Overestimated Pressure Broadening Misleads Model Spectra in Cool M Dwarf Stars

TL;DR

This study demonstrates that overestimated pressure broadening in stellar models leads to inaccurate spectra of cool M dwarf stars, and adjusting broadening parameters improves model-observation agreement.

Contribution

The paper shows that revising pressure broadening assumptions significantly enhances the accuracy of synthetic spectra for ultracool M dwarfs.

Findings

Minimal van der Waals broadening matches observed FeH features

Current broadening prescriptions are invalid for low-mass stars

Refined broadening treatments improve stellar and exoplanet characterization

Abstract

Available one-dimensional stellar models fail to reproduce the observed spectrum of the ultracool M dwarf TRAPPIST-1. In particular, current models predict strong iron hydride (FeH) absorption due to the Wing-Ford bands at 0.99$\mu$m, yet this spectral feature is only weakly present in TRAPPIST-1 and other mid-to-late M dwarf stars. Additionally, the shape of the continuum between the water bands in the near-infrared does not match between models and observations. Here, we show that assumptions about pressure broadening, specifically van der Waals broadening, have a dramatic effect on modeled broadband spectral features. We use Merged Parallelized Simplified-ATLAS to generate synthetic spectra over a range of van der Waals broadening strengths, adopting 1D PHOENIX temperature-pressure structures. We find that minimal broadening best matches the observed FeH profile at 0.99$\mu$m and in the pseudocontinuum between the large water bands. These results suggest that broadening prescriptions derived for Sun-like stars are not valid for lower-mass stars and that pressure broadening for molecular lines in cool stellar atmospheres must be reevaluated. Refining pressure broadening treatments will improve the accuracy of M dwarf spectral models, enabling more reliable determinations of stellar properties and atmospheric compositions of planets orbiting M dwarfs.