Non-LTE Calculation for the Be Star Decretion Disk

TL;DR

This study models the non-LTE hydrogen gas state in Be star decretion disks, revealing the importance of stellar radiation and temperature minima, and compares observable predictions with actual star data.

Contribution

It introduces an iterative non-LTE calculation method for Be star disks, incorporating radiation transfer and temperature structure analysis, advancing understanding of disk physics.

Findings

Stellar Balmer continuum significantly influences the non-LTE state.

Disk temperature exhibits double minima in optically thick regions.

Model reproduces observed continuum quantities but not strong Hα emission.

Abstract

The non-LTE state of the hydrogen gas in isothermal transonic decretion disks around a B1V star has been calculated by an iterative method in order to explore the basic physical process in the disk. This dynamical model is characterized by a density law in the equatorial plane of $\rho(R) \propto R^{-3.5}$. The continuous radiation is calculated with the $\Lambda$ iteration in the integral form, while we adopt a single-flight escape probability for lines. We describe the non-LTE state, the radiation flow and conversion in the disk. We conclude that the stellar Balmer continuum plays a key role in the non-LTE state of the disk. The examination of the local energy gain and loss suggests that the disk temperature has double minima along the equatorial plane in the optically thick case: the intermediate region caused by deficient ultraviolet radiation and the outer Lyman $\alpha$ cooling region. We have also calculated some observable quantities, such as the spectral energy distribution, the $UBV$ colors, the infrared excess and the Balmer line profiles. Our calculations with the mass loss rate less than $10^{-10} M_{\odot} {\rm yr}^{-1}$ reproduce the observed continuum quantities. However, we could not get large H$\alpha$ emission strength observed in Be stars. We suggest that the density gradient of the Be star disk is slower than that of the isothermal decretion disk.