Magnetic grain trapping and the hot excesses around early-type stars

TL;DR

This paper proposes that weak stellar magnetic fields can trap tiny dust grains near early-type stars, explaining the hot excess infrared emission observed, especially around rapidly rotating stars.

Contribution

It introduces a magnetic trapping mechanism for small dust grains close to stars, providing a natural explanation for hot excesses and their correlation with stellar rotation.

Findings

Magnetic fields can trap <200 nm dust grains near stars.

Trapping explains hot excess emission in interferometric observations.

Magnetic trapping is more effective in rapidly rotating stars.

Abstract

A significant fraction of main sequence stars observed interferometrically in the near infrared have slightly extended components that have been attributed to very hot dust. To match the spectrum appears to require the presence of large numbers of very small (< 200 nm in radius) dust grains. However, particularly for the hotter stars, it has been unclear how such grains can be retained close to the star against radiation pressure force. We find that the expected weak stellar magnetic fields are sufficient to trap nm-sized dust grains in epicyclic orbits for a few weeks or longer, sufficient to account for the hot excess emission. Our models provide a natural explanation for the requirement that the hot excess dust grains be smaller than 200 nm. They also suggest that magnetic trapping is more effective for rapidly rotating stars, consistent with the average vsini measurements of stars with hot excesses being larger (at about 2 sigma) than those for stars without such excesses.