Photoionization Models of the Inner Gaseous Disk of the Herbig Be Star BD+65 1637

TL;DR

This study models the inner gaseous disk of the Herbig Be star BD+65 1637 using non-LTE codes and observed spectra to constrain its physical properties, revealing the need for more complex density models.

Contribution

It introduces a detailed non-LTE modeling approach to analyze the inner disk of BD+65 1637, highlighting limitations of simple power-law density models.

Findings

Single power-law density models cannot fit all emission lines simultaneously.

Metal lines, especially Ca II IR triplet, indicate higher disk densities.

A shallower density fall-off better matches Ca II line strengths.

Abstract

We attempt to constrain the physical properties of the inner, gaseous disk of the Herbig Be star BD+65 1637 using non-LTE, circumstellar disk codes and observed spectra (3700 to 10,500 \r{A}) from the ESPaDOnS instrument on CFHT. The photoionizing radiation of the central star is assumed to be the sole source of input energy for the disk. We model optical and near-infrared emission lines that are thought to form in this region using standard techniques that have been successful in modeling the spectra of Classical Be stars. By comparing synthetic line profiles of hydrogen, helium, iron and calcium with the observed line profiles, we try to constrain the geometry, density structure, and kinematics of the gaseous disk. Reasonable matches have been found for all line profiles individually; however, no disk density model based on a single power-law for the equatorial density was able to simultaneously fit all of the observed emission lines. Amongst the emission lines, the metal lines, especially the Ca II IR triplet, seem to require higher disk densities than the other lines. Excluding the Ca II lines, a model in which the equatorial disk density falls as $10^{-10} (R_{*}/R)^3 g\,cm^{-3}$ seen at an inclination of 45{\deg} for a $50\,R_{*}$ disk provides reasonable matches to the overall line shapes and strengths. The Ca II lines seem to require a shallower drop off as $10^{-10} (R_{*}/R)^2 g\,cm^{-3}$ to match their strength. More complex disk density models are likely required to refine the match to the BD+65 1637 spectrum.