Magnetic Activity as a Driver of Low-gravity-like Spectral Morphology in Late-M Dwarfs

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~$\mu$m and FeH${}_J$ at 1.2~$\mu$m, 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 $p\le 1.0\times {10}^{-4}$, 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.