CHAPTER ONE

INTRODUCTION

Background to the study

Astrophysical jets are observed in the Universe in a large variety of environments and under a wide range of sizes and powers. They are generated in active galactic nuclei (AGNs) and YSOs, can travel up to a few thousands of Megaparsecs, and reach the largest powers observed in the Universe (up to ∼1047−48 erg s−1), (Zanni et al., 2003; Godfrey and Shabala, 2013). Astrophysical jets can be found in giant molecular clouds, emanating in the vicinities of young stellar objects (YSOs), and reaching distances of some parsecs (Reipurth and Bally, 2001). They are also located near neutron stars in galactic X-ray binary star systems, such as GRS 1915 + 105 that behave as microquasars generating relativistic jets (Fender, 2004). Astrophysical jets can be found in the asymptotic giant branch (post-AGB) stars as well in pre-planetary and planetary nebulae. Opposite, precessing jets are observed in the SS433 binary source, leading to a peculiar phenomenology (Frank, 2011). A jet-like structure is observed, at X-ray energies, inside the Crab Nebula departing from the embedded pulsar (Hester, 2008). Finally, jets can be at the base of the phenomenology of gamma-ray bursts, observed at the highest radiation energies that are still elusive phenomena because of their extreme distances (Granot, 2007).

Statement of problem

In recent years, we have seen many laboratory experiments that try to reproduce some aspects of the jet phenomenology, with particular concern towards YSO jets. These aspects are mainly related to the jet origin, propagation, and emission (Bellan et al 2005, 2009, Falize et al 2011, Gregory et al 2008, González et al 2009, Hartigan et al 2009, Lebedev et al 2005, Suzuki-Vidal 2010, Rus et al 2002).

With the goal to address the propagation of YSO jets and their interaction with the ambient medium, Tordella et al (2011) (Paper I) and Belan et al (2013) (Paper II) studied in the laboratory and by numerical means hypersonic hydrodynamic (HD) flows. In Paper I the authors considered as main jet parameters the Mach number M and the jet-to-ambient density ratio η and studied, both experimentally and numerically, representative cases of high and small values of density ratio. In Paper II they analyzed several cases with Mach number and density ratios consistent with those derived from the observations of YSO jets.

Objective of the study

1. In this paper we focus our attention on the measure of the spreading of the laboratory jets and the comparison with the results obtained by numerical simulations.

2. We also discuss the appearance of sinuous structures observed in some laboratory jets and the extent to which these features may develop as the response to non-axial perturbations in numerical simulations.

Significance of the study

We discuss observations used to infer the presence of accretion disks in astrophysical sources and the disks’ association with evidence for astrophysical jets. We highlight some important results from past and current literature that show parallels between the temporal behavior of an active galaxy (NGC 5128) and a galactic microquasar (GRS 1915+105). In addition, we note the remarkable observations of the time history of SCO X-1 from VLBI data at radio frequencies.

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