been followed to address the phenomenon: high angular resolution observations to pinpoint their properties, analytical treatment to formulate the appropriate physical context, and numerical simulations to explore their complicated time-dependent dynamics. Although several studies have linked any two of the above approaches, namely, theory and observations (e.g., Cabrit et al 1999;Ferreira et al 2006;Sauty et al 2011), simulations and observations (e.g., Massaglia et al 2005;Teşileanu et al 2009Teşileanu et al , 2012Staff et al 2010), and theory and simulations (e.g., Gracia et al 2006;Stute et al 2008;Čemeljić et al 2008;Matsakos et al 2008Matsakos et al , 2009Matsakos et al , 2012Sauty et al 2012), there has not yet been any significant effort to combine all three of them.…”

“…The first one assumes that the ejection takes place below the computational box, so the flow properties are specified as boundary conditions on the lower ghost zones. This approach is appropriate to modeling the large scale jet structure and allows a wide parameter space to be explored (Ouyed & Pudritz 1997a,b, 1999Krasnopolsky et al 1999;Anderson et al 2005;Pudritz et al 2006;Fendt 2006Fendt , 2009Matt & Pudritz 2008;Matsakos et al 2008Matsakos et al , 2009Matsakos et al , 2012Staff et al 2010;Teşileanu et al 2012). The second approach has the flow evolve together with the dynamics of the disk, and thus is very demanding computationally.…”