# Photoionization Models for the Inner Gaseous Disks of Herbig Be Stars: Evidence Against Magnetospheric Accretion?

## TL;DR

This study models the inner gaseous disks of Herbig Be stars to determine if optical and near-IR emission lines can be explained without magnetospheric accretion, suggesting a simple disk model suffices.

## Contribution

It demonstrates that a single power-law disk model can qualitatively reproduce emission lines without invoking magnetospheric accretion in Herbig Be stars.

## Key findings

- Good line profile matches achieved for individual lines
- A specific power-law density model best fits the data
- Inner disk mass estimated at approximately 10^-9 solar masses

## Abstract

We investigate the physical properties of the inner gaseous disks of the three, hot, Herbig B2e stars, HD 76534, HD 114981 and HD 216629, by modelling CFHT-ESPaDOns spectra using non-LTE radiative transfer codes. We assume that the emission lines are produced in a circumstellar disk heated solely by the photospheric radiation from the central star in order to test if the optical and near-IR emission lines can be reproduced without invoking magnetospheric accretion. The inner gaseous disk density was assumed to follow a simple power-law in the equatorial plane, and we searched for models that could reproduce observed lines of HI (H$\alpha$ and H$\beta$), HeI, CaII and FeII. For the three stars, good matches were found for all emission line profiles individually; however, no density model based on a single power-law was able to reproduce all of the observed emission lines. Among the single power-law models, the one with the gas density varying as $\sim 10^{-10}\,(R_{*}/R)^3\, \rm g\,cm^{-3}$ in the equatorial plane of a 25 R$_{*}$ ($0.78$ AU) disk did the best overall job of representing the optical emission lines of the three stars. This model implies a mass for the H$\alpha$-emitting portion of the inner gaseous disk of $\sim 10^{-9} M_*$. We conclude that the optical emission line spectra of these HBe stars can be qualitatively reproduced by a $\approx\,1$ AU, geometrically thin, circumstellar disk of negligible mass compared to the central star in Keplerian rotation and radiative equilibrium.