Unveiling stellar aurorae: Simulating auroral emission lines in hot stars induced by high-energy irradiation

TL;DR

This study simulates auroral emission lines in hot stars caused by high-energy irradiation, identifying potential infrared lines, especially He ii 69458 A, and compares these with observations but finds no detections in FUSE spectra.

Contribution

The paper introduces a novel simulation approach for auroral lines in hot stars using TLUSTY, highlighting infrared emission lines induced by stellar wind interactions.

Findings

He ii 69458 A line appears in models with 15-30 kK temperatures.

Infrared excess is caused by high-energy irradiation.

No auroral lines were detected in FUSE observations.

Abstract

Auroral emission lines result from the interaction between magnetic field and stellar wind, offering valuable insights into physical properties and processes occurring within magnetospheres of celestial bodies. While extensively studied in planetary and exoplanetary atmospheres, in ultra-cool dwarfs, and as radio emission from early-type stars, the presence of specific auroral emission lines in hot star spectra remains unexplored. In this study, we utilized TLUSTY code to simulate the auroral lines, while modelling the effect of the interaction between stellar wind and magnetosphere through X-ray irradiation. Utilizing high-resolution synthetic spectra generated from model atmospheres, we identified potential candidate lines indicative of auroral emission, which were absent in non-irradiated spectra. Emission lines in synthetic spectra were present primarily in the infrared domain. The most prominent line generated by irradiation was He ii 69458 A, which appeared in all our model atmospheres with effective temperatures ranging from 15 kK to 30 kK. We also calculated the minimum irradiation required to detect emission in this most prominent line. The presence of emission lines was interpreted by considering changes in the population of different excited states of given atoms. Besides the appearance of infrared emission lines, high-energy irradiation causes infrared excess. To complement our simulations, we also searched for auroral lines in Far Ultraviolet Spectroscopic Explorer (FUSE) observations, which are deposited in the Multimission Archive at Space Telescope (MAST) catalogue. The comparison of observed spectra with synthetic spectra did not identify any possible candidate emission lines in FUSE spectra.