The SPHINX M-dwarf Spectral Grid. II. Fundamental Properties of Mid-to-Late M-dwarfs

Low-mass M-dwarfs are the dominant stellar class representing a significant population of our Galaxy. Spanning a wide range of masses to the hydrogen burning limit 0.08-0.6 solar mass, and cooler temperatures (2000 - 3800 K), they endure varying physical processes in their interiors and atmospheres. Standard stellar atmosphere models struggle to characterize dust/cloud condensation, convection, and magnetic activity-induced photospheric effects such as spots. Part I of this paper series introduced a new stellar synthesis grid of model atmospheres and spectra-SPHINX. We validated SPHINX solving for fundamental stellar properties of early-M-dwarfs, both part of benchmark FGK+binary systems and those with interferometric angular diameter measurements. In part II, we expand the capability of the model grid and present SPHINX+, which includes a gray cloud opacity causing a continuum effect in the atmosphere mimicking suppression of flux in the shorter wavelengths of the spectra and reddening/redistribution at longer wavelengths. We also include model spectra and atmospheres assuming a lower convective mixing length value, reducing the region in the stellar atmosphere with efficient convection. Altering these properties in the model produces improved fits relevant for mid-to-late type M-dwarfs. We perform a series of validation tests for SPHINX+ by acquiring fundamental atmospheric properties for 32 mid-to-late-type M-dwarfs from low-resolution data taken from the SpeX Prism Library Database and with SpeX IRTF spectra of 39 mid-to-late-type M-dwarfs that are companions to main-sequence FGKM primaries observed by Mann et al. 2014. Our model-derived values have comparable precision as the empirical constraints, with 0.078 dex in metallicities and 13% precision in M-dwarf C/O ratios.