Gravity-sensitive Spectral Indices in Ultracool Dwarfs: Investigating Correlations with Metallicity and Planet Occurrence using SpeX and FIRE Observations

Abstract: We present a near-infrared spectroscopic analysis (0.9-2.4 micron) of gravity indices for 57 ultracool dwarfs (spectral types M5.5 to L0), including exoplanet hosts TRAPPIST-1, SPECULOOS-2, SPECULOOS-3, and LHS 3154. Our dataset includes 61 spectra from the SpeX and FIRE spectrographs. Using gravity-sensitive indices such as FeH absorption (at 0.99, 1.20, and 1.55 microns), the VO band at 1.06 microns, the H-band continuum, and alkali lines like K I (at 1.17 and 1.25 microns), we investigate correlations between surface gravity, stellar metallicity, and the presence of close-in transiting planets. All four planet-hosting stars show intermediate-gravity spectral signatures despite indicators of field age. However, a volume-corrected logistic regression reveals no significant association between gravity class and planet occurrence. Among individual indices, FeH_z is the most promising tracer of planet-hosting status. We tentatively identify a correlation between FeH_z (0.99 micron) and planet presence at the 2-sigma level, though this may reflect observational biases including transit probability, small-number statistics, and detection sensitivity. More robustly, we find a significant anti-correlation between FeH_z and metallicity ([Fe/H]) at 3.3 sigma. A Kruskal-Wallis test shows no significant metallicity difference across gravity classes, suggesting the observed FeH_z-metallicity trend is not driven by bulk metallicity differences. We propose this anti-correlation reflects interplay between age, gravity, and composition: higher-metallicity objects may be systematically younger with lower gravities, suppressing FeH absorption. While our results only hint at a link between gravity-related characteristics and planet occurrence among late-M dwarfs, they underscore the need for caution when using spectral diagnostics to infer properties of planet-hosting ultracool dwarfs.