Laboratory experiments to recreate the accretion in T Tauri stars

Accretion is a ubiquitous phenomenon in the Universe, occurring over a wide rande of scales: From the small T Tauri stars to the enormous black holes in the center of galaxies. In particular, T Tauri stars are very young stars (a few million years old) still surrounded by a disk, called protoplanetary disk, and still acrreting gas from their disks. The rate at which T Tauri stars accrete material from their disks typically ranges from 10-8 a 10-10 solar masses per year, even if more extreme and transitory cases have been observed with accretion rates in the order of 10-5-10-6 solar masses per year. Accretion in T Tauri stars is mediated by the magnetic field connecting star and disk. The accreting material is in fact funneled by the magnetic field and forced to fall onto the star at free-fall velocity of several hundreds km/sec. Because of this high velocities, both the region of the star nearby the accretion columns and the gas in the accreting funnels is heated up to a few 105-106 degrees.

 

Several important phenomena are involved in the accretion process in T Tauri star. For instance, the magnetic interaction between star and disk, the influence of accretion over stellar atmosphere (photopshere, chromosphere, and corona), the thermal stratification setting along the accretion funnels and the shocks triggered inside the funnels, the temporal evolution of the magnetic field and of the accretion process in such complex and rapidly rotating systems. Astronomers study accretion in T Tauri stars mainly adopting three approaches. One method is based on the analysis of the high-energy optical, UV, and X-ray emission from the accreting material and the stellar surface heated up by accretion. Another method is based on the spectroscopic analysis of lines which are created or affected by the accretion process. This spectroscopic analysis allows us also to quantify the accretion rate and study the properties, such as temperature and pressure, of the gas inside the accretion funnels. Another approach is purely theoretical and it consists in the developing of magneto-hydrodynamic models of the accretion phenomenon.

 

An original method to study the accretion onto T Tauri stars was developed by the team of researchers led by XXX, with the participation also of the astronomers R. Bonito, C. Argiroffi, and S. Orlando of INAF – Astronomical Observatory of Palermo. This new method relies on laboratory experiments to reproduce the physical processes involved in the accretion onto T Tauri stars. In these experiments, plasma is created by using high-power lasers (1013 W/cm2). The plasma is then funneled in an external magnetic field and forced to impact an obstacle at velocities of hundreds km/sec.  To date, these experiments led to three publications. In the study “Laboratory unravelling of matter accretion in young stars” of G. Revet (Institute of Applied Physics, Nizhny Novgorod, Russia; École Polytechnique, CEA: Université Paris-Saclay), appeared on 2017 on the prestigious journal Science Advances, the authors have observed that the accreting plasma literally bounces over the impacted surface, and then it is funneled again by the magnetic field. In this way, the accretion funnels are surrounded by a layer of warmer and more dense plasma. Scaling up these results to match the astronomical case of the T Tauri stars, the authors suggest that this process can led to a severe absorption of the X-ray emission from the base of the accreting funnel. This could be the cause of the observed discrepancy of the accretion rates estimated from diagnostics based on optical and X-ray data. These results also led to new experiments described in the paper: “Laboratory evidence for asymmetric accretion structure upon slanted matter impact in young stars” of K. Burdonov (École Polytechnique, CEA: Université Paris-Saclay), accepted for publication on the journal XXX. In this paper the authors study how the inclination between the direction of the accreting funnels and the impacted surface affects the properties of the accretion process. In particular, they discovered that when the plasma is not accreting perpendicularly to the stellar surface, part of the material escapes from the base of the accretion funnels, and thus a larger portion of the stellar surface is affected by the accretion. This effect increases at increasing the inclination between the stellar surface and the direction of the accretion funnels. The recent study “Laboratory disruption of scaled astrophysical outflows by a misaligned magnetic field” of G. Revet (Institute of Applied Physics, Nizhny Novgorod, Russia; École Polytechnique, CEA: Université Paris-Saclay), accepted for publication in the journal XXX, describes how the experiments were used to study the mechanisms that led to the collimation of the jets observed in T Tauri stars. The authors have demonstrated that collimation depends on the inclination of the magnetic field with respect the direction of the jet.

 

The figure (click here to visualize the entire figure) shows the experimental setup used to study the accretion along a perpendicular direction with respect to the surface of the obstacle, and a simulation of the plasma flow at the impact.

 

Mario Giuseppe Guarcello  ( follow mguarce)