Spectroscopic and interferometric signatures of magnetospheric accretion in young stars

TL;DR

This study uses spectroscopic and interferometric modeling to analyze magnetospheric accretion signatures in young stars, revealing how magnetic obliquity affects emission region size and line profile modulation.

Contribution

It introduces a novel application of non-LTE line formation modeling combined with interferometric simulations to study magnetospheric accretion in young stars.

Findings

Line profiles vary with rotational phase, showing inverse P Cygni features.

The emission region size decreases with increasing magnetic obliquity.

Interferometric size estimates are more compact than actual magnetosphere size.

Abstract

Methods. We use the code MCFOST to solve the non-LTE problem of line formation in non-axisymmetric accreting magnetospheres. We compute the Br{\gamma} line profile originating from accretion columns for models with different magnetic obliquities. We also derive monochromatic synthetic images of the Br{\gamma} line emitting region across the line profile. This spectral line is a prime diagnostics of magnetospheric accretion in young stars and is accessible with the long baseline near-infrared interferometer GRAVITY installed at the ESO Very Large Telescope Interferometer. Results. We derive Br{\gamma} line profiles as a function of rotational phase and compute interferometric observables, visibilities and phases, from synthetic images. The line profile shape is modulated along the rotational cycle, exhibiting inverse P Cygni profiles at the time the accretion shock faces the observer. The size of the line's emission region decreases as the magnetic obliquity increases, which is reflected in a lower line flux. We apply interferometric models to the synthetic visibilities in order to derive the size of the line-emitting region. We find the derived interferometric size to be more compact than the actual size of the magnetosphere, ranging from 50 to 90\% of the truncation radius. Additionally, we show that the rotation of the non-axisymmetric magnetosphere is recovered from the rotational modulation of the Br{\gamma}-to-continuum photo-centre shifts, as measured by the differential phase of interferometric visibilities.