Expanding the SPISEA Stellar Population Synthesis Software to the Substellar Regime

We present an extension of the SPISEA stellar population synthesis framework that adds brown dwarfs to the existing range of stellar mass objects, enabling physically consistent modeling of brown dwarfs within synthetic star clusters. Previous versions of SPISEA included limited substellar support, relying on outdated initial mass functions and incomplete atmospheric and evolutionary coverage below the hydrogen-burning limit. This was addressed through the implementation of a modern substellar initial mass function based on robust observational constraints, the introduction of merged atmospheric grids that smoothly transition between stellar and brown dwarf regimes, and the construction of unified evolutionary tracks spanning the lowest-mass brown dwarf objects through massive stars at solar metallicity. The updated framework was validated by comparing simulated color-magnitude diagrams to observational data from the Pleiades, Upper Scorpius, and M44 clusters using Gaia, UKIDSS, and 2MASS photometry. The new models allow for generation of user-specified isochrones and clusters that reproduce observed stellar behaviors while enabling realistic population synthesis in the brown dwarf regime. This work extends SPISEA's applicability to substellar science cases, including young cluster studies and microlensing simulations, and provides a foundation for future incorporation of planetary-mass objects and non-solar metallicities.