X-ray spectroscopy evidence for plasma shell formation in experiments modeling accretion columns in young stars

E. D. Filippov,J. Fuchs,S. N. Chen,A. Ciardi,D. Khaghani,G. Revet,S. A. Pikuz,D. P. Higginson,B. Khiar,I. Yu. Skobelev

doi:10.1063/1.5124350

Abstract

Recent achievements in laboratory astrophysics experiments with high-power lasers have allowed progress in our understanding of the early stages of star formation. In particular, we have recently demonstrated the possibility of simulating in the laboratory the process of the accretion of matter on young stars [G. Revet et al., Sci. Adv. 3, e1700982 (2017)]. The present paper focuses on x-ray spectroscopy methods that allow us to investigate the complex plasma hydrodynamics involved in such experiments. We demonstrate that we can infer the formation of a plasma shell, surrounding the accretion column at the location of impact with the stellar surface, and thus resolve the present discrepancies between mass accretion rates derived from x-ray and optical-radiation astronomical observations originating from the same object. In our experiments, the accretion column is modeled by having a collimated narrow (1 mm diameter) plasma stream first propagate along the lines of a large-scale external magnetic field and then impact onto an obstacle, mimicking the high-density region of the stellar chromosphere. A combined approach using steady-state and quasi-stationary models was successfully applied to measure the parameters of the plasma all along its propagation, at the impact site, and in the structure surrounding the impact region. The formation of a hot plasma shell, surrounding the denser and colder core, formed by the incoming stream of matter is observed near the obstacle using x-ray spatially resolved spectroscopy.

Highlights

Nowadays laser plasma is an excellent tool to simulate various magnetized supersonic and hydrodynamic flows.1–3 Recently, for example, collimated long-scaled plasma streams were used in experiments to simulate jets in Young Stellar Objects (YSOs).4 In these laboratory experiments, the most interesting processes occur far away from the laserirradiated surface of the source target, when the plasma is no longer heated by the laser
The experiments dedicated to the study of accretion dynamics in Classical T Tauri Stars (CTTSs)11 were performed at the nanosecond laser facility ELFIE at Ecole Polytechnique (France)
We developed a combined approach to characterize the dynamics of a magnetized plasma flow when it collides with a solid obstacle, mimicking the formation of accretion columns in young stars

Summary

Introduction

Nowadays laser plasma is an excellent tool to simulate various magnetized supersonic and hydrodynamic flows. Recently, for example, collimated long-scaled plasma streams were used in experiments to simulate jets in Young Stellar Objects (YSOs). In these laboratory experiments, the most interesting processes occur far away from the laserirradiated surface of the source target, when the plasma is no longer heated by the laser. The common steady-state models used to analyze x-ray spectroscopy observations are not relevant, and new models geared towards recombining plasmas are actively being developed This new method of x-ray spectroscopy was applied in the investigation of magnetized supersonic flows and has shown remarkable results.. The combination of external magnetic fields and laser-produced plasmas allowed us to create a stable, collimated plasma stream which, in turn, let us investigate more complicated hydrodynamic phenomena. This setup can be exploited to investigate the impact of a magnetized plasma column with an additional medium, e.g., a gas jet, solid obstacle, or another plasma. It has allowed us, by placing a solid obstacle in the path of the plasma column, as seen in Fig. 1, to simulate the astrophysical processes related to matter accretion onto a young star.

Objectives

Methods

Conclusion