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.
Highlights
Low-to-intermediate mass protostars (0.1−8 M ) accrete their mass from the material inside the circumstellar disc
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
The first group are FU Orionis objects, which are characterized by bursts with ∆V ∼ 6−8 mag, duration of decades, accretion rates of 10−5−10−4 M yr−1, and spectra dominated by absorption lines
Summary
Low-to-intermediate mass protostars (0.1−8 M ) accrete their mass from the material inside the circumstellar disc. This is done using two different methods and facilities, namely a plasma gun and a high-power laser, the details of which are presented in Sect. 4, we discuss the scalability of the laboratory experiment to the astrophysical phenomena of interest and compare the laboratory parameters with the particular object representing the episodic accretion event; and, we discuss the results and draw our conclusions regarding the magnetic field at play in the scaled EXor situations
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