Sloan Digital Sky Survey IV MaStar: Quantification and Abatement of Interstellar Absorption in the Largest Empirical Stellar Spectral Library

We assess the impact of Ca ii λ λ3934, 3969 and Na i λ λ5891, 5897 absorption arising in the interstellar medium (ISM) on the Sloan Digital Sky Survey-IV MaNGA Stellar Library (MaStar) and produce corrected spectroscopy for 80% of the 24,162-star catalog. We model the absorption strength of these transitions as a function of the stellar distance, Galactic latitude, and dust reddening based on high-spectral resolution studies. With this model, we identify 6342 MaStar stars that have negligible ISM absorption (W ^ISM(Ca ii K) < 0.07 Å and W ^ISM(Na i 5891) < 0.05 Å). For 12,110 of the remaining stars, we replace their Na i D profile (and their Ca ii profile for effective temperatures T _eff > 9000 K) with a coadded spectrum of low-ISM stars with similar T _eff, surface gravity, and metallicity. For 738 additional stars with T _eff > 9000 K, we replace these spectral regions with a matching ATLAS9-based BOSZ model. This results in a mean reduction in W(Ca ii K) (W(Na i D)) of 0.4-0.7 Å (0.6-1.1 Å) for hot stars (T _eff > 7610 K), and a mean reduction in W(Na i D) of 0.1-0.2 Å for cooler stars. We show that interstellar absorption in the simple stellar population (SSP) model spectra constructed from the original library artificially enhances W(Ca ii K) by ≳20% at young ages (<400 Myr); dramatically enhances the strength of stellar Na i D in starbursting systems (by ≳50%); and enhances stellar Na i D in older stellar populations (≳10 Gyr) by ≳10%. We provide SSP spectra constructed from the cleaned library and discuss the implications of these effects for stellar population synthesis analyses constraining the stellar age, [Na/Fe] abundance, and initial mass function.