Overestimated Pressure Broadening Misleads Model Spectra in Cool M Dwarf Stars
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| Format: | Preprint |
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2026
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| _version_ | 1866908754620973056 |
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| author | Glidden, Ana Witzke, Veronika Shapiro, Alexander I. Seager, Sara |
| author_facet | Glidden, Ana Witzke, Veronika Shapiro, Alexander I. Seager, Sara |
| contents | 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$μ$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$μ$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. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2601_05313 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Overestimated Pressure Broadening Misleads Model Spectra in Cool M Dwarf Stars Glidden, Ana Witzke, Veronika Shapiro, Alexander I. Seager, Sara Solar and Stellar Astrophysics Earth and Planetary Astrophysics 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$μ$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$μ$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. |
| title | Overestimated Pressure Broadening Misleads Model Spectra in Cool M Dwarf Stars |
| topic | Solar and Stellar Astrophysics Earth and Planetary Astrophysics |
| url | https://arxiv.org/abs/2601.05313 |