Herbig Stars' Near-Infrared Excess: An Origin in the Protostellar Disk's Magnetically-Supported Atmosphere

TL;DR

This paper proposes that the near-infrared excess observed in Herbig Ae and Be stars originates from the magnetically-supported atmosphere of their protostellar disks, which reprocesses stellar light into infrared radiation.

Contribution

It introduces a model where the disk's magnetically-supported atmosphere explains the large near-infrared excess, supported by synthetic spectral energy distributions matching observations.

Findings

The atmosphere can be optically thick and tall enough to reprocess stellar light.

Synthetic SEDs match observed Herbig star spectra, especially in the near-infrared.

Grain composition near the disk's inner rim influences the infrared flux.

Abstract

Young stars with masses 2-8 Suns, called the Herbig Ae and Be stars, often show a near-infrared excess too large to explain with a hydrostatically-supported circumstellar disk of gas and dust. At the same time the accretion flow carrying the circumstellar gas to the star is thought to be driven by magneto-rotational turbulence, which according to numerical MHD modeling yields an extended low-density atmosphere supported by the magnetic fields. We demonstrate that the base of the atmosphere can be optically-thick to the starlight and that the parts lying near 1 AU are tall enough to double the fraction of the stellar luminosity reprocessed into the near-infrared. We generate synthetic spectral energy distributions (SEDs) using Monte Carlo radiative transfer calculations with opacities for sub-micron silicate and carbonaceous grains. The synthetic SEDs closely follow the median Herbig SED constructed recently by Mulders and Dominik, and in particular match the large near-infrared flux, provided the grains have a mass fraction close to interstellar near the disk's inner rim.