Magnetic Activity as a Driver of Low-gravity-like Spectral Morphology in Late-M Dwarfs
Abstract
We present a systematic investigation of the connection between magnetic activity and the low-surface-gravity-like near-infrared spectral morphology of late-M dwarfs. Our sample includes TRAPPIST-1 observed with JWST NIRISS/SOSS and 22 late-M dwarfs observed with IRTF/SpeX, together with flare observations from K2, TESS, JWST NIRISS/SOSS, and literature-reported X-ray luminosities. Using one-component PHOENIX/NewEra spectral modeling, we find that all targets are better reproduced by lower-surface-gravity model spectra than by models with the expected high gravities of field late-M dwarfs. The retrieved surface gravities moderately correlate with independently measured gravity-sensitive spectral indices, with the strongest trends seen for FeHz at 0.998~μm and FeHJ at 1.2~μm, both showing statistically significant Pearson and Spearman correlations with p<0.005. They also show strong anti-correlations with flare luminosity and X-ray luminosity: for both relations, Pearson and Spearman coefficients are <−0.9, with p ≤ 1.0×10−4, indicating that more active stars tend to have spectra favoring lower-gravity morphologies. Time-series spectra of TRAPPIST-1 during two large flares show no clear short-timescale variations in selected photospheric indices, suggesting that persistent small-scale magnetic activity may play a more important role than large flares in modifying the photospheric temperature--pressure structure. A simplified radiative-transfer experiment further shows that activity-associated heating can produce lower-gravity-like spectral features even when the input surface gravity is fixed. Our results suggest that underestimated surface gravities in late-M dwarfs may reflect atmospheres modified by frequent, small-scale magnetic activity, such as microflares, producing emergent spectra that mimic lower-gravity morphology.
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BibTeX
@misc{lin2026magnetic,
title = {{Magnetic Activity as a Driver of Low-gravity-like Spectral Morphology in Late-M Dwarfs}},
author = {Chia-Lung Lin and Daniel Apai and Benjamin V. Rackham and Prajwal Niraula and Mark S. Giampapa and Ana Glidden and Julien de Wit and Valeriy Vasilyev and Nadiia Kostogryz and Alexander Shapiro},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.31647},
eprint = {2610.31647},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.31647},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Magnetic Activity as a Driver of Low-gravity-like Spectral Morphology in Late-M Dwarfs
AU - Chia-Lung Lin
AU - Daniel Apai
AU - Benjamin V. Rackham
AU - Prajwal Niraula
AU - Mark S. Giampapa
AU - Ana Glidden
AU - Julien de Wit
AU - Valeriy Vasilyev
AU - Nadiia Kostogryz
AU - Alexander Shapiro
PY - 2026
DA - 2026/10/22
PB - arXiv
DO - 10.48550/arxiv.2610.31647
UR - https://arxiv.org/abs/2610.31647
AB - We present a systematic investigation of the connection between magnetic activity and the low-surface-gravity-like near-infrared spectral morphology of late-M dwarfs. Our sample includes TRAPPIST-1 observed with JWST NIRISS/SOSS and 22 late-M dwarfs observed with IRTF/SpeX, together with flare observations from K2, TESS, JWST NIRISS/SOSS, and literature-reported X-ray luminosities. Using one-component PHOENIX/NewEra spectral modeling, we find that all targets are better reproduced by lower-surface-gravity model spectra than by models with the expected high gravities of field late-M dwarfs. The retrieved surface gravities moderately correlate with independently measured gravity-sensitive spectral indices, with the strongest trends seen for FeH_z at 0.998~μm and FeH_J at 1.2~μm, both showing statistically significant Pearson and Spearman correlations with p<0.005. They also show strong anti-correlations with flare luminosity and X-ray luminosity: for both relations, Pearson and Spearman coefficients are <−0.9, with p ≤ 1.0×10^−4, indicating that more active stars tend to have spectra favoring lower-gravity morphologies. Time-series spectra of TRAPPIST-1 during two large flares show no clear short-timescale variations in selected photospheric indices, suggesting that persistent small-scale magnetic activity may play a more important role than large flares in modifying the photospheric temperature--pressure structure. A simplified radiative-transfer experiment further shows that activity-associated heating can produce lower-gravity-like spectral features even when the input surface gravity is fixed. Our results suggest that underestimated surface gravities in late-M dwarfs may reflect atmospheres modified by frequent, small-scale magnetic activity, such as microflares, producing emergent spectra that mimic lower-gravity morphology.
ER -