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

¹ LULI – CNRS, CEA, UPMC Univ. Paris 06, Sorbonne Université, Ecole Polytechnique, Institut Polytechnique de Paris, 91128 Palaiseau Cedex, France
e-mail: konstantin.burdonov@polytechnique.edu
² Sorbonne Université, Observatoire de Paris, PSL Research University, LERMA, CNRS UMR 8112, 75005 Paris, France
³ IAP, Russian Academy of Sciences, 603950 Nizhny Novgorod, Russia
⁴ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
⁵ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monteporzio Catone, Italy
⁶ Department of Physics and Chemistry, University of Palermo, 90133 Palermo, Italy
⁷ LNCMI, UPR 3228, CNRS-UGA-UPS-INSA, 31400 Toulouse, France
⁸ ELI-NP, ‘Horia Hulubei’ National Institute for Physics and Nuclear Engineering, 30 Reactorului Street, 077125 Bucharest-Magurele, Romania
⁹ National Research Nuclear University “MEPhI”, 115409 Moscow, Russia
¹⁰ Joint Institute for High Temperatures, RAS, 125412 Moscow, Russia

Received: 1 December 2020
Accepted: 6 February 2021

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 G. 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.