Inferring possible magnetic field strength of accreting inflows in EXor-type objects from scaled laboratory experiments

TL;DR

This study uses scaled laboratory plasma experiments to estimate the magnetic field strength in accretion streams of EXor-type objects, providing new insights into their magnetic properties and accretion processes.

Contribution

It introduces a novel laboratory approach to infer magnetic field strengths in EXor accretion streams, bridging experimental plasma physics with astrophysical observations.

Findings

Estimated magnetic field in accretion stream: ~100 gauss

Inferred stellar surface magnetic field: a few kilogauss

Supported the hypothesis of effective plasma propagation across magnetic fields

Abstract

Aims. EXor-type objects are protostars that display powerful UV-optical outbursts caused by intermittent and powerful events of magnetospheric accretion. These objects are not yet well investigated and are quite difficult to characterize. Several parameters, such as plasma stream velocities, characteristic densities, and temperatures, can be retrieved from present observations. As of yet, however, there is no information about the magnetic field values and the exact underlying accretion scenario is also under discussion. Methods. We use laboratory plasmas, created by a high power laser impacting a solid target or by a plasma gun injector, and make these plasmas propagate perpendicularly to a strong external magnetic field. The propagating plasmas are found to be well scaled to the presently inferred parameters of EXor-type accretion event, thus allowing us to study the behaviour of such episodic accretion processes in scaled conditions. Results. We propose a scenario of additional matter accretion in the equatorial plane, which claims to explain the increased accretion rates of the EXor objects, supported by the experimental demonstration of effective plasma propagation across the magnetic field. In particular, our laboratory investigation allows us to determine that the field strength in the accretion stream of EXor objects, in a position intermediate between the truncation radius and the stellar surface, should be of the order of 100 gauss. This, in turn, suggests a field strength of a few kilogausses on the stellar surface, which is similar to values inferred from observations of classical T Tauri stars.