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

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.

Laser-generated supersonic plasma jets and shocks in a transverse magnetic field

“…These observations are supported by numerical simulations of the jet (see section 6), which show the formation of a cavity surrounded by an enhanced magnetic field. It is also in agreement with experiments studying the simpler case of a spherical/semi-spherical laser ablated plasma expanding into a transverse magnetic field [6,[9][10][11]14]. While the jets observed in this experiment differ in that they are created independently of the magnetic field, the peripheral plasma exhibits comparable structures.…”

“…Laboratory jets and shocks are studied through a number of different approaches, such as using high-current Z-pinch facilities [3,4], plasma guns [5,6], high power lasers combined with complex target configurations [7,8], as well as through the interaction of a spherically expanding laser-generated plasma with an externally applied magnetic field [9][10][11][12][13][14].…”