The <tt>SPHINX</tt> M Dwarf Spectral Grid. II. New Model Atmospheres and Spectra to Derive the Fundamental Properties of Mid-to-late Type M Dwarfs

Abstract M dwarfs are the most dominant stars in the Galaxy. Their interiors and atmospheres exhibit complex processes including dust condensation, convective feedback, and magnetic activity-driven heterogeneity. Standard stellar characterization methods often struggle to capture these coupled effects. Paper I of this series introduced SPHINX I , a validated grid of self-consistent radiative-convective model atmospheres and spectra for M dwarfs with up-to-date molecular opacities suitable for early-to-mid M dwarfs. Here, we present SPHINX II , which extends the model grid to cover mid-to-late type M dwarfs, including both gray and physically motivated condensate cloud treatments and shorter convective mixing lengths. We validate SPHINX II using 39 benchmark FGK+M binary systems observed with SpeX/IRTF and apply it to 32 mid-to-late type M dwarfs from the SpeX Prism Library. SPHINX II yields improved fits that are statistically consistent with empirical benchmarks, achieving precisions of 0.078 dex in metallicity and 0.13 dex in C/O. Across the model grid, condensate cloud mass peaks between 2100 and 2400 K, decreasing sharply toward both cooler and hotter temperatures. We find the onset of the cloud-free regime around ∼2900 K, and below 2100 K, we see the formation of deep/buried clouds. As a case study, we also model Trappist-1 and show that even mass-limited silicate grains subtly modify its emergent spectrum, suppressing near-infrared flux and reddening the mid-infrared slope via shallow cloud formation near 10 −2 bar. In sum, SPHINX II provides an improved framework for constraining the fundamental properties of mid-to-late M dwarfs.