Accretion funnels onto weakly magnetized young stars

Abstract

Aims. We re-examine the conditions required to steadily deviate an accretion flow from a circumstellar disc into a magnetospheric funnel flow onto a slow rotating young forming star. Methods. New analytical constraints on the formation of accretion funnels flows due to the presence of a dipolar stellar magnetic field disrupting the disc are derived. The Versatile Advection Code is used to confirm these constraints numerically. Axisymmetric MHD simulations are performed, where a stellar dipole field enters the resistive accretion disc, whose structure is self-consistently computed.Results. The analytical criterion derived allows to predict a priori the position of the truncation radius from a non perturbative accretion disc model. Accretion funnels are found to be robust features which occur below the co-rotation radius, where the stellar poloidal magnetic pressure becomes both at equipartition with the disc thermal pressure and is comparable to the disc poloidal ram pressure. We confirm the results of Romanova et al. (2002, ApJ, 578, 420) and find accretion funnels for stellar dipole fields as low as 140 G in the low accretion rate limit of 10 −9 M yr −1 . With our present numerical setup with no disc magnetic field, we found no evidence of winds, neither disc driven nor X-winds, and the star is only spun up by its interaction with the disc. Conclusions. Weak dipole fields, similar in magnitude to those observed, lead to the development of accretion funnel flows in weakly accreting T Tauri stars. However, the higher accretion observed for most T Tauri stars (Ṁ ∼ 10 −8 M yr −1 ) requires either larger stellar field strength and/or different magnetic topologies to allow for magnetospheric accretion.

Accretion onto stars with octupole magnetic fields: Matter flow, hot spots and phase shifts

“…The main term, r (0) m = µ 4/7 1Ṁ −2/7 (GM) −1/7 , exactly coincides with the main term for spherical accretion (see also Bessolaz et al 2008). However, here the coefficient k 1 depends on α and the ratio c s /v K .…”

“…Comparison of numerical results for r m with Eqn. A.2 made in Long et al (2005) shows that for their set of simulations, k 1 ≈ 0.5 (see more comparisons in Bessolaz et al 2008).…”

Super-Eddington accretion on to a magnetized neutron star

“…Analytical models of disc accretion onto magnetized neutron star predict ξ from ≃ 0.5 (Ghosh et al 1977;Kluzniak & Rappaport 2007;Koenigl 1991) to 1 (Wang 1996). MHD simulations for small magnetospheres of young stars and white dwarfs give ξ = 0.4 − 0.5 (Long et al 2005;Bessolaz et al 2008). Recent simulations by Kulkarni & Romanova (2013) show variable ξ ≃ 0.77(µ 2 /Ṁ ) −0.086 that may be understood as different scaling of the magnetosphere size with the basic parameter set, roughly as Rin ∝ (µ 2 /Ṁ ) 1/5 .…”

MHD Simulations of Magnetospheric Accretion, Ejection and Plasma-field Interaction

“…We noticed that the condition β = 8πp/B 2 = 1 can also be used, in particular, in cases where v disk ≈ v m , and also for finding the position of the funnel streams, which are at slightly larger distances than the inner edge of the disk. [14] derived the magnetospheric radius from different conditions, and found only small difference between different radii r m . The magnetospheric radius has also been derived theoretically in analogy with the Alfvén radius in the case of spherical accretion (e.g., [15]):…”