Dynamics of magnetic flux tubes in accretion disks of Herbig Ae/Be stars

TL;DR

This paper models the behavior of magnetic flux tubes in Herbig Ae/Be star accretion disks, revealing their periodic rise and oscillations that may explain observed infrared variability.

Contribution

It introduces a comprehensive MHD model of MFT dynamics in Herbig Ae/Be disks, accounting for buoyancy, drag, heat exchange, and magnetic fields, and links oscillations to IR variability.

Findings

MFTs rise at speeds up to 12 km/s from the inner disk regions.

Oscillation periods increase with distance from the star, from hours to months.

Oscillations may explain the IR variability observed in Herbig Ae/Be stars.

Abstract

The dynamics of magnetic flux tubes (MFTs) in the accretion disk of typical Herbig Ae/Be star with fossil large-scale magnetic field is modeled taking into account the buoyant and drag forces, radiative heat exchange with the surrounding gas, and the magnetic field of the disk. The structure of the disk is simulated using our magnetohydrodynamic (MHD) model, taking into account the heating of the surface layers of the disk with the stellar radiation. The simulations show that MFTs periodically rise from the innermost region of the disk with speeds up to $10-12$ km s$^{-1}$. MFTs experience decaying magnetic oscillations under the action of the external magnetic field near the disk's surface. The oscillation period increases with distance from the star and initial plasma beta of the MFT, ranging from several hours at $r=0.012$ au up to several months at $r=1$ au. The oscillations are characterized by pulsations of the MFT's characteristics including its temperature. We argue that the oscillations can produce observed IR-variability of Herbig Ae/Be stars, which would be more intense than in the case of T Tauri stars, since the disks of Herbig Ae/Be stars are hotter, denser and have stronger magnetic field.